A continuation of the "Why does the 115V-24V transformer keep blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the wiring
diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the 115V
leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This third
unit is most certainly a proper replacement, and it's most certainly indicative
of a real problem I have somewhere else in the unit. I didn't see any such
charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower spins
freely when I turn it by hand. Start capacitor on the motor maybe? Relay on
the control board perhaps?

On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the wiring
diagrams that I also copied to my above flickr site as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the 115V
leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This third
unit is most certainly a proper replacement, and it's most certainly indicative
of a real problem I have somewhere else in the unit. I didn't see any such
charring on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the xfmr? I'd
put it on the secondary side. That way, if there is an overload, the
fuse will blow instead.


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On 4/8/2011 7:58 PM, David Nebenzahl wrote:
On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the wiring
diagrams that I also copied to my above flickr site as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the xfmr? I'd put it
on the secondary side. That way, if there is an overload, the fuse will blow
instead.

Thinking maybe I should spring for a transformer with a manual reset, like this
one:

http://www.pexsupply.com/White-Rodge...unt-14937000-p

Would that protect me from having to replace the damn transformer every time?
It looks like the circuit breaker is on the 24V output side; I'm a little fuzzy
on how that would protect the input side of the transformer...?

On 4/8/2011 8:22 PM Steve Turner spake thus:

On 4/8/2011 7:58 PM, David Nebenzahl wrote:

On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible
charring of the 115V leads going into the transformer, and of
course the 115V circuit is open (again). If you didn't see my
first thread, this is the third transformer the unit has blown.
In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second
blew because it wasn't a proper replacement (poor quality, made
in China, etc.). This third unit is most certainly a proper
replacement, and it's most certainly indicative of a real problem
I have somewhere else in the unit. I didn't see any such charring
on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the
xfmr? I'd put it on the secondary side. That way, if there is an
overload, the fuse will blow instead.

Thinking maybe I should spring for a transformer with a manual reset, like this
one:

http://www.pexsupply.com/White-Rodge...unt-14937000-p

Would that protect me from having to replace the damn transformer every time?
It looks like the circuit breaker is on the 24V output side; I'm a little fuzzy
on how that would protect the input side of the transformer...?

Why in the world would you think you need to protect the primary side?

Unless you've got a *really* weird problem--namely, severe overvoltage
spikes--there's no way the damage is coming from that side. It's got to
be because of overcurrent on the secondary side.

Unless there's something really messed up with your house's wiring ...


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On 4/8/2011 11:38 PM, David Nebenzahl wrote:
On 4/8/2011 8:22 PM Steve Turner spake thus:

On 4/8/2011 7:58 PM, David Nebenzahl wrote:

On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible
charring of the 115V leads going into the transformer, and of
course the 115V circuit is open (again). If you didn't see my
first thread, this is the third transformer the unit has blown.
In the previous discussion, it was discussed that perhaps the first one
just blew because of old age (6 years), and the second
blew because it wasn't a proper replacement (poor quality, made
in China, etc.). This third unit is most certainly a proper
replacement, and it's most certainly indicative of a real problem
I have somewhere else in the unit. I didn't see any such charring
on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the
xfmr? I'd put it on the secondary side. That way, if there is an
overload, the fuse will blow instead.

Thinking maybe I should spring for a transformer with a manual reset, like
this one:

http://www.pexsupply.com/White-Rodge...unt-14937000-p


Would that protect me from having to replace the damn transformer every time?
It looks like the circuit breaker is on the 24V output side; I'm a little
fuzzy on how that would protect the input side of the transformer...?

Why in the world would you think you need to protect the primary side?

Well I'm no electrician so I don't *know*. What I really mean is that I want
to protect *myself* from having to drop $20 or $30 a pop on a half-dozen
transformers while I'm trying to figure out what's wrong. So again, would the
transformer referenced above protect me from that? Many helpful things are
being suggested, but unfortunately they all seem to require testing the unit
while it's operational, and I can't do that unless I have a working transformer
installed.

Unless you've got a *really* weird problem--namely, severe overvoltage
spikes--there's no way the damage is coming from that side. It's got to be
because of overcurrent on the secondary side.

Ok, and how does that condition come into existence? Is there anything in the
wiring diagram that jumps out at you as a potential candidate? I have two
identical Trane AC/furnace units; one upstairs and one down. Should I swap
control boards between units to see if the problem follows the control board?

Unless there's something really messed up with your house's wiring ...

The house is only six years old. Nothing's changed in terms of wiring and
everything's been fine up until now.

In article ,
Steve Turner wrote:

snip

You might start at the beginning and make sure everything is hooked up
correctly. I mean, once you spring for yet another transformer. For
example, your latest fried transformer has six wires. The schematic
shows the secondary wire colors as blue and red. Given the location of
the blue and red wires on the pic of the transformer you have, that's
not quite intuitive.

And, it may not even be right. So I'd buy a new one, and check some
voltages on the bench. Put 120 into the black and white, and then
measure every other pair to make sure you understand which two wires are
really putting out 24 V.

Obviously, when you go to install it, clip and shrink tube the unused
wires, and make sure that leads are dressed so that they won't short
anywhere.

Then I'd be monitoring the damn thing, preferably with a voltmeter
perhaps on the primary and an ammeter on the secondary.

On 4/9/2011 12:38 AM, David Nebenzahl wrote:
On 4/8/2011 8:22 PM Steve Turner spake thus:

On 4/8/2011 7:58 PM, David Nebenzahl wrote:

On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible
charring of the 115V leads going into the transformer, and of
course the 115V circuit is open (again). If you didn't see my
first thread, this is the third transformer the unit has blown.
In the previous discussion, it was discussed that perhaps the first
one just blew because of old age (6 years), and the second
blew because it wasn't a proper replacement (poor quality, made
in China, etc.). This third unit is most certainly a proper
replacement, and it's most certainly indicative of a real problem
I have somewhere else in the unit. I didn't see any such charring
on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the
xfmr? I'd put it on the secondary side. That way, if there is an
overload, the fuse will blow instead.

