Blew another damn transformer on my Trane XB80

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On 4/9/2011 2:21 PM Steve Turner spake thus:

Well, it's not exactly rocket science. And you can't hurt anything, unless you get a fuse that's too big (i.e., rated at too many amps).
First thing you gotta figure out is how much current your transformer is rated at. Since your wiring diagrams don't say, we have to take an educated guess.
The only thing connected to the transformer is a thingamajig called the "integrated furnace control" (IFC). Even without seeing this thing, I can guess with confidence that it's an electronic circuit board which controls the operation of the furnace. Since it undoubtedly does so through relays (either mechanical or electronic), it's safe to assume that it doesn't draw much current.
Let's say 1 amp to start. We could be wrong, but that's a good starting point. So what we need is a fuse rated at *at least* 24 volts (could be much more, up to 120 or 240 volts), and *exactly* 1 amp. To be on the safe side, let's get a "fast blow" fuse. Hey, fuses are cheaper than transformers, right?
So install such a fuse, run the unit and see what happens.
If the fuse blows right away, it's probably too small, so use the next bigger fuse; let's say 1.5 A.
Now you want to stop at some point, let's say 2-3 amps, because now you're getting up to where you may be exposing whatever fault there is with the unit and risking blowing the transformer again. If this happens, you're going to have to bite the bullet and find out what's wrong with the damn thing. But at least your 3rd transformer will still be intact.
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On 4/9/2011 4:37 PM, David Nebenzahl wrote:

Great information David. Thanks!
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On Sat, 09 Apr 2011 14:37:10 -0700, David Nebenzahl

We don't need to guess here. The transformer has a rating. NEVER fuse above the transformer rating. If the transformer is a 40va 24 volt transformer the MAXIMUM fuse rating is 40/24=1.66 amps - so a 1.5 amp fuse will protect the transformer - a 2 amp fuse will NOT.

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On 4/9/2011 9:08 PM, snipped-for-privacy@snyder.on.ca wrote:

And on the primary we have a max of .332 amps, I'd say a 1/4amp slow blow would work.
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On Sun, 10 Apr 2011 19:51:34 -0400, Tony Miklos

As noted before, correct.
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On 4/9/2011 2:21 PM Steve Turner spake thus:

Guess I missed that.
35 VA/24 V = 1.45A, so you need a 1.5A fuse. Fast-blow.
Get the kind that'll fit in an in-line fuse holder (the little glass jobbies with metal contacts on the ends). Should be available even at Radio Shack.
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On 4/9/2011 5:21 PM, Steve Turner wrote:

http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p
It's not that hard, or that critical.
Put this on the primary: http://www.radioshack.com/product/index.jsp?productId !03752
.25A
This on the secondary:
http://www.radioshack.com/product/index.jsp?productId !02740&numProdsPerPage`
1.5A
You can put them in this:
http://www.radioshack.com/product/index.jsp?productId !02782&numProdsPerPage`
But first do as I suggested in another thread,look carefully for anything burnt and check the polarity of the line. Make sure the neutral is neutral. You can measure from the ground prong. See the note in the schematic. I'm not so sure this didn't arc to the transformer shell, from looking at the pic. You may wish to measure voltage (AC) from the transformer shell to a guaranteed neutral.
It is OK for the transformer to run a little warm, it shouldn't be hot.
There are two kinds of failures, one is a long term thermal failure (which seemed likely when you said years had gone by), the other is a transient short, or maybe not so transient. If that is the case then something else will be bad. It may look burnt, or if it is a diode or transistor it may just be shorted. You can Google how to test those.
Don't be afraid to put a quick finger on a transistor to see if it is hot, or on the transformer.
As far as calculating what size fuse, you should know Ohms law. Current * voltage is watts, or in this case V(olt)A(mps). 120V * 1/4A = 30 VA, on the out side, 24V * 1.5 = 36 VA.
Note that there is a small surge when you turn on a device. So it may peak higher at the instant of turning on. A fuse usually can absorb that. The white ceramics are faster blowing and the slow blow ones usually have a little coil inside the fuse to show it down and are marked slow blow. When/if you fix this, leave in the fuses and we can give you some guidance on more properly sizing them. But some fuse in the ballpark is definitely better than nothing.
Jeff
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wrote:

It's AC so there is no polarity. It's single output, non grounded, so it doesnt matter which direction either the primary or secondary are connected. Good to have the white on neutral and the bloack on live, but it will have NO EFFECT on the operation. - or the life of the transformer.

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On 4/9/2011 9:41 PM, snipped-for-privacy@snyder.on.ca wrote:

Except that the schematic has a note at the top that says:
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

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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.

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wrote:

I'm sure I am as baffled by that as you are. AC in not "polarity sensitive" and as long as neither side of the primary or secondary is grounded the control should not know which way the primary (or secondary) is connected..
I would SUSPECT, but do not know for sure, that all the relays etc are DC coils and all the circuitry on the control board is running on DC provided by a full wave rectifier bridge on the circuit board., and all the sensors also run on DC (and quite possibly - even likely - on 5 vdc, not 24)

It would not surprise me there is a problem with the board. Trane has "upgraded" the board from a white-rogers unit with their part number CNT0377 to a new replacement kit, part number cnt05122 - which might lead one to ask the question "WHY".
This can be pretty well determined by installing the fuse I (and others) have recommended several times in the past.. If it blows the 1.5 amp fast blow fuse it is safe to say there MAY be a board problem (or a shorted relaycoil, etc).

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On 4/11/2011 12:59 AM, snipped-for-privacy@snyder.on.ca wrote:

I have a guess, maybe the board has to be in sync with another power source. Would reversed polarity make it 180 out of sync?
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On 4/11/2011 6:27 AM Tony Miklos spake thus:

Now you're *really* grasping at straws ...
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On 4/11/2011 2:27 PM, David Nebenzahl wrote:

I am because I don't know why else it would have to be polarized.
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On 4/12/2011 8:10 AM Tony Miklos spake thus:

Well, it *is* a mystery ...
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On Mon, 11 Apr 2011 09:27:43 -0400, Tony Miklos

Does not APPEAR to be another power source - - -
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On 4/9/2011 5:21 PM, Steve Turner wrote:

http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p

Here is the fuse holder, available at Radio Shack for a couple bucks: http://www.radioshack.com/product/index.jsp?productId !02784
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.
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On 4/9/2011 5:21 PM, Steve Turner wrote:

http://www.pexsupply.com/White-Rodgers-90-T60C3-Transformer-60VA-50-60-Hz-120-208-240V-Primary-24V-Secondary-Foot-Mount-14937000-p
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.
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On Fri, 08 Apr 2011 19:52:06 -0500, Steve Turner

The blower motor is NOT running on the 24 volt transformer, so will have NO effect on the transformer.
You NEED to fuse the secondary - then IF it is an overload problem you will just pop the fuse, and not the transformer. 24 volt, 35 VA = 1,5 amp FAST BLOW fuse.
If the fuse does not blow and the transformer does, it PROVES, almost beyond a doubt, that you have bad luck getting a good transformer.
The other option is to closely monitor the primary current and see what you have.
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On Sat, 9 Apr 2011 17:46:06 -0400, "Stormin Mormon"

He needs to fuse the secondary and meter the primary, and have someone switch all the loads while he watches the meter. This will tell if he has a neutral issue, because a load on the "other phase" will cause the voltage to go up while a load on the "same phase" will cause the voltage to go down.
Even simpler, when the blower motor comes on, if he has a neutral issue the voltage on the primary WILL DROP. Absolutely no question. (and more than 1 volt - virtually guaranteed)
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