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.


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