Thinking maybe I should spring for a transformer with a manual reset,
like this one:

http://www.pexsupply.com/White-Rodge...unt-14937000-p


Would that protect me from having to replace the damn transformer
every time? It looks like the circuit breaker is on the 24V output
side; I'm a little fuzzy on how that would protect the input side of
the transformer...?

Why in the world would you think you need to protect the primary side?

Look at the picture. The primary side has been overheated, the
secondary side looks fine. Looks like an over voltage problem to me.

On 4/9/2011 8:30 AM, Tony Miklos wrote:
On 4/9/2011 12:38 AM, David Nebenzahl wrote:
On 4/8/2011 8:22 PM Steve Turner spake thus:

On 4/8/2011 7:58 PM, David Nebenzahl wrote:

On 4/8/2011 5:52 PM Steve Turner spake thus:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible
charring of the 115V leads going into the transformer, and of
course the 115V circuit is open (again). If you didn't see my
first thread, this is the third transformer the unit has blown.
In the previous discussion, it was discussed that perhaps the first
one just blew because of old age (6 years), and the second
blew because it wasn't a proper replacement (poor quality, made
in China, etc.). This third unit is most certainly a proper
replacement, and it's most certainly indicative of a real problem
I have somewhere else in the unit. I didn't see any such charring
on the previous two units, at least not like this.

How 'bout installing an in-line fuse next time you replace the
xfmr? I'd put it on the secondary side. That way, if there is an
overload, the fuse will blow instead.

Thinking maybe I should spring for a transformer with a manual reset,
like this one:

http://www.pexsupply.com/White-Rodge...unt-14937000-p



Would that protect me from having to replace the damn transformer
every time? It looks like the circuit breaker is on the 24V output
side; I'm a little fuzzy on how that would protect the input side of
the transformer...?

Why in the world would you think you need to protect the primary side?

Look at the picture. The primary side has been overheated, the secondary
side looks fine. Looks like an over voltage problem to me.

I wonder if our intrepid OP has done the basics involved in
troubleshooting instead of flitting around from place to place?
In the case of a power problem, you always start at the source by
measuring voltage and current. It could be a very simple problem
like a bit of insulation skinned off a thermostat wire that only
shorts out when everything is buttoned up and vibration from the
running unit causes the short circuit. If you ever watched the TV
program MASH, you would see the surgeons run the intestines looking
for a nick in the wall of the organ. Wiring is the same way sometimes.
I'll inspect the wiring and often find a small cut or tiny area where
insulation is missing and sometimes the wire is bent over a sharp edge
of the cabinet where it's fine until you close things up. I wonder if
the supply voltage to the transformer is what it's supposed to be? Most
domestic household air handler/furnaces are powered by 120vac but some
may require 240vac if there are electric heat strips. I would check the
voltage feeding the transformer first.

TDD

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again). If you didn't see my first thread, this is the
third transformer the unit has blown. In the previous discussion, it was
discussed that perhaps the first one just blew because of old age (6
years), and the second blew because it wasn't a proper replacement (poor
quality, made in China, etc.). This third unit is most certainly a
proper replacement, and it's most certainly indicative of a real problem
I have somewhere else in the unit. I didn't see any such charring on the
previous two units, at least not like this.

I never got a real chance to test out the system after installing this
third transformer. We had cool weather for several days, and I never
tried to force the system to come on so I could monitor it; that was
probably a mistake. Unfortunately, I was also absent from the premises
during the extended times when the unit was most likely operational, so
that didn't help either. However, my family tells me that it WAS working
and cooling the house rather nicely, for at least a day, perhaps two.
I'm getting 115V in all the right places, so it doesn't look like an
over-voltage condition to me. Perhaps it's an overheating condition? It
looks to me like the only real load on this circuit is the blower motor;
could the motor be causing this? The blower spins freely when I turn it
by hand. Start capacitor on the motor maybe? Relay on the control board
perhaps?
Hi,
Is the x-former gets hot when in use? Unless it is under rated for the
application, it should npot blow so often. It is just 120-24V AC step
down tranny. Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

On 4/8/2011 8:44 PM, Tony Hwang wrote:


Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.
....

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

On Apr 8, 9:05*pm, dpb wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

Put a voltmeter on the secondary side and measure the voltage when it
is not running. THen turn on the AC and see what the secondary
voltage does. IF it is supposed to operate a relay, and the current
draw is appropriate for the size transformer you have, I would not
expect the voltage to drop by more than 20%. Then leaving the AC on,
for at least 1/2 hour, keep monitoring the transformer tempoerature.
If it is a gradual heating, see how hot it gets. If there is a severe
overload, the transform may blow out before it even gets hot to the
touch. If it just gets warm over 10 - 20 minutes, then that is
probably about normal. IF you can find out what the correct current
drain is supposed to be, I would go for a transformer with double that
amperage ratinbg just ot be ultra-safe. But you have to be there and
monitor things, coming back 48 hours later is not going to work!!!

On Apr 8, 10:05*pm, dpb wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

I'd agree. Responding to his first post and before he put this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for advice
and instead of listening, he justs puts another transformer in
and blows it too..... Go figure. Time to either get educated
or hire a pro.

On 4/9/2011 9:12 AM, wrote:
On Apr 8, 10:05 pm, wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

I'd agree. Responding to his first post and before he put this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for advice
and instead of listening, he justs puts another transformer in
and blows it too..... Go figure. Time to either get educated
or hire a pro.

I don't understand the lack of a temporary fuse or circuit breaker in
troubleshooting this unit. The supply houses sell a little push to reset
breaker that can be installed in series with the transformer. Heck, a
box of fuses is less expensive than a transformer. :-)

TDD

On 4/9/2011 9:12 AM, wrote:
On Apr 8, 10:05 pm, wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

I'd agree. Responding to his first post and before he put this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for advice
and instead of listening, he justs puts another transformer in
and blows it too..... Go figure. Time to either get educated
or hire a pro.

