Blew another damn transformer on my Trane XB80

[John Robertson]

So the problem is the joint of the transformer wiring to the lugs. They may have cold solder joints, or the lug may be slightly loose - enough to crack the solder connection over time. Your transformer may still be good!

When you are examining the transformer wire make sure it is stripped enough to get good solder coverage. Burn off the insulation (enamel) - do not sand, scrape, or try to erode it because if you scratch the wire it will break at the scratch.

Re-solder the transformer connectors after first making sure the wire has at least two complete wraps around the solder lug and the solder makes a good meniscus joint to the wire and lug. Also use a good grade of solder, not regular 60/40, find some 63/37 solder (Kester is best) - this solder resists cold solder joints due to movement when cooling by solidifying very fast.

John :-#)#

[Phil Allison]

"robb"

** From the damage to the tranny visible in the photos - the primary has developed an internal short ( due to insulation failure) and since there is no fuse link in series, the resulting current was high enough to make an exposed wire leading to the terminals explode.

An internal short could develop due to heat alone because of an overload on the tranny - but this requires a fault to exist on the secondary side which seems not to be the case.

High voltage spikes on the primary could also cause insulation failure leading to the damage seen in the pics - lightning does this sort of thing. So also could back emfs from the blower fan if the is a bad connection in the AC supply feed.

I suggest you provide the next replacement for that vulnerable tranny with some "protection" - firstly an in-line fuse of say 1/4 amp AND a capacitor wired across the primary of say 1uF rated for continuous use across the AC supply.

If there is an overload on the tranny, the fuse will blow.

The 1uF capacitor should suppress spike voltages enough to save the tranny from harm.

..... Phil

[Congoleum Breckenridge]

That 3rd transformer looks like a dual voltage (120/240) input. If it is, it has two primary windings, and both need to be wire up to work. Like this:

[Mark]

To OP this is a long shot but... I noticed on the Trane wiring diagram that you posted that the connections for the IGNITION circuit are shown to be near to the connections for the primary of the transformer.

If there is some insulation problem on the ignition wires and the VERY high voltage from the ignition circuit is sparking or jumping over to the transformer primary circuit, this could cause the symptoms you are seeing.

Make sure the wires that are part of the ignition circuit are not even close to the anything else. and inspect them (with the power off) for any sigh of cracks or other faults. These have very high voltage like in a car spark plug and can jump several inches if there is an insulation problem. And the problem would be intermittent. The transformer primary voltage would look perfect except when the high voltage spark jumps over to it and this could easily damage the insulation on the transformer which is not designed for very high voltages.

If you are an electrical novice, you may want to think about calling in for some help at this point.

Mark

[Jeff Thies]

Except that the schematic has a note at the t Important Integrated Control is polarity sensitive. Hot leg of 120V power supply must be connected to the black power lead...

Whats with that?

Otherwise I would fully agree with you.

It seems to me there is a flaw in the integrated control and whereas I don't see how this could be a problem, it is easy enough to scratch off the list. Troubleshooting is little more than eliminating possibilities.

Jeff

[Phil Allison]

"Phil Allison"

** Must be a "slo-blo" type fuse.

.... Phil

[Smarty]

Here is the fuse holder, available at Radio Shack for a couple bucks:

Ask the Radio Shack clerk to sell you some compatible fuses which are fast acting approx 1 or 1.5 amp size. If your transformer is 35VA rated, it should be able to pump out an amp continuously without damage. A 1 amp fuse will allow it to provide 24 VA (24 volts X 1 amp) which is well below the rating.

Your contactor should not draw anywhere near 24VA, and if it does, the problem needs to be in either the contactor or the wiring to the contactor.

Good luck.

[Jeff Thies]

>

1 uF sounds a little high.

I would think .01 uF at 400V would be a better bet. Or an MOV (metal oxide varistor) or three (across the line and then from each side to ground).

I don't think this is a voltage spike problem though. Voltage spikes take out other components first.

