Blew another damn transformer on my Trane XB80

On Tue, 19 Apr 2011 08:05:12 -0500, Steve Turner

Step #1. Measure the voltage on the primary, BEFORE you connect the transformer Step #2 VERIFY it is between 100 and 127 volts. Step #3 If it is not, FIND OUT WHY.
Step #4 If it IS VERIFY the proper connection of the transformer.
Step #5 - If it is properly connected CHECK THE TRANSFORMER. 3 bad ones in a row is stretching creduality, but not TOTALLY impossible.
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What series 260? Mine uses fuses that are similar to and only slightly cheaper than the Fluke fuses. I ended up ordering from McMaster-Carr after x-ref'ing part nos. and now I have a couple extras of both but they are Not Cheap.
What I really hated to see was when I bought my 260 (used) the fuses in it were completely the wrong size... yeah, that's a great idea, let's fry an expensive meter next time you have a brain fart because you're too damn cheap to buy the right fuse. (or kill yourself, potentially, although that's less likely.)
nate
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On Mon, 18 Apr 2011 21:41:43 -0500, Steve Turner

Just a note - was going to mention it earlier but didn't - a short in the thermostat wiring is unlikely to be the cause - all THAT would do is turn on the heating or cooling. A short to GROUND would likewize not cause a problem, assuming the secondary side of the transformer is not grounded.

I'm betting on a bad board at this point.
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To the op.
here is another troubleshooting tip..
Since you have wired a fuse in series with the pri input and they blow, you can try the next thing,
instead of the fuse, use an ordinary incendescent light bulb (Not a CFL) . Wire a 40W light bulb in series instead of the fuse. (or if you prefer, wire it in series with the fuse too.
If there is a problem, the bulb will light bright, if all is well it may glow a bit.
With this in place, you can carefuly try all modes and move things etc, anytime the bulb goes bright, you have a problem.
Wire it carfully so that there isno chance for the leads to short to anything else.
You may need to use a 60 Watt or more. Note that no matter what else happens, no more then 1/2 Amp can flow through a 60 Watt bulb long term. And even if there is a fault, 1/2 Amp will not fry anything in less than a few minutes.
Mark
This is
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On Apr 19, 7:40 pm, snipped-for-privacy@snyder.on.ca wrote: .com/webcache/un/furnaces%20%28furn%29/product/22-166...

True of many common thermostats, but not all thermotats. My Honeywell VisionPro, for example, has a backlight which you can power all the time by using a common wire from the furnace. Bad thermostat or short in the wire and you have an overload on the 24V transformer.
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On Friday, April 8, 2011 5:52:06 PM UTC-7, Steve Turner wrote:

my Trane XB80?" discussion I started on 04/02/2011.Yep, My A/C unit blew a nother transformer. Pictures (and wiring diagram) here: http://www.flickr.c om/photos/bbqboyee/sets/72157626457562742/Trane's manual for the unit is he re (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/furna ces%20%28furn%29/product/22-1666-07_04012009.pdf As you can probably see in the pictures, there is visible charring of the 115V leads going into the t ransformer, and of course the 115V circuit is open (again). If you didn't s ee my first thread, this is the third transformer the unit has blown. In th e previous discussion, it was discussed that perhaps the first one just ble w because of old age (6 years), and the second blew because it wasn't a pro per replacement (poor quality, made in China, etc.). This third unit is mos t certainly a proper replacement, and it's most certainly indicative of a r eal problem I have somewhere else in the unit. I didn't see any such charri ng on the previous two units, at least not like this.I never got a real cha nce 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 com e on so I could monitor it; that was probably a mistake. Unfortunately, I w as also absent from the premises during the extended times when the unit wa s most likely operational, so that didn't help either. However, my family t ells me that it WAS working and cooling the house rather nicely, for at lea st a day, perhaps two. I'm getting 115V in all the right places, so it does n'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 bl ower 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). Th e second time I replaced it with a 40VA because I assumed the one i replace d was the correct size. The Primary side keeps getting burned up. What caus es that?
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On Tuesday, June 17, 2014 9:34:20 AM UTC-4, snipped-for-privacy@gmail.com wrote:

on my Trane XB80?" discussion I started on 04/02/2011.Yep, My A/C unit blew another transformer. Pictures (and wiring diagram) here: 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 naces%20%28furn%29/product/22-1666-07_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 b lew because of old age (6 years), and the second blew because it wasn't a p roper replacement (poor quality, made in China, etc.). This third unit is m ost 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 char ring on the previous two units, at least not like this.I never got a real c hance to test out the system after installing this third transformer. We ha d cool weather for several days, and I never tried to force the system to c ome 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 l east a day, perhaps two. I'm getting 115V in all the right places, so it do esn't look like an over-voltage condition to me. Perhaps it's an overheatin g 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 contro l board perhaps?

er 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 repla ced was the correct size. The Primary side keeps getting burned up. What ca uses that?
If the correct size was 70 and you replaced it once with a 40, then you hav e 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 probl em. The tranformer doesn't power blowers, it's used for the thermostat cir cuit, contactor circuit, control board power, etc.
Have you measured the current draw and voltage on the output of the transfo rmer? 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 b ecause 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 fa n. That one smoked when it was first turned on. When I finally did what y ou 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 possibl e.
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if the OP has the original failed transformer they could try looking for a thermal fuse buried in the primary wiring coil.
OP sghould look at the temperature where the transformer lives. perhaps the temp in the area is too hot, causing the transformer to fail.
OP could shop for a higher current transformer rather than 40 or 70 watts how about 150 watts.
the transformer with the obvious charring was likely a manufacturing defect.
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snipped-for-privacy@gmail.com wrote:

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 mot or maybe? Relay on the control board perhaps?

Hmmm, I never had 24V AC x-former blown in any appliances. If it keeps blowing, take logical steps B4 just keep replacing it. It must have a reason why it is blowing. Probably overloading, El Cheapo x-former, Bad or partially failing component on the load side(control board) When the unit is running, does it feel hot? Did a careful inspection on the board for any component with color change, sign of over heating, cracking, etc.? Checked thermostat inside? Measured the current in full load to see it is within specs.? I perused eBay for a x-former. Price varies from 5.00 to 100.00. Try to buy a NOS good stuff paying for it's value. That is even a fire hazard. I'd also Google the issue to see if it is common problem. Good luck.
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Tony Hwang wrote:

P.S. I wonder why fuse did not blow B4 x-former blew? Make sure all the connections related to x-former are not loose including crimped pins inside Molex connector. Triad Magnetics old vintage x-formers are floating around at eBay. They are good stuff.
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On Friday, April 8, 2011 5:52:06 PM UTC-7, Steve Turner wrote:

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?
*Is there any chance that water is somehow dripping on the transformer? Moisture could cause it to burn out over time. Maybe try relocating it.
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On Tuesday, June 17, 2014 2:45:58 PM UTC-4, John G wrote:

That seems highly unlikely. Condensate is the possible water source and fresh water, though eventually damaging, would be obvious long before it did any real damage to the transformer.
Either he's using too small/too cheap transformers or something is drawing too much current.
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jamesgang posted for all of us...
And I know how to SNIP

+1
--
Tekkie

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On Tue, 17 Jun 2014 06:34:20 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

Use a 90va or better transformer and fuse the secondary. Buy a good brand name transformer. Eiuther you are overloading the transformer or the transformer is junk.
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snipped-for-privacy@snyder.on.ca wrote:

Hi, There are a few listings at eBay for a brand name NOS x-formers at around 90VA rating. Not Chinese El Cheapo ones. For ~25.00.
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<stuff snipped>

too much current.
Is the circuit board possibly being subjected to back EMF from the furnace motor? It's hard to tell without seeing a wiring diagram what's connected to what but this seems to be a very intermittent issue. What series of events could come together to burn out a transformer every few months?
One thing that comes to mind is that all day yesterday, my UPSs were chirping like demented crickets. It was the first very hot day of the seasons and ACs were drawing so much juice that the power company was reducing the voltage substantially during the day. That's the kind of rare event I would look at when considering why a transformer took so long between replacements to burn out.
--
Bobby G.