In case you hadn't noticed, I've been getting LOTS of (much appreciated) advice
from many people, but it's also been very contradictory with no single
suggested approach. It's been an interesting and informative discussion, and
I've been happy that nobody's been a jerk about it... until *now*. And I DID
listen; plenty of people suggested that the original transformer was likely to
be under-rated and to replace it with something more heavy-duty. Also, it
seems to me that having a working transformer is a pre-requisite to following
your advice of measuring the amp draw on the low voltage side, no?
Unfortunately, time constraints prevented me from being there to perform those
measurements when the unit was running, and unfortunately again the transformer
blew in my absence. But of course, I'm repeating myself...

On 4/9/2011 10:48 AM, Steve Turner wrote:
On 4/9/2011 9:12 AM, wrote:
On Apr 8, 10:05 pm, wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is
wrong.

I'd venture something is already wrong...

--

I'd agree. Responding to his first post and before he put this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for advice
and instead of listening, he justs puts another transformer in
and blows it too..... Go figure. Time to either get educated
or hire a pro.

In case you hadn't noticed, I've been getting LOTS of (much appreciated)
advice from many people, but it's also been very contradictory with no
single suggested approach. It's been an interesting and informative
discussion, and I've been happy that nobody's been a jerk about it...
until *now*. And I DID listen; plenty of people suggested that the
original transformer was likely to be under-rated and to replace it with
something more heavy-duty. Also, it seems to me that having a working
transformer is a pre-requisite to following your advice of measuring the
amp draw on the low voltage side, no? Unfortunately, time constraints
prevented me from being there to perform those measurements when the
unit was running, and unfortunately again the transformer blew in my
absence. But of course, I'm repeating myself...


Steve I really hope you can figure out the problem with the furnace and
I can understand the bit about time constraints. Installing a fuse in
series with with either side of the transformer is cheap insurance to
keep from losing another transformer. The fuse holders are inexpensive
and it's a lot less of a hassle to replace a blown fuse.

TDD

On Apr 9, 11:48*am, Steve Turner
wrote:
On 4/9/2011 9:12 AM, wrote:





On Apr 8, 10:05 pm, *wrote:
On 4/8/2011 8:44 PM, Tony Hwang wrote:

Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

...

... Buy one of hevier one(bigger size and really heavier in
weight than stock) If it blows again, something on the load side is wrong.

I'd venture something is already wrong...

--

I'd agree. *Responding to his first post and before he put this
* latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
* * This is electricity 101. * So, he comes here asking for advice
* and instead of listening, he justs puts another transformer in
* and blows it too..... * Go figure. * Time to either get educated
or hire a pro.

In case you hadn't noticed, I've been getting LOTS of (much appreciated) advice
from many people, but it's also been very contradictory with no single
suggested approach. *It's been an interesting and informative discussion, and
I've been happy that nobody's been a jerk about it... until *now*.

OK, so now I'm a jerk for suggesting that you should have taken some
basic
measurements, starting with the transformer output current, instead of
just
continuing to replace transformers and watch them blow. A
component that supplies power fails by burning up. You've replaced
it
several times now. You think just MAYBE it's a good idea to see how
much power it's be asked to supply instead of just buyng new
transformers?
If a fuse blew out, would you just keep putting in new fuses or would
you
measure the current and see what it is? I'd have measured the
voltages
and currents on both sides of the transformer after the first one
failed.



*And I DID
listen; plenty of people suggested that the original transformer was likely to
be under-rated and to replace it with something more heavy-duty.

I find it hard to believe that Trane uses transformers in it's
furnaces that are
so under-rated that they burn out in a day. I also question the
soundness of
anyone telling you to start installing transformers capable of
delivering more
power without even taking some basic measurements and finding out
what's
drawing current and if it's excessive for some reason. If you had a
light
circuit that was blowing fuses, would your approach be to put in a
bigger fuse
or would it be to find out what's really going on? If you have a
short somewhere
and you put in a large enough transformer, you think you might
eventually heat
something up enough to start a fire, like maybe in the thermostat
wiring in
your walls?



*Also, it
seems to me that having a working transformer is a pre-requisite to following
your advice of measuring the amp draw on the low voltage side, no?
Unfortunately, time constraints prevented me from being there to perform those
measurements when the unit was running, and unfortunately again the transformer
blew in my absence.

So, you had the time to obtain a new transformer, put it in, but
didn't have the
1 minute it would take to use a VOM to measure the current and voltage
on
the secondary? If it were me and I saw transformers burning up, I
wouldn't
leave it on until I had some confidence as to what was going on.



*But of course, I'm repeating myself


Yes, by putting in each new transformer and watching it fail.

It does sound like there is unidentified problem. As the
fellows on alt hvac are often heard to say "replace the
thermostat".

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


wrote in message
...


I'd venture something is already wrong...

--

I'd agree. Responding to his first post and before he put
this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for
advice
and instead of listening, he justs puts another transformer
in
and blows it too..... Go figure. Time to either get
educated
or hire a pro.

On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's very unusual for the primary to go bad from a short on the
secondary side, and the picture shows that the secondary wasn't
overheated like the primary was. Where's all those electricians talking
about an open neutrals when you need them? If it measures 120vac tie a
light bulb into the primary side and leave it where it can be seen.
Have the family let you know if it ever gets brighter than normal. Or
better yet, call a pro.

On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at times. It's
very unusual for the primary to go bad from a short on the secondary side, and
the picture shows that the secondary wasn't overheated like the primary was.
Where's all those electricians talking about an open neutrals when you need
them? If it measures 120vac tie a light bulb into the primary side and leave it
where it can be seen. Have the family let you know if it ever gets brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could happen. The
unit is on a dedicated 115V circuit tied to a single-pole breaker. Where would
the possibility exist for the neutral to become hot? In the breaker box?

On 4/9/2011 12:19 AM, Steve Turner wrote:
On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf




As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the secondary
side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when you
need
them? If it measures 120vac tie a light bulb into the primary side and
leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. The unit is on a dedicated 115V circuit tied to a single-pole
breaker. Where would the possibility exist for the neutral to become
hot? In the breaker box?


