On Tuesday, June 17, 2014 9:34:20 AM UTC-4, wrote:
On Friday, April 8, 2011 5:52:06 PM UTC-7, 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/bbqboyee/sets/72157626457562742/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?



Hello, I am having the same exact problem and have replaced the transformer a third time with one of the correct rating as the manufacturer (70VA). The second time I replaced it with a 40VA because I assumed the one i replaced was the correct size. The Primary side keeps getting burned up. What causes that?

If the correct size was 70 and you replaced it once with a 40, then you have one or two unexplained failures, not 3. You can ignore the last part of that thread where he speculates about the blower motor load being the problem. The tranformer doesn't power blowers, it's used for the thermostat circuit, contactor circuit, control board power, etc.

Have you measured the current draw and voltage on the output of the transformer?
Made sure it's not exceeding the 70VA? My first suspicion would be that it's just cheap, crap parts. The other fellow talked about that. And just because you get it from Trane, doesn't mean it's not a cheap, China part. A few years ago I had a Sears dehumidifier where the fan died. It was only a few years old, so I bought a new fan for $50 from Sears. That fan lasted about a year. Still not having learned my lesson, I bought yet another fan. That one smoked when it was first turned on. When I finally did what you did, ie google, I
found that for 7 years people had been having the exact same thing happen. It's clearly a case of bad parts and Sears either not having a process to even know it, or not caring.

If it were me, I'd see if I could find a transformer that will fit from
another source, preferably American made, definitely avoid China if possible.