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) here:
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):
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?
Thinking maybe I should spring for a transformer with a manual reset, like this
Would that protect me from having to replace the damn transformer every time?
It looks like the circuit breaker is on the 24V output side; I'm a little fuzzy
on how that would protect the input side of the transformer...?
Why in the world would you think you need to protect the primary side?
Unless you've got a *really* weird problem--namely, severe overvoltage
spikes--there's no way the damage is coming from that side. It's got to
be because of overcurrent on the secondary side.
Unless there's something really messed up with your house's wiring ...
The current state of literacy in our advanced civilization:
Well I'm no electrician so I don't *know*. What I really mean is that I want
to protect *myself* from having to drop $20 or $30 a pop on a half-dozen
transformers while I'm trying to figure out what's wrong. So again, would the
transformer referenced above protect me from that? Many helpful things are
being suggested, but unfortunately they all seem to require testing the unit
while it's operational, and I can't do that unless I have a working transformer
Ok, and how does that condition come into existence? Is there anything in the
wiring diagram that jumps out at you as a potential candidate? I have two
identical Trane AC/furnace units; one upstairs and one down. Should I swap
control boards between units to see if the problem follows the control board?
The house is only six years old. Nothing's changed in terms of wiring and
everything's been fine up until now.
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
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
If you are an electrical novice, you may want to think about calling
in for some help at this point.
Only one almost obvious flaw with that arguement - the ignition is NOT
used when the system is running the "cool" cycle - which is what the
OP says was running when the second transformer popped.
Also, the secondary resistance will be low enough, and the impedence
of the ignitor (assuming it is spark ignited, and not hot surface)
high enough, that the voltage across the primary would most likely be
clamped to a reasonable level - and if not the electronics on the
control board would be damaged, long before the voltage induced in the
primary would get high enough to damage the windings or the insulation
If, on the other hand, it is a hot surface ignitor, they run on line
voltage (on any I've seen anyway) and a short as you envision would
ALMOST DEFINITELY fry the control board.
On Sat, 09 Apr 2011 22:24:10 -0400, Congoleum Breckenridge
True - SOME dual voltage primaries are wired that way, and need both
windings connected either in series or parallel to operate.
Others have a center tapped winding - 110 on one half, 220 across the
full winding - and still others have 2 separate windings that operate
Real easy to find out - even in this case with the 110v primary open.
Connect 110 across the 220 (or 120 across the 240 - let's not get
nit-picky over voltage ratings) or "high voltage" primary and check
the secondary voltage. If he only gets 12 volts it is a totally
separate and independent high voltage primary. If on the other hand he
gets 24 volts, you hit the jackpot and HE screwed up..
This type of "dual primary" circuit is very "polarity" sensitive and
the right colour wires need to be connected together, both in series
and in parallel, or the "magic smoke" comes out very quickly.
Again , like I posted before - READ the instructions.
FOLLOW the instructions, and VERIFY the result.
Part of verifying the result, or erifying an undocummented
transformer, is to check the voltage on ALL winding, used or not, to
fully understand their intended function.
If 110 across the black and white produces 110 across another pair,
that other pair is is a "complimentary" primary and should, in all
likelihood, be connected, phase for phase, across the black/white
I wonder if our intrepid OP has done the basics involved in
troubleshooting instead of flitting around from place to place?
In the case of a power problem, you always start at the source by
measuring voltage and current. It could be a very simple problem
like a bit of insulation skinned off a thermostat wire that only
shorts out when everything is buttoned up and vibration from the
running unit causes the short circuit. If you ever watched the TV
program MASH, you would see the surgeons run the intestines looking
for a nick in the wall of the organ. Wiring is the same way sometimes.
I'll inspect the wiring and often find a small cut or tiny area where
insulation is missing and sometimes the wire is bent over a sharp edge
of the cabinet where it's fine until you close things up. I wonder if
the supply voltage to the transformer is what it's supposed to be? Most
domestic household air handler/furnaces are powered by 120vac but some
may require 240vac if there are electric heat strips. I would check the
voltage feeding the transformer first.
Yes, I've done all those things. The supply voltage is fine. I've inspected
the wiring at great length and can find no evidence of problems; it all looks
pristine. It's a gas furnace, and there are no 220 circuits running anywhere
near that area of the house.
Put a voltmeter on the secondary side and measure the voltage when it
is not running. THen turn on the AC and see what the secondary
voltage does. IF it is supposed to operate a relay, and the current
draw is appropriate for the size transformer you have, I would not
expect the voltage to drop by more than 20%. Then leaving the AC on,
for at least 1/2 hour, keep monitoring the transformer tempoerature.
If it is a gradual heating, see how hot it gets. If there is a severe
overload, the transform may blow out before it even gets hot to the
touch. If it just gets warm over 10 - 20 minutes, then that is
probably about normal. IF you can find out what the correct current
drain is supposed to be, I would go for a transformer with double that
amperage ratinbg just ot be ultra-safe. But you have to be there and
monitor things, coming back 48 hours later is not going to work!!!
I'd agree. Responding to his first post and before he put this
latest transformer in I recommended measuring
how many amps are being drawn on the low voltage side.
This is electricity 101. So, he comes here asking for advice
and instead of listening, he justs puts another transformer in
and blows it too..... Go figure. Time to either get educated
or hire a pro.
On 4/9/2011 9:12 AM, email@example.com wrote:
I don't understand the lack of a temporary fuse or circuit breaker in
troubleshooting this unit. The supply houses sell a little push to reset
breaker that can be installed in series with the transformer. Heck, a
box of fuses is less expensive than a transformer. :-)
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