It happens all the time.

What's the process, there? Neutral opens up, between the box
and the pole, and the power feeds in from the other leg?

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Tony Miklos" wrote in message
...
On 4/9/2011 12:19 AM, Steve Turner wrote:


That very thought crossed my mind, but I'm not sure how
that could
happen. The unit is on a dedicated 115V circuit tied to a
single-pole
breaker. Where would the possibility exist for the neutral
to become
hot? In the breaker box?


It happens all the time.

"Steve Turner" wrote in message
...
On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible charring
of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the
secondary side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when
you need
them? If it measures 120vac tie a light bulb into the primary side
and leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. The unit is on a dedicated 115V circuit tied to a
single-pole breaker. Where would the possibility exist for the
neutral to become hot? In the breaker box?


A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is used
on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric company
or electrician retorque the main panel lug nuts to the panel
manufacturers tightness specifications in inch lbs. Note POWER IS
ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming from
the breaker panel. A loose neutral connection with that one wire and
same problem.

When the entire panel is having this problem, you would see lights get
brighter as the problem occurs.

If just a problem with one MWBC, then only those circuits would have
the problem.

On Apr 9, 9:53*am, "Bill" wrote:
"Steve Turner" wrote in message

...





On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the
wiring diagrams that I also copied to my above flickr site as jpg
images):

http://www.trane.com/webcache/un/fur...product/22-166....

As you can probably see in the pictures, there is visible charring
of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the
secondary side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when
you need
them? If it measures 120vac tie a light bulb into the primary side
and leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. *The unit is on a dedicated 115V circuit tied to a
single-pole breaker. *Where would the possibility exist for the
neutral to become hot? *In the breaker box?

A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is used
on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric company
or electrician retorque the main panel lug nuts to the panel
manufacturers tightness specifications in inch lbs. Note POWER IS
ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming from
the breaker panel. A loose neutral connection with that one wire and
same problem.

When the entire panel is having this problem, you would see lights get
brighter as the problem occurs.

If just a problem with one MWBC, then only those circuits would have
the problem.- Hide quoted text -

- Show quoted text -

The problem with that theory is that the 120V circuit that supplies
power
to the transformer also powers the rest of the furnace, ie the inducer
blower, main blower, etc. If it were getting 240V, one would think by
now one of those other components would have failed. Also, with
240V it would be delivering 48V to the logic board, which one would
think would have opened a fuse on it or destroyed some electronics
by now.

I can't say I can make much sense of what is going on either. But
until someone with some basic electric skills capable of taking
some measurements looks at it, I doubt we're gonna fix it by
remote analysis.

On Apr 9, 10:21*am, "
wrote:
On Apr 9, 9:53*am, "Bill" wrote:



"Steve Turner" wrote in message

...

On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the
wiring diagrams that I also copied to my above flickr site as jpg
images):

http://www.trane.com/webcache/un/fur...product/22-166...

As you can probably see in the pictures, there is visible charring
of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the
secondary side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when
you need
them? If it measures 120vac tie a light bulb into the primary side
and leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. *The unit is on a dedicated 115V circuit tied to a
single-pole breaker. *Where would the possibility exist for the
neutral to become hot? *In the breaker box?

A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is used
on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric company
or electrician retorque the main panel lug nuts to the panel
manufacturers tightness specifications in inch lbs. Note POWER IS
ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming from
the breaker panel. A loose neutral connection with that one wire and
same problem.

When the entire panel is having this problem, you would see lights get
brighter as the problem occurs.

If just a problem with one MWBC, then only those circuits would have
the problem.- Hide quoted text -

- Show quoted text -

The problem with that theory is that the 120V circuit that supplies
power
to the transformer also powers the rest of the furnace, ie the inducer
blower, main blower, etc. *If it were getting 240V, one would think by
now one of those other components would have failed. *Also, with
240V it would be delivering 48V to the logic board, which one would
think would have opened a fuse on it or destroyed some electronics
by now.

I can't say I can make much sense of what is going on either. *But
until someone with some basic electric skills capable of taking
some measurements looks at it, I doubt we're gonna fix it by
remote analysis.

To OP,
I'm not going to wade through all the previous posts...

you pictures show a transformer with 6 leads.
the wiring diagrams show a transformer with 4 leads

how are you connecting the transformer..

Do you have a voltmeter... when you connected the transformer does it
blow up immediately or after a time?

Did you check the pri and sec voltages..

A failure like shown in the pic with big black scorch marks would
indicate a MAJOR overload like you have connected the transformer
completely wrong ...or there is some metal box or something that
shorts to the terminals. Does the main breaker blow too?

Find a ham radio guy or someone that knows a bit about electricity to
help you.

Mark

Agree, sounds like electrical problem, and the OP isn't
quite skilled enough to find it.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


wrote in message
...

The problem with that theory is that the 120V circuit that
supplies
power
to the transformer also powers the rest of the furnace, ie
the inducer
blower, main blower, etc. If it were getting 240V, one
would think by
now one of those other components would have failed. Also,
with
240V it would be delivering 48V to the logic board, which
one would
think would have opened a fuse on it or destroyed some
electronics
by now.

I can't say I can make much sense of what is going on
either. But
until someone with some basic electric skills capable of
taking
some measurements looks at it, I doubt we're gonna fix it by
remote analysis.

On Sat, 9 Apr 2011 06:53:25 -0700, "Bill"
wrote:

"Steve Turner" wrote in message
...
On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible charring
of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the
secondary side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when
you need
them? If it measures 120vac tie a light bulb into the primary side
and leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. The unit is on a dedicated 115V circuit tied to a
single-pole breaker. Where would the possibility exist for the
neutral to become hot? In the breaker box?


A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is used
on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric company
or electrician retorque the main panel lug nuts to the panel
manufacturers tightness specifications in inch lbs. Note POWER IS
ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming from
the breaker panel. A loose neutral connection with that one wire and
same problem.