Jeff

[trader4]

I recently installed a Rheem and the installation manual clearly stated that the furnace would not start and would come up with a fault condition if the hot and neutral were reversed. Don;t know exactly how that one is wired or designed vs the Trane, but at least for Rheem it does make a difference. But I suspect the main effect is that the controller board detects it and just causes a fault. I don't see how a reversed hot/neutral would be causing this guys problem, but it is one thing I would have checked long ago, or if I didn't understand the basic steps, I would have called in a pro.

[trader4]

earthlink.com:

It also could be powering a thermostat. Some of them will take power from the transformer circuit. For example, I have a Honeywell VisionPro that has an optional 24V connection to power the thermostat, with batteries then being the backup. Doing it that way allows the display to be backlit 24/7 too.

Also, it's not unusual to have humdifiers tied into the 24V circuit too.

Again, he needs to start doing some basic current measurements, starting at the transformer and find out how much current the whole thing is drawing and then if it's high, work to isolate it. If the current is normal, then I'd put in a 1.5 amp fuse in the secondary temporarily. Contrary to the suggestions to use a fast blow, I'd probably just use a regular one, as whatever is capable of destroying a transformer should be most capable of opening any fuse.

That's a good idea too.

I don't see how he could have a waveform that is so out of shape that it burns up this one specific transformer, yet there are no apparent complaints of any other problems in the house. Or how you'd get such a badly distorted waveform in the house in the first place.

[Mark]

To OP this is a long shot but... I noticed on the Trane wiring diagram that you posted that the connections for the IGNITION circuit are shown to be near to the connections for the primary of the transformer.

If there is some insulation problem on the ignition wires and the VERY high voltage from the ignition circuit is sparking or jumping over to the transformer primary circuit, this could cause the symptoms you are seeing.

Make sure the wires that are part of the ignition circuit are not even close to the anything else. and inspect them (with the power off) for any sigh of cracks or other faults. These have very high voltage like in a car spark plug and can jump several inches if there is an insulation problem. And the problem would be intermittent. The transformer primary voltage would look perfect except when the high voltage spark jumps over to it and this could easily damage the insulation on the transformer which is not designed for very high voltages.

If you are an electrical novice, you may want to think about calling in for some help at this point.

Mark

>

[David]

Everything I see on the diagrams says this is a HSI (Hot Surface Igniter) type of furnace. There is no high voltage in that type of igniter. David

[Mark]

David, I think you are right...

well it sounded good anyway....

regards Mark

[Tony Miklos]

>>>>>>>>

>>>>>>

You are correct, I mentioned that back a day or three ago.

[Tony Miklos]

>>

It appears the breaker/reset is on the secondary side, so yes please buy that transformer and install it. That will at least settle the debate (well mostly me) of those who says the problem is on the primary side. If it burns up that transformer then I'm correct. If it only trips the breaker, well... it's probably on the secondary side.

[Phil Allison]

>>

** No it ain't.

** Think again.

** Bad idea.

** Not on the AC supply they don't.

..... Phil

[Tony Miklos]

>>>>

And on the primary we have a max of .332 amps, I'd say a 1/4amp slow blow would work.

[Tony Miklos]

Hi John, fancy meeting you here! Wait, I didn't realize this is cross posted to sci.electronics.repair Now I know why you are here. Glad you liked the package.

Tony

[Tony Miklos]

>

Oh my gawd, someone agrees with me. Looking at the picture says a lot. Burnt on the primary side, looks like new on the secondary. And this is the third transformer with an open primary!

I don't know about a the 1uF cap, seems way too high so I'll delete what was below and pretend I didn't see it.

[Phil Allison]

"Tony Miklos don't know much"

** Why ?????

It ( likely ) needs to absorb a transient back emf generated by a powerful blower fan.

Non puny size cap will ever do that and using 1uF cases no harm whatsoever.

** Nothing like doing the old Ostrich trick when the brain gets overloaded.

Wot a jerk off.

.... Phil