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On Wednesday, June 18, 2014 8:09:19 AM UTC-4, Robert Green wrote:

If it was, you'd expect components on the electronic board to fail, not the transformer. You would think any decent transformer is going to be among the components most resistant to a surge from a motor like that.
It's hard to tell without seeing a wiring diagram what's connected

Crappy, cheap Chinese parts? As I stated previously, I saw 3 fan motors for a Sears dehumidifier fail. Original lasted a few years, one lasted a year, one smoked when installed and turned on. Just simple two speed 120V motors, nothing to account for those failures either. And when I googled, I found a lot of people with exactly the same thing. Nothing I know of that accounts for a new 120V fan smoking right out of the box, except that it's cheap China junk.
Not saying for sure that's what's going on here, only that it's not unheard of and I don't see anything obvious. But then the OP hasn't taken any current readings either.
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On Wednesday, June 18, 2014 8:09:19 AM UTC-4, Robert Green wrote:

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On the vast majority of air handlers the furnace blower motor is started vi a a cube style relay on the control board. The contactor out on the compre ssor is a pretty big relay (hvac guys call them contactors) and it's pulled by 24vac via the thermostat calling for cooling and what little emf it gen erates should not hurt a transformer. Once pulled, the outside contactor c urrent draw is pretty small as the impedance goes up a good bit with the ar m pulling into the coil.
Generally speaking hvac systems electrical/electronics is designed to be pr etty forgiving as it has to operate in a wild range of conditions and is ex pected to work 24/365 without issue for many years.
A current check would be a good test. As others have said installing a kno wn quality high current transformer may solve the problem. It might also c ause a failure at whatever is drawing too much current. But then the probl em will be more obvious.
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On Friday, April 8, 2011 at 6:52:06 PM UTC-6, Steve Turner wrote:

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Our XR90 furnace did the same thing, I installed a 50W transformer instead of the stock 40W just in case it was underrated. I measured the operating o utput current during cool or heat and it maxes at .6 amps when 2.0 would be the limit. I cleaned up all the sloppy wiring from the installation and so far no blown fuses but with such an intermittent problem you never feel li ke you got it for sure. The only problem I could find was during heat call the red light would blink for 3 times indicating a problem with the safety pressure switch (enough gas pressure?). I temporarily bypassed the switch t hen it would latch and the heat would work fine. The only thing I can figur e is that it the unit tries many times to turn on the heat and the pressure switch never makes the board commits suicide and deliberately blows the fu se so that you have to call a service man???? Sounds crazy but I wonder. I' ll replace the pressure switch and see if the problem ever returns. For now I have a bunch of 2 amp (instead of 5A) fuses on hand to keep from blowing the transformer again. From the many web posts this seems like a common pr oblem.
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On Wednesday, June 21, 2017 at 7:15:34 PM UTC-4, snipped-for-privacy@gmail.com wrote:

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d of the stock 40W just in case it was underrated. I measured the operating output current during cool or heat and it maxes at .6 amps when 2.0 would be the limit. I cleaned up all the sloppy wiring from the installation and so far no blown fuses but with such an intermittent problem you never feel like you got it for sure. The only problem I could find was during heat cal l the red light would blink for 3 times indicating a problem with the safet y pressure switch (enough gas pressure?). I temporarily bypassed the switch then it would latch and the heat would work fine. The only thing I can fig ure is that it the unit tries many times to turn on the heat and the pressu re switch never makes the board commits suicide and deliberately blows the fuse so that you have to call a service man???? Sounds crazy but I wonder. I'll replace the pressure switch and see if the problem ever returns. For n ow I have a bunch of 2 amp (instead of 5A) fuses on hand to keep from blowi ng the transformer again. From the many web posts this seems like a common problem.
so i know this is an old thread but i will add this
I've had 24V xformer burn out and it was due to a shorted wire.
There is a lot of vibration in HVAC equipment and if there is a wire touchi ng a bracket or anything, it may eventually wear through the insulation and create a short, often these are intermittent and hard to see. The short bl ows the xformer.
I was lucky to spot the bad wire in my case. I repaired the short, instal led a new xformer and added a PTC fuse device in series with the secondary just in case. A regular fuse rated for an amp or so above the xformer rati ng is a good idea. Also tie wrapped all the loose wires so none were rubb ing on anything. m
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