MWBC is not legal for a furnace and never has been (at least in
Canada) and a main panel neutral problem should have manifested
iteself with light bulbs flaring/dimming or other indications of
"wonky" voltage elsewhere in the house.

When the entire panel is having this problem, you would see lights get
brighter as the problem occurs.

If just a problem with one MWBC, then only those circuits would have
the problem.

On 4/9/2011 2:42 PM spake thus:

On Sat, 9 Apr 2011 06:53:25 -0700, "Bill"
wrote:

A neutral in a breaker box could have a loose connection. These
connections can heat up and cool down as more/less electricity is
used on the panel in question. And there may be a point where the
connection fails momentarily or arcs. Then if you lose the neutral
connection, 120 volt circuits become 240 volt circuits via other
circuits turned on in the panel on the other leg.

The solution to that problem is usually to have the electric
company or electrician retorque the main panel lug nuts to the
panel manufacturers tightness specifications in inch lbs. Note
POWER IS ALWAYS HOT TO THESE CONNECTIONS and this is NOT a DIY
thing!

A similar problem can be with what is called a MWBC or Multi-Wire
Branch Circuit. These are 2 separate 120 V circuits run to the same
area that share one neutral wire. So 2 hots and 1 neutral coming
from the breaker panel. A loose neutral connection with that one
wire and same problem.

MWBC is not legal for a furnace and never has been (at least in
Canada) and a main panel neutral problem should have manifested
iteself with light bulbs flaring/dimming or other indications of
"wonky" voltage elsewhere in the house.

I agree. Suggesting that the OP's problem is due to a buggy Edison
circuit (and why don't they just call them that, since that's what
everyone else calls 'em?) is just grasping at straws. Possible, sure,
but highly unlikely.


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On 4/9/2011 12:19 AM, Steve Turner wrote:
On 4/8/2011 10:16 PM, Tony Miklos wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf




As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again).

Three times it fried the primary? I'd guess it's getting 240vac at
times. It's
very unusual for the primary to go bad from a short on the secondary
side, and
the picture shows that the secondary wasn't overheated like the
primary was.
Where's all those electricians talking about an open neutrals when you
need
them? If it measures 120vac tie a light bulb into the primary side and
leave it
where it can be seen. Have the family let you know if it ever gets
brighter
than normal. Or better yet, call a pro.

That very thought crossed my mind, but I'm not sure how that could
happen. The unit is on a dedicated 115V circuit tied to a single-pole
breaker. Where would the possibility exist for the neutral to become
hot? In the breaker box?

Yes, it's called an open neutral. Can be quite dangerous.

On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram)
he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg images):

It would seem to me that you are going to have to get yet another
transformer. Put a fuse on the primary, 1/2 A I would think. Slow blow,
perhaps less.

Then I'd check a couple things.

Note where it says that the integrated controller is polarity
sensitive. Check to make sure it is correct.

Then I would look to see if anything looks overheated on the
integrated controller.

I doubt it is any of the motors as those probably all run on line
voltage.

Jeff



http://www.trane.com/webcache/un/fur...7_04012009.pdf

On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing
on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the
wiring diagrams that I also copied to my above flickr site as jpg
images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of
the 115V leads going into the transformer, and of course the 115V
circuit is open (again). If you didn't see my first thread, this is
the third transformer the unit has blown. In the previous discussion,
it was discussed that perhaps the first one just blew because of old
age (6 years), and the second blew because it wasn't a proper
replacement (poor quality, made in China, etc.). This third unit is
most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I
didn't see any such charring on the previous two units, at least not
like this.

I never got a real chance to test out the system after installing this
third transformer. We had cool weather for several days, and I never
tried to force the system to come on so I could monitor it; that was
probably a mistake. Unfortunately, I was also absent from the premises
during the extended times when the unit was most likely operational,
so that didn't help either. However, my family tells me that it WAS
working and cooling the house rather nicely, for at least a day,
perhaps two. I'm getting 115V in all the right places, so it doesn't
look like an over-voltage condition to me. Perhaps it's an
overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The
blower spins freely when I turn it by hand. Start capacitor on the
motor maybe? Relay on the control board perhaps?

Steve,

If you are not comfortable with taking current measurements, or do not
have an ammeter, or do not wish to sit there waiting for some unusual
condition which is drawing too much current, you could temporarily
install an in-line fuse holder and automotive cartridge fuse rated at or
above the secondary side amperage which the transformer is rated. With
this fuse installed on the secondary side, in series with the load, you
could then determine if the load is indeed drawing too much current from
the secondary and burning out the transformer, versus primary side
excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing
about a quarter to a half an amp of current at 24V. The transformer
secondary should not need to source a lot more than that amount of
current to provide adequate power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in
the coil itself, a breakdown of the coil insulation allowing a short to
ground when the coil heats up or cools or vibrates, etc.

Smarty

On 4/9/2011 3:43 PM, Smarty wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of the wiring
diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

Steve,

If you are not comfortable with taking current measurements, or do not have an
ammeter, or do not wish to sit there waiting for some unusual condition which
is drawing too much current, you could temporarily install an in-line fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing about a
quarter to a half an amp of current at 24V. The transformer secondary should
not need to source a lot more than that amount of current to provide adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in the
coil itself, a breakdown of the coil insulation allowing a short to ground when
the coil heats up or cools or vibrates, etc.

Smarty

Yes Smarty, I would very much like to try this approach because I don't want to
zap any more transformers. Lots of people have suggested it, but there appear
to be more types of fuses than there are insects, and I have no idea what
*exact* type to look for. That's why I asked earlier in this thread if this
particular transformer:

http://www.pexsupply.com/White-Rodge...unt-14937000-p

with a built-in manual reset would be a much cleaner solution that serves the
same purpose, but I got no responses.

