Blew another damn transformer on my Trane XB80

[trader4]

More likely this transformer burned up the truck with it.

[Steve Turner]

LOL!

[Steve Turner]

Ok, *finally* some news to report!

First, I'd already procured the fuse holders and various fuses, and whilst waiting for the replacement transformer to arrive I decided to run some innocuous experiments on my other *working* A/C unit. Several days ago I installed (as recommended by several) a 1/4-amp fast-acting fuse on the 115V input side of the transformer, and the unit has been running fine ever since. So I know that a 1/4-amp fuse will carry the load on a properly working system; what I didn't know for *sure* was if that fuse was rated too high to protect a transformer in the failing system...

The replacement transformer arrived this evening, so I got to work installing connectors and testing the two circuits for continuity. I installed a 1/4-amp fuse in the 115V primary side and (once again, as recommended) a 1.5-amp fuse (both fast acting) on the 24V secondary side. I also hooked my meter up in series on the secondary side to measure the current draw (I taught myself how to do this earlier using my little Dremel tool; it draws about 1/2 amp when spinning freely). All set, I closed the cover interlock switch to apply 115V to the system... The red LED on the circuit board lights up momentarily, then "piff"; the 1/4 fuse blows. Didn't get any chance to measure the current on the meter.

Some people mentioned possible shorts in the wiring leading to the thermostat, so my next move was to to eliminate that as a possibility. Once again, the wiring diagrams can be found here:

I removed all wires from the thermostat ("YWRGC") connector block on the circuit board. The wiring diagram doesn't show it (at least it's not obvious to me), but the wires that lead to the external A/C unit also connect to this block; I removed them all (after I marked them :-) ). I also disconnected the meter from the secondary circuit just to remove that from the equation. Once again, I closed the cover interlock switch, and once again the 1/4-amp fuse on the primary side opens up.

At this point, I'm 95% sure there's something wrong with the logic board, and in anticipation of this I already had one on order; it's scheduled to arrive tomorrow. I've toyed with the idea of swapping logic boards between the two blower units to see if the problem follows the board (I used this approach to isolate a similar problem on a Trane unit at my previous residence about 10 years ago; it helped, one of the boards was bad), but at this point I'm calling it a night and will pick it up again tomorrow.

Your comments are welcome, and hopefully by tomorrow evening I'll be back in business!

[trader4]

Sounds like it was many of us thought, ie that something was drawing too much current. I assume you didn't have the system turned on, so the theory that it could be caused by a surge on the AC line from the blower motor is gone. So is the possibility of the AC contactor, as you say you had that disconnected too.

Did you look around the controller board and see if there is anything that looks like it's been running hot? Sometimes the AC to DC power section uses parts, like a full wave rectifier, that are easily identifiable and can be checked. If it were something like that it might be possible to repair it using a part from Radio Shack. On the other hand, if the new board is reasonable price, that will most likely solve it.

[Steve Turner]

Dammit, I missed a step! One thing I forgot to do was verify that I was getting 24V on the output side of the transformer when 115V was applied to the input side. I just came back from testing for that, and now I'm confused again. I removed both wires from the output side and connected my meter to check for voltage, closed the interlock switch, and "piff", the 1/4-amp fuse (my last one) blew! Apart from the fact that the input voltage comes to the transformer via output connectors on the logic board (which are just tied directly to input connectors on the board as far as I can see), the fuse is blowing and the logic board isn't even a part of the equation... What do you make of that?

[trader4]

The first thing I would do is replace the fast blowing fuses with slow blowing ones. I thought all along using fast ones was a bad idea. If its an overload that's causing the problem, the slow-blow will still fail long before the transformer. And if it's some mysterious voltage spike that causing it, which I doubt, even the fast acting fuse may not prevent damage. It's possible the fast acting ones are getting creamed by the initial start-up current.

I'd also consider useing a fuse temporarily thats 1.5 or 2X the current expected. The transformer should be able to handle that for more than the minute it takes to get a current measurement. And if you measure voltage on the secondary, if it's an overload, I'd expect you'd see less than 24V, as the excessive load pulls the voltage down.

[Steve Turner]

Sure, I can try some different fuses (but I have to go make a run to buy some; that might not happen until later today). But now with the connectors removed from the 24V output side of the transformer, haven't I removed all the load? Wouldn't that mean there is something wrong with the input side of the circuit?

