On 4/2/2011 3:28 PM, wrote:
On Sat, 02 Apr 2011 15:03:52 -0500, Steve Turner
wrote:

On 4/2/2011 3:37 PM, wrote:
On Sat, 02 Apr 2011 14:10:01 -0500, Steve Turner
wrote:

A couple of months ago the heater quit coming on in my upstairs Trane XB80 HVAC
unit. It's been off warranty for about 2 years, and I am enough of a
do-it-yourselfer that I'm comfortable with simple repairs. I found that the
LED on the control board was not lighting up, and subsequently found that I was
not getting 24V output from the 115V-24V transformer. Sure enough, the 24V
circuit was open, so I found a local replacement transformer (not the exact
form factor, but same specs) and things worked fine after I replaced it. Now,
I have the very same problem (only this time it's the A/C that's trying to come
on; not that it matters), and once again 24V is missing on the output side of
the transformer. This time the 24V circuit has continuity, but the 115V
circuit is open. Of course, I can replace the transformer AGAIN, but I'd sure
like to know what could be causing this problem... Any ideas would be
appreciated. Thanks.

You didn;t say how long the original one lasted. It could be that the
replacement was not strong enough. Ratings are not always everything,
they may mean PEAK output, which is only for short periods of time.
Besides that, they're likely something made in China or some other
foreign country and are cheaply made. If the original lasted for
years, try to get another one of them. Otherwise find a replacement
with a higher amperage. They're more durable.

Of course check your AC input (line) voltage. You may have a loose
connection on a neutral causing higher than normal voltages. And of
course lighting can cause surges to destroy things like this too.

The original lasted about 6 years. It was made in Mexico by Basler (model
BE25214009 class 2 XFMR; it also has C340041P04 printed on it). The primary
leg says "115V 60HZ 115V", and the secondary says "24V 35VA 24V". I can't seem
to find a replacement for it online anywhere.

The replacement was indeed made in China. It's an Edwards model 599:

http://www.edwards-signals.com/index...d=104&level=15

Perhaps (as you say) it's just cheaply made an not up to the task, and its
subsequent failure has nothing to do with the failure of the first.

Any suggestions on what to do about securing a replacement? When the first one
failed I made quite a few calls to local electrical supply houses with no luck
(and I live in Austin; it shouldn't be hard to find a suitable replacement)
before I found this Edwards transformer. We were having a hard freeze at the
time and I was just happy to find something I thought would work. This time
I'd rather be sure I get something that's up to the task.

Thanks for the help.


Get yourself an industrial grade control transformer, like a triad
f107z or f108u available from Newark for about $19 or $34
respectively. The 107 is 48va, the 108 is 96. Personally I'd go for
the 108 because of it's form factor and it has leads, not solder
terminals.

A Hammond166l24 fir about $25 or a 166N24 for about $36 might be even
better (simpler connections) from the same source

What you do NOT want is another "home depot" bell or thermostat
transformer.

Whatever you use must be "Class 2". That means there is not enough power
available on the secondary to cause a fire. Using a class 2 transformer
allows using light weight thermostat wire instead of power wiring.

Class 2 heating system transformers can be easily installed to be safe,
with primary wiring not exposed - may or may not be true of the
transformers above.

Modifying a class 2 transformer by replacing internal fuses with
external fuses makes the source not class 2. If you knew what you were
doing you could probably protect the secondary with a fuse. I wouldn't.

I agree with looking at the secondary current.

--
bud--