The specs on the original transformer we Class 2, 115V primary (60hz), 24V
(35VA) secondary. Any chance you could point me to some _exact_ fuse solutions
for this application? There's a virtual six-pack of beer in it for you. :-)

On 4/9/2011 2:21 PM Steve Turner spake thus:

On 4/9/2011 3:43 PM, Smarty wrote:

On 4/8/2011 8:52 PM, Steve Turner wrote:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

If you are not comfortable with taking current measurements, or do not have an
ammeter, or do not wish to sit there waiting for some unusual condition which
is drawing too much current, you could temporarily install an in-line fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing about a
quarter to a half an amp of current at 24V. The transformer secondary should
not need to source a lot more than that amount of current to provide adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in the
coil itself, a breakdown of the coil insulation allowing a short to ground when
the coil heats up or cools or vibrates, etc.

Yes Smarty, I would very much like to try this approach because I don't want to
zap any more transformers. Lots of people have suggested it, but there appear
to be more types of fuses than there are insects, and I have no idea what
*exact* type to look for.

Well, it's not exactly rocket science. And you can't hurt anything,
unless you get a fuse that's too big (i.e., rated at too many amps).

First thing you gotta figure out is how much current your transformer is
rated at. Since your wiring diagrams don't say, we have to take an
educated guess.

The only thing connected to the transformer is a thingamajig called the
"integrated furnace control" (IFC). Even without seeing this thing, I
can guess with confidence that it's an electronic circuit board which
controls the operation of the furnace. Since it undoubtedly does so
through relays (either mechanical or electronic), it's safe to assume
that it doesn't draw much current.

Let's say 1 amp to start. We could be wrong, but that's a good starting
point. So what we need is a fuse rated at *at least* 24 volts (could be
much more, up to 120 or 240 volts), and *exactly* 1 amp. To be on the
safe side, let's get a "fast blow" fuse. Hey, fuses are cheaper than
transformers, right?

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so use the next
bigger fuse; let's say 1.5 A.

Now you want to stop at some point, let's say 2-3 amps, because now
you're getting up to where you may be exposing whatever fault there is
with the unit and risking blowing the transformer again. If this
happens, you're going to have to bite the bullet and find out what's
wrong with the damn thing. But at least your 3rd transformer will still
be intact.


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

Yo! Neb!
S'appenin
In earlier post, he said it's 24 VAC, 35 VA.
Tnx
Ywlcm
by

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"David Nebenzahl" wrote in message
s.com...


Well, it's not exactly rocket science. And you can't hurt
anything,
unless you get a fuse that's too big (i.e., rated at too
many amps).

First thing you gotta figure out is how much current your
transformer is
rated at. Since your wiring diagrams don't say, we have to
take an
educated guess.

The only thing connected to the transformer is a thingamajig
called the
"integrated furnace control" (IFC). Even without seeing this
thing, I
can guess with confidence that it's an electronic circuit
board which
controls the operation of the furnace. Since it undoubtedly
does so
through relays (either mechanical or electronic), it's safe
to assume
that it doesn't draw much current.

Let's say 1 amp to start. We could be wrong, but that's a
good starting
point. So what we need is a fuse rated at *at least* 24
volts (could be
much more, up to 120 or 240 volts), and *exactly* 1 amp. To
be on the
safe side, let's get a "fast blow" fuse. Hey, fuses are
cheaper than
transformers, right?

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so
use the next
bigger fuse; let's say 1.5 A.

Now you want to stop at some point, let's say 2-3 amps,
because now
you're getting up to where you may be exposing whatever
fault there is
with the unit and risking blowing the transformer again. If
this
happens, you're going to have to bite the bullet and find
out what's
wrong with the damn thing. But at least your 3rd transformer
will still
be intact.


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On 4/9/2011 4:37 PM, David Nebenzahl wrote:
On 4/9/2011 2:21 PM Steve Turner spake thus:

On 4/9/2011 3:43 PM, Smarty wrote:

On 4/8/2011 8:52 PM, Steve Turner wrote:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf


As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

If you are not comfortable with taking current measurements, or do not have an
ammeter, or do not wish to sit there waiting for some unusual condition which
is drawing too much current, you could temporarily install an in-line fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing about a
quarter to a half an amp of current at 24V. The transformer secondary should
not need to source a lot more than that amount of current to provide adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in the
coil itself, a breakdown of the coil insulation allowing a short to ground when
the coil heats up or cools or vibrates, etc.

Yes Smarty, I would very much like to try this approach because I don't want
to zap any more transformers. Lots of people have suggested it, but there
appear to be more types of fuses than there are insects, and I have no idea
what *exact* type to look for.

Well, it's not exactly rocket science. And you can't hurt anything, unless you
get a fuse that's too big (i.e., rated at too many amps).

First thing you gotta figure out is how much current your transformer is rated
at. Since your wiring diagrams don't say, we have to take an educated guess.

The only thing connected to the transformer is a thingamajig called the
"integrated furnace control" (IFC). Even without seeing this thing, I can guess
with confidence that it's an electronic circuit board which controls the
operation of the furnace. Since it undoubtedly does so through relays (either
mechanical or electronic), it's safe to assume that it doesn't draw much current.

Let's say 1 amp to start. We could be wrong, but that's a good starting point.
So what we need is a fuse rated at *at least* 24 volts (could be much more, up
to 120 or 240 volts), and *exactly* 1 amp. To be on the safe side, let's get a
"fast blow" fuse. Hey, fuses are cheaper than transformers, right?

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so use the next bigger
fuse; let's say 1.5 A.

Now you want to stop at some point, let's say 2-3 amps, because now you're
getting up to where you may be exposing whatever fault there is with the unit
and risking blowing the transformer again. If this happens, you're going to
have to bite the bullet and find out what's wrong with the damn thing. But at
least your 3rd transformer will still be intact.

Great information David. Thanks!

As a heating and AC installer and repair tech, I assure you
that David's writings sound fancy, but he's not competent to
comment.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


"Steve Turner" wrote in
message ...
On 4/9/2011 4:37 PM, David Nebenzahl wrote:

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so
use the next bigger
fuse; let's say 1.5 A.