[Smarty]

Steve,

I am (I think) the person who originally suggested getting the inline fuse holder and fuses from Radio Shack, and am glad to see you are pursuing this. Since you have a disconnected secondary, and a fused primary, and the transformer blows the 1/4 amp fuse, it certainly sounds like the primary supply voltage is wrong. Have you actually measured the primary voltage and found it to be about 110 VAC? If so, then the transformer, with no secondary load, must be defective if it immediately blows a 1/4 amp fuse with no secondary load, unless for some reason your logic board is providing some odd waveform other than purely alternating

60 Hz power. Perhaps, for example, some big DC voltage is also present?

It is very hard to believe that the several transformers you have tried are ALL defective. I am way more suspicious of the supply voltage.

[Steve Turner]

>>

Yes, the supply voltage is correct (actually, it's 120V, which is what pretty much every other "115V" circuit in the house is receiving). To be safe this time, I ordered more than one replacement transformer (they were cheap) and I just hooked another one to a completely different circuit (on my workbench) with a 1/2-amp fast-acting fuse on the input side and no load on the secondary, and the fuse held and I got a constant 26V on the output side. I will be going to procure some more 1/4-amp fuses (both fast and slow blow) and I will remove the exact transformer I intend to use from the blower unit (it has all the right connectors attached to the wires) and attempt the same experiment on my workbench using both 1/2-amp and 1/4-amp fuses just to be sure I can completely rule out the transformer. If that works, I will take that same transformer back to the blower unit and run the same test there, bypassing the circuit board entirely. More to come...

[Smarty]

[trader4]

He's talking about it blowing without the transformer connected to anything except the AC. And I doubt the logic board provides the power, typically it gets it's power from power from the transformer which is wired directly to the incoming AC line.

I believe he said he measured it in the past and it was 120V. Is the new transformer fastened to the metal cabinet? I'd seperate it from the cabinet and see if there is any voltage between the two and see if it still blows the fuse. My first thought though is the fuse may be close to the limit of the transformer and being fast-blow it might be blowing just on some initial start-up current.

[Steve Turner]

(brevity snip)

I'm back. The saga continues.

Ok, I got some more 1/4-amp fast-acting fuses (sorry trader4; neither Radio Shack nor Home Depot had any slow-blow fuses rated less than 1.5 amps). I took the transformer out of the blower unit, brought it to my workbench and tested it there (on a completely different 115V circuit). The 1/4-amp fuse held fine, and I got a solid 26 volts on the output side. So far so good.

Reinstalled it in the blower unit (screwed it to the frame) and unhooked the input lines from the logic board (to eliminate it completely from the equation) and sent them straight into the transformer (with the fuse on the black hot wire) and with my meter measuring for 24V on the output side. Closed the cover interlock switch, and "poof"; the fuse blew. That pretty much tells me something is wrong with the incoming power.

I unhooked the transformer and measured the voltage across the hot and neutral wires with the interlock switch closed: 120V. I opened the switch, and the meter still wants to read *something*, like about a volt or so. What the hell? I metered on the *other* side of the interlock switch: nothing. So I hooked the transformer back up (it's still bolted to the frame) with the switch bypassed, and PRESTO; the fuse holds and I have 26V on the output side of the transformer. Bad interlock switch? I wondered if it was shorting to the frame, but it's completely encased in plastic. It seems to meter out fine, and I was going to try to open it up to see if I could find any problem, but it's completely sealed so I gave up.

Anyway, the switch looks like the culprit, so I turn off the breaker at the panel, hook everything back up, reconnecting all the wires the way they are supposed to be, with the exception that the interlock switch is bypassed (not really comfortable with that) and the 1/4-amp fuse still installed on the hot line going into the transformer. Turn the breaker back on, and "poof", the fuse blows again. What the hell? Unhook the transformer output wires from the logic board to remove any load (and back up to the meter to check for 24V output), install a new fuse and repeat; the fuse blows AGAIN, and now I'm just confused. At this point, the "something might be shorted to the frame" theory is starting to look like a distinct possibility. Thoughts?

[Steve Turner]

One more interesting thing I just discovered. trader4 asked if the transformer was connected to the frame. I just went back and took it out of the frame, set it on the plywood floor, then tested it again. Everything was hooked up normally on the input side (the input wires leading into the logic board (interlock switch still bypassed), then in turn leading to the transformer), but I left the output wires unhooked. Applied power (heh, it turns out there is a local switch in the vicinity to do that, rather than me traipsing to the breaker box - duh), and the fuse *held* (not the case when the transformer was installed in the frame). Switch off, connected the meter to the output leads, switch on; fuse held, 26 volts of output. Switch off, connected the output leads to the logic board, switch on; momentary red LED, then "poof", the fuse blows.... Houston, we have a problem.

