Blew another damn transformer on my Trane XB80

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"Steve Turner" wrote in message

Just a thought,
When you put the new transformer in does the "Diagnostic Light" LED turn on ? Does the LED blink or flash in one of the patterns indicated in the "diagnostic codes" section that you posted a link to ?
The "diagnostic codes" imply that the control board can detect several of the problems mentioned by others. Maybe a good starting point as there is not enough time to test with test equipment.
robb
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a bad start cap would not affect the 24v control transformer,that just powers the controller board and relays. either something is loading the transformer or a problem with input voltage.

a line voltage monitor may be needed to see if there's some short-term overvoltage applied to the transformer,and a oscilloscope may determine if the input waveform is sinusoidal or otherwise,because 60hz iron core tranformers don't like extreme distortion on their input,it gets converted to heat,not output voltage. a DMM will not show line distortion or short term overvoltages. are the wires burned close to the transformer,or over their entire length?
--
Jim Yanik
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wrote:

Or just another crappy transformer. You do not just ASSume it is the transformer

Transient overvoltages of less than 20% or there-abouts are unlikely to blow the transformer primary of the furnace without manifesting themselves elsewhere in the house - and over 20% would definitely manifest themselves elsewhere.
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On 4/9/2011 7:29 PM, Jim Yanik wrote:

I peeled the insulation back from the hot and neutral wires and the burning only occurred right at the terminals, no more than 1/4" into the wiring.
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Steve Turner wrote:

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 :-#)#
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On 4/9/2011 10:06 PM, John Robertson wrote:

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
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Tony Miklos wrote:

Hi Tony,
Yes, we enjoyed the peanut brittle...
I do not think the original poster noticed my suggestion above, that failure mode is really not going to be too much load or some weird AC situation - it's either a short to the metal case the transformer is mounted in or a problem with the wire to solder lug joint.
Would be nice to know if that idea was checked out...
John :-#)#
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On 4/11/2011 9:31 PM, John Robertson wrote:

primary went open on 3 transformers, it sure looks like a problem on the primary side. If the load was too much, those wires on the secondary side would most likely have been at the very least discolored from the heat.
Tony
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Tony Miklos wrote:

Yes, I looked at the pictures (rather poor resolution). The failure looks like a connection issue because there is no evidence of windings overheating. If the output was shorted the transformer would burn up - assuming it could sink enough current, but it would be unlikely to flare at one spot. This is a pretty standard 115VAC transformer, I've seen many of them...
If someone tried to pump 5000 volts into the transformer then possibly you could get an arc like that to the transformer frame, so that would mean a serious miss-wiring of the furnace where the igniter somehow was connected to the primary of the transformer and then turned on.
Hard to see that happening as the transformer powers the logic board that would enable the igniter to fire up!
John :-#)#
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wrote:

further removes that scenario from any semplance of possibility
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This makes me think the most likely causes are:
* Improperly connecting the transformer (such as using only 1 of the 2 primaries of a 120/240V dual primary transformer)
* DC flowing through the secondary. That can occur if the tranny's load has a fullwave rectifier with one diode open. If the fullwave rectifier has discrete diodes or a dual diode, the problem may be a bad solder joint at one of the diodes.
DC through the secondary is well known to be able to cause transformer cores to saturate. That can lead to the primary drawing excessive current.
* Less likely - there is a magnet on the transformer core. That combined with the tranny's normal magnetic flux can cause core saturation, similar to the effect of DC in a winding although this usually only causes part of the core to saturate.
However, a magnet on the core appears to me "probably unlikely to be an actual problem" unless the tranny is of marginal design, line voltage is on the high side, or the magnet is especially powerful.
There is the matter of voltage spikes on the primary blowing the insulation. However, I consider that unlikely if nothing else has been blown while 3 transformers blew.
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- Don Klipstein ( snipped-for-privacy@misty.com)

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Don Klipstein wrote:

That's not true. an open rectifier does not allow DC current to flow through the secondary. It's just 1/2 of the power line cycle. During the opposite half of the cycle, no current flows in the secondary. True, it's unidirectional current, but it's an intermittent current, not constant DC. If your statement were true, then half-wave rectifiers wouldn't be feasible.
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"Dave M"

** So the average value is non zero and that means there is a DC component to the current.

** It will show a nice, steady reading on a DC current meter.
The interesting thing is how there is no corresponding DC component in the primary current.
.... Phil
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wrote:

Unless the half wave rectifier is on the PRIMARY side. I don't see any evidence of that on any of the diagrams I saw, but is there something else on this furnace circuit? By code there cannot be - but we don't know know this to be the case - or what is on the circuit if there is.

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To the OP... another long shot..
it looks like the primary of the transformer is wired in series with the door safety switch. Is it possible that the door safety switch is loose and arcing. This could cause a high voltage to appear at the transformer and cause the insulation to fail...
Mark
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There won't be a rectifier feeding the primary of a transformer with DC. A transformer does not pass DC from one winding to another the way it does with AC.
Changing AC that transformers work with to DC that electronics work with has to occur downstream of the transformer.

I have yet to look at these diagrams, but is the circuit board powered by this tranny shown to "board level" as opposed to "component level"? If so, then the board can have, probably does have, a rectifier not shown in the diagram.
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On 4/12/2011 9:23 PM Don Klipstein spake thus:

With all the pontificating you've been doing here, Don, I would've thought you'd at least had glanced at the wiring diagrams the OP posted, way up there somewhere. Sheesh.
The controller is shown as a block. It most certainly has at least one rectifier on it, as it contains electronics that no doubt requires DC power to operate. Thought you'd have figured it out. (Not just a relay board.)
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With such indication even as described here giving low indication as to rectification having a 1-component bridge rectifier, 4 discrete diodes, a fed-with-center-tap 2-diode fullwave scheme, 2 separate diodes or a single 3-lead dual-diode used for that...
If the controller is only shown as a block, how well does it show the rectifier scheme, as in whether the rectifier's diodes are discrete individual diodes or integrated into one rectifier package?
If anyone here sees that noted to such extent that it's not a waste of my time to take a look there, please pipe up!
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On Thu, 14 Apr 2011 04:43:29 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

The block diagram does not show it, but a picture of a replacement board I found on the net seems to show 4 discrete diodes on the board - which is what I have found on most control boards I have actually had my hands on. Being a 2 wire transformer secondary, the center tapped full wave scenario is a non-starter, leaving either a 4 wire full wave bridge rectifier or 4 discrete diodes as the only real options.
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On Wed, 13 Apr 2011 04:23:08 +0000 (UTC), snipped-for-privacy@manx.misty.com (Don Klipstein) wrote:

I fully understand that, byt what happens if, say, the Mrs plugs her cheap blow drier into the same circuit as the primary of the transformer is plugged in to, and she puts the blow drier on low (it uses a half wave rectifier to drop the power to the heater) - and you end up with a DC component on the primary - which saturates the primary without any increase in secondary (load) current or power.

Again, fully aware that the circuit board WILL have a rectifier which is not shown on the diagram, and better than 95% chance it is full wave bridge rectifier. There are precious few solid state control devices that run on AC.
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