Now you want to stop at some point, let's say 2-3 amps,
because now you're
getting up to where you may be exposing whatever fault
there is with the unit
and risking blowing the transformer again. If this
happens, you're going to
have to bite the bullet and find out what's wrong with the
damn thing. But at
least your 3rd transformer will still be intact.

Great information David. Thanks!

On Sat, 09 Apr 2011 14:37:10 -0700, David Nebenzahl
wrote:

On 4/9/2011 2:21 PM Steve Turner spake thus:

On 4/9/2011 3:43 PM, Smarty wrote:

On 4/8/2011 8:52 PM, Steve Turner wrote:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

If you are not comfortable with taking current measurements, or do not have an
ammeter, or do not wish to sit there waiting for some unusual condition which
is drawing too much current, you could temporarily install an in-line fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing about a
quarter to a half an amp of current at 24V. The transformer secondary should
not need to source a lot more than that amount of current to provide adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in the
coil itself, a breakdown of the coil insulation allowing a short to ground when
the coil heats up or cools or vibrates, etc.

Yes Smarty, I would very much like to try this approach because I don't want to
zap any more transformers. Lots of people have suggested it, but there appear
to be more types of fuses than there are insects, and I have no idea what
*exact* type to look for.

Well, it's not exactly rocket science. And you can't hurt anything,
unless you get a fuse that's too big (i.e., rated at too many amps).

First thing you gotta figure out is how much current your transformer is
rated at. Since your wiring diagrams don't say, we have to take an
educated guess.

The only thing connected to the transformer is a thingamajig called the
"integrated furnace control" (IFC). Even without seeing this thing, I
can guess with confidence that it's an electronic circuit board which
controls the operation of the furnace. Since it undoubtedly does so
through relays (either mechanical or electronic), it's safe to assume
that it doesn't draw much current.

Let's say 1 amp to start. We could be wrong, but that's a good starting
point. So what we need is a fuse rated at *at least* 24 volts (could be
much more, up to 120 or 240 volts), and *exactly* 1 amp. To be on the
safe side, let's get a "fast blow" fuse. Hey, fuses are cheaper than
transformers, right?

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so use the next
bigger fuse; let's say 1.5 A.

We don't need to guess here.
The transformer has a rating. NEVER fuse above the transformer rating.
If the transformer is a 40va 24 volt transformer the MAXIMUM fuse
rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect the
transformer - a 2 amp fuse will NOT.

Now you want to stop at some point, let's say 2-3 amps, because now
you're getting up to where you may be exposing whatever fault there is
with the unit and risking blowing the transformer again. If this
happens, you're going to have to bite the bullet and find out what's
wrong with the damn thing. But at least your 3rd transformer will still
be intact.

You should contact the manufacturers of furnaces, and tell
them that they are doing it wrong. They will thank you.
Don't take no for an answer.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..



wrote in message
...


So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so
use the next
bigger fuse; let's say 1.5 A.

We don't need to guess here.
The transformer has a rating. NEVER fuse above the
transformer rating.
If the transformer is a 40va 24 volt transformer the MAXIMUM
fuse
rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect
the
transformer - a 2 amp fuse will NOT.

On 4/9/2011 9:08 PM, wrote:
On Sat, 09 Apr 2011 14:37:10 -0700, David Nebenzahl
wrote:

On 4/9/2011 2:21 PM Steve Turner spake thus:

On 4/9/2011 3:43 PM, Smarty wrote:

On 4/8/2011 8:52 PM, Steve Turner wrote:

A continuation of the "Why does the 115V-24V transformer keep
blowing on my Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing
of the wiring diagrams that I also copied to my above flickr site
as jpg images):

http://www.trane.com/webcache/un/fur...7_04012009.pdf

As you can probably see in the pictures, there is visible charring of the
115V leads going into the transformer, and of course the 115V circuit is open
(again). If you didn't see my first thread, this is the third transformer the
unit has blown. In the previous discussion, it was discussed that perhaps the
first one just blew because of old age (6 years), and the second blew because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most certainly
indicative of a real problem I have somewhere else in the unit. I didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing this third
transformer. We had cool weather for several days, and I never tried to force
the system to come on so I could monitor it; that was probably a mistake.
Unfortunately, I was also absent from the premises during the extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the right
places, so it doesn't look like an over-voltage condition to me. Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

If you are not comfortable with taking current measurements, or do not have an
ammeter, or do not wish to sit there waiting for some unusual condition which
is drawing too much current, you could temporarily install an in-line fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing about a
quarter to a half an amp of current at 24V. The transformer secondary should
not need to source a lot more than that amount of current to provide adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short in the
coil itself, a breakdown of the coil insulation allowing a short to ground when
the coil heats up or cools or vibrates, etc.

Yes Smarty, I would very much like to try this approach because I don't want to
zap any more transformers. Lots of people have suggested it, but there appear
to be more types of fuses than there are insects, and I have no idea what
*exact* type to look for.

Well, it's not exactly rocket science. And you can't hurt anything,
unless you get a fuse that's too big (i.e., rated at too many amps).

First thing you gotta figure out is how much current your transformer is
rated at. Since your wiring diagrams don't say, we have to take an
educated guess.

The only thing connected to the transformer is a thingamajig called the
"integrated furnace control" (IFC). Even without seeing this thing, I
can guess with confidence that it's an electronic circuit board which
controls the operation of the furnace. Since it undoubtedly does so
through relays (either mechanical or electronic), it's safe to assume
that it doesn't draw much current.

Let's say 1 amp to start. We could be wrong, but that's a good starting
point. So what we need is a fuse rated at *at least* 24 volts (could be
much more, up to 120 or 240 volts), and *exactly* 1 amp. To be on the
safe side, let's get a "fast blow" fuse. Hey, fuses are cheaper than
transformers, right?

So install such a fuse, run the unit and see what happens.

If the fuse blows right away, it's probably too small, so use the next
bigger fuse; let's say 1.5 A.

We don't need to guess here.
The transformer has a rating. NEVER fuse above the transformer rating.
If the transformer is a 40va 24 volt transformer the MAXIMUM fuse
rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect the
transformer - a 2 amp fuse will NOT.