[Steve Turner]

Ok, now I'm just depressed. I could not reproduce the scenario of the fuse blowing when the transformer was mounted to the frame. I traced every last bit of wiring leading to the transformer input; I loosened it from its mounting clamps where it feeds into the junction box, and I separated all wires so none were touching each other or the frame, and I tightened the wire nuts. There are no breaches anywhere, and there just isn't any place where the input wiring can possibly be shorting to the frame of the blower unit.

I reached the point where the only time the fuse blows is when I plug the 24V output wires into the logic board, and since the new logic board arrived earlier today I decided to go ahead and do the swap. Careful labeling of all wires, and careful reading of the documentation, and I know the board is installed correctly. This board is a newer model, and it has an interesting feature:

"The 50A55 has only one serviceable part - an automotive type fuse, which protects the low voltage transformer from damage if its output is short-circuited. If the fuse has opened up, remove whatever caused the short circuit and replace the fuse with only a 3 Amp automotive type fuse. If the fuse is not the cause of the control's problem, replace the entire 50A55 control. There are no other user serviceable parts."

This is the installation manual for the new board:

So guess what happened when I applied power after installing the board? Yep, the on-board 3-amp fuse popped. The 1/4-amp fuse I left on the input side did not.

So the more I piddle-fart around, the more apparent it becomes that I have an electrical problem like none I've personally encountered before. I love fixing stuff myself (it's how I was raised) and I love the challenge of tracking down problems, but now I'm growing weary of the chase...

If you're one of those guys waiting in the wings to say "I TOLD you you should have called in a professional!"... Yeah yeah; save it. :-) I've got some friends of friends (and family too) that are electricians by trade that I can lure in with the promise of beer, and if I need to call one in I will be right by their side shadowing their every move, and I will be *learning* from the experience, not just throwing some money at a local service company.

[clare]

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.

[clare]

>

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.

[trader4]

What 120V current does the transformer draw when it's on the bench and not driving anything?

Let me make sure I understand this. Normally the 120V goes from the furnace junction box, through the door switch, then to the logic board. From there the logic board in turn sends 120V to the transformer and also gets 24V back from it via the low voltage connections. Correct?

And what you did above as a test was to instead connect the transformer directly so that the 120V goes from junction box, to door switch, to transformer, with the secondary not connected to anything.

Correct?

That''s probably just due to the high impedance of the meter.

Confused here. The other side of the interlock is connected right to the incoming AC junction box, right? So it should have 120V.

It is very strange. Even though it may not be the real problem, one thing I would do is use bigger fuses. I don't know what the transformer is rated at, but using 2X fuses at least for a few minutes to do some testing should not burn out the transformer. Then you could at least get it to the point you could do some voltage and current measurements, expecially on the secondary.

[clare]

The ontly thing you are forgetting is the half amp fast blow DID work on the furnace that works - which is basically identical - so there is NO REASON the same fuse should not work on this one. Half amp is already about 50% oversized for the transformer at full rated output.

[trader4]

Again, what is the rating of the transformer? The above would suggest that it's at least 72VA and that in turn means that the 1/4 amp fuse on the primary is way to small. Combine that with that it's fast acting and that could be why you're seeing primary fuses blow, adding to the confusion. If they have a 3A on the 24V side, I'd put a 3/4 amp one on the primary, or maybe even a 1 amp temprarily so you can get the thing debugged. Why the 1/4 primary would work on the bench, but not with the same transformer with no load in the furnace, remains a mystery.

At this point, I'd get the bigger fuse in the primary and hopefully it will stop blowing. Then I'd be looking for what else the logic board drives with

24V and start disconnecting those wires one by one until it stops blowing fuses. My best guess would be that something else it supplies with 24v, ie contactor relay, etc is bad, or that a 24v wire is shorting out somewhere. Did you try disconnecting the thermostat wires, for example?

I think if you called a professional you could very well be at the same point, possibly with an even bigger bill for other parts and certainly lots of labor.

[trader4]

But he's using a 1/4 amp fuse, not 1/2 amp. He just posted further down in this thread that the new logic board has a 3A 24V fuse on it. In which case, it would seem his 1/4 amp primary fuse is too small. I don;t see the harm in at least temporarily going to a larger fuse so he can get the damn thing going enough to take some measurements.