And on the primary we have a max of .332 amps, I'd say a 1/4amp slow
blow would work.

On 4/9/2011 2:21 PM Steve Turner spake thus:

The specs on the original transformer we Class 2, 115V primary (60hz), 24V
(35VA) secondary. Any chance you could point me to some _exact_ fuse solutions
for this application? There's a virtual six-pack of beer in it for you. :-)

Guess I missed that.

35 VA/24 V = 1.45A, so you need a 1.5A fuse. Fast-blow.

Get the kind that'll fit in an in-line fuse holder (the little glass
jobbies with metal contacts on the ends). Should be available even at
Radio Shack.


--
The current state of literacy in our advanced civilization:

yo
wassup
nuttin
wan2 hang
k
where
here
k
l8tr
by

- from Usenet (what's *that*?)

On 4/9/2011 5:21 PM, Steve Turner wrote:
On 4/9/2011 3:43 PM, Smarty wrote:
On 4/8/2011 8:52 PM, Steve Turner wrote:
A continuation of the "Why does the 115V-24V transformer keep
blowing on my
Trane XB80?" discussion I started on 04/02/2011.

Yep, My A/C unit blew another transformer. Pictures (and wiring
diagram) he

http://www.flickr.com/photos/bbqboye...7626457562742/

Trane's manual for the unit is here (for perhaps better viewing of
the wiring
diagrams that I also copied to my above flickr site as jpg images):


http://www.trane.com/webcache/un/fur...7_04012009.pdf



As you can probably see in the pictures, there is visible charring of
the
115V leads going into the transformer, and of course the 115V circuit
is open
(again). If you didn't see my first thread, this is the third
transformer the
unit has blown. In the previous discussion, it was discussed that
perhaps the
first one just blew because of old age (6 years), and the second blew
because
it wasn't a proper replacement (poor quality, made in China, etc.). This
third unit is most certainly a proper replacement, and it's most
certainly
indicative of a real problem I have somewhere else in the unit. I
didn't see
any such charring on the previous two units, at least not like this.

I never got a real chance to test out the system after installing
this third
transformer. We had cool weather for several days, and I never tried
to force
the system to come on so I could monitor it; that was probably a
mistake.
Unfortunately, I was also absent from the premises during the
extended times
when the unit was most likely operational, so that didn't help either.
However, my family tells me that it WAS working and cooling the house
rather
nicely, for at least a day, perhaps two. I'm getting 115V in all the
right
places, so it doesn't look like an over-voltage condition to me.
Perhaps it's
an overheating condition? It looks to me like the only real load on this
circuit is the blower motor; could the motor be causing this? The blower
spins freely when I turn it by hand. Start capacitor on the motor maybe?
Relay on the control board perhaps?

Steve,

If you are not comfortable with taking current measurements, or do not
have an
ammeter, or do not wish to sit there waiting for some unusual
condition which
is drawing too much current, you could temporarily install an in-line
fuse
holder and automotive cartridge fuse rated at or above the secondary side
amperage which the transformer is rated. With this fuse installed on the
secondary side, in series with the load, you could then determine if
the load
is indeed drawing too much current from the secondary and burning out the
transformer, versus primary side excess voltage being the problem.

A rough guess would be that the contactor coil should maybe be drawing
about a
quarter to a half an amp of current at 24V. The transformer secondary
should
not need to source a lot more than that amount of current to provide
adequate
power to the coil of the contactor.

Fuses are a lot cheaper than transformers......

There may be an intermittent short in the wiring to the coil, a short
in the
coil itself, a breakdown of the coil insulation allowing a short to
ground when
the coil heats up or cools or vibrates, etc.

Smarty

Yes Smarty, I would very much like to try this approach because I don't
want to zap any more transformers. Lots of people have suggested it, but
there appear to be more types of fuses than there are insects, and I
have no idea what *exact* type to look for. That's why I asked earlier
in this thread if this particular transformer:

http://www.pexsupply.com/White-Rodge...unt-14937000-p


with a built-in manual reset would be a much cleaner solution that
serves the same purpose, but I got no responses.

The specs on the original transformer we Class 2, 115V primary
(60hz), 24V (35VA) secondary. Any chance you could point me to some
_exact_ fuse solutions for this application? There's a virtual six-pack
of beer in it for you. :-)


It's not that hard, or that critical.

Put this on the primary:
http://www.radioshack.com/product/in...ductId=2103752

..25A

This on the secondary:

http://www.radioshack.com/product/in...rodsPerPage=60

1.5A

You can put them in this:

http://www.radioshack.com/product/in...rodsPerPage=60

But first do as I suggested in another thread,look carefully for
anything burnt and check the polarity of the line. Make sure the neutral
is neutral. You can measure from the ground prong. See the note in the
schematic. I'm not so sure this didn't arc to the transformer shell,
from looking at the pic. You may wish to measure voltage (AC) from the
transformer shell to a guaranteed neutral.

It is OK for the transformer to run a little warm, it shouldn't be hot.

There are two kinds of failures, one is a long term thermal failure
(which seemed likely when you said years had gone by), the other is a
transient short, or maybe not so transient. If that is the case then
something else will be bad. It may look burnt, or if it is a diode or
transistor it may just be shorted. You can Google how to test those.

Don't be afraid to put a quick finger on a transistor to see if it is
hot, or on the transformer.

As far as calculating what size fuse, you should know Ohms law. Current
* voltage is watts, or in this case V(olt)A(mps). 120V * 1/4A = 30 VA,
on the out side, 24V * 1.5 = 36 VA.

Note that there is a small surge when you turn on a device. So it may
peak higher at the instant of turning on. A fuse usually can absorb
that. The white ceramics are faster blowing and the slow blow ones
usually have a little coil inside the fuse to show it down and are
marked slow blow. When/if you fix this, leave in the fuses and we can
give you some guidance on more properly sizing them. But some fuse in
the ballpark is definitely better than nothing.

Jeff