Blew another damn transformer on my Trane XB80

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On 4/9/2011 9:12 AM, snipped-for-privacy@optonline.net wrote:

In case you hadn't noticed, I've been getting LOTS of (much appreciated) advice from many people, but it's also been very contradictory with no single suggested approach. It's been an interesting and informative discussion, and I've been happy that nobody's been a jerk about it... until *now*. And I DID listen; plenty of people suggested that the original transformer was likely to be under-rated and to replace it with something more heavy-duty. Also, it seems to me that having a working transformer is a pre-requisite to following your advice of measuring the amp draw on the low voltage side, no? Unfortunately, time constraints prevented me from being there to perform those measurements when the unit was running, and unfortunately again the transformer blew in my absence. But of course, I'm repeating myself...
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On 4/9/2011 10:48 AM, Steve Turner wrote:

Steve I really hope you can figure out the problem with the furnace and I can understand the bit about time constraints. Installing a fuse in series with with either side of the transformer is cheap insurance to keep from losing another transformer. The fuse holders are inexpensive and it's a lot less of a hassle to replace a blown fuse.
TDD
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On 4/9/2011 10:59 AM, The Daring Dufas wrote:

Yes, I would be more than willing to do that, but my electricity training is not quite up to the 101 level that trader4 seems to think I should have; can you suggest a specific fuse rating that I should use? Thanks!
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On 4/9/2011 12:23 PM, Steve Turner wrote:

What were the specs of the original transformer?
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On 4/9/2011 11:23 AM, Steve Turner wrote:

Most control boards have a place to plug in a low voltage blade fuse just like the ones introduced to automobiles by GM years ago. The value will vary from 3 to 5 amps. A one amp fuse should be sufficient for the primary/120volt side. The primary fuse should be installed in series with the black/hot wire of the transformer and the low voltage fuse should be installed in series with the red/R wire OOPS! I just noticed something from the pictures of the burned transformer! The wire colors in the control system of air handler/furnace wiring can vary a little from manufacturer to manufacturer and I just saw something that may be a problem causing confusion. The transformer in the picture has a YELLOW wire in place of the RED low voltage output. The black/hot and white/neutral are for the 120vac connection and the yellow/R and blue/C are on the other side of the transformer. The red and orange wires should be taped up because they are for 208-240 volt connections. If the red wire on that transformer were hooked to the red wire connections shown in the wiring diagram, it will burn up. Yellow is usually the low voltage wire color used for safety switches, pressure switches and interlocks. The blue wire is the common 24vac and is often grounded to the metal cabinet of the furnace. A seasoned HVAC tech would have seen the anomaly immediately and it wouldn't have posed a problem.
TDD
TDD
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On 4/9/2011 4:52 PM, The Daring Dufas wrote:

Not to worry; I read the directions. The red and orange wires were indeed taped up and not used; only the white and black wires on the input side were hooked up in the normal fashion. The blue and yellow wires on the 24V output side of the transformer were hooked to the blue and red wires (respectively) leading to the control board. I measured input and output voltages after installing the transformer, and I ensured that I was getting 24V on the output side before I hooked those leads to the control board. The control board then came up with it the flashing red LED to indicate normal ("no call for heat") operation.
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On 4/9/2011 5:05 PM, Steve Turner wrote:

Well, that blew that theory! ^_^
TDD
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On Sat, 09 Apr 2011 11:23:47 -0500, Steve Turner

primary if the original was 35va. full rated load would be about .3 amps, so a 250ma would be boarderline. (but the transformer should NOT be running full rated load, either)
Take the va rating and devide by the voltage to get the current rating in amps. 40va at 24 volts= 1.66 amps. 40va at 115 volts = .347 amps. 40 va at 127 volts = 0.315, etc.
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On Saturday, April 9, 2011 12:23:47 PM UTC-4, Steve Turner wrote:

Fuse depends on the transformer rating. Typically they specify a va rating on the secondary. Sometimes an amperage. If you have a secondary amperag e then that's the fuse size you want. If they give you a va rating, say 50 divide that by the voltage to get the fuse amps. You can get an inline fu se holder and a fuse at radio shack. Use a slo-blo because there will be a surge when the outside unit contactor gets pulled. Most likely you will e nd up with a 2 amp fuse.
In the ac mode the transformer is powering the control board and the compre ssor contactor in the outside unit. If you have a smart thermostat that do esn't run on batteries then it's powering that too.
Most vom's won't measure amperage high enough to check it on the secondary side. You can measure the amperage on the primary and multiple by 4.8 to g et the approximate secondary amperage.
You have a gas furnace so you only have 115 at the furnace. So fluctuating 115 would have also causes light bulbs to burn out around the house.
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On Tuesday, June 17, 2014 10:47:48 AM UTC-4, jamesgang wrote:

He said it's 70VA.
Sometimes an amperage. If you have a secondary amperage then that's the fu se size you want. If they give you a va rating, say 50 divide that by the voltage to get the fuse amps. You can get an inline fuse holder and a fuse at radio shack. Use a slo-blo because there will be a surge when the outs ide unit contactor gets pulled. Most likely you will end up with a 2 amp f use.

I agree, that's a good idea, except it should be 3 or 4 amps. It would rul e out any long over-current draw as the cause.

doesn't run on batteries then it's powering that too.

70VA is 3 amps at 24V. IDK about most. Maybe some won't do that, but all the ones I've used will, including cheap ones. Any decent one should. But I guess all that's a moot point, based on his last response.
You can measure the amperage on the primary and multiple by 4.8 to get the approximate secondary amperage.

You would think so and also, voltage would have to fluctuate a lot to screw the transformer. Given that others are having similar problem, my money w ould be on these just being cheap, crap parts, but who knows.
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wrote:

OK, so now I'm a jerk for suggesting that you should have taken some basic measurements, starting with the transformer output current, instead of just continuing to replace transformers and watch them blow. A component that supplies power fails by burning up. You've replaced it several times now. You think just MAYBE it's a good idea to see how much power it's be asked to supply instead of just buyng new transformers? If a fuse blew out, would you just keep putting in new fuses or would you measure the current and see what it is? I'd have measured the voltages and currents on both sides of the transformer after the first one failed.

I find it hard to believe that Trane uses transformers in it's furnaces that are so under-rated that they burn out in a day. I also question the soundness of anyone telling you to start installing transformers capable of delivering more power without even taking some basic measurements and finding out what's drawing current and if it's excessive for some reason. If you had a light circuit that was blowing fuses, would your approach be to put in a bigger fuse or would it be to find out what's really going on? If you have a short somewhere and you put in a large enough transformer, you think you might eventually heat something up enough to start a fire, like maybe in the thermostat wiring in your walls?

So, you had the time to obtain a new transformer, put it in, but didn't have the 1 minute it would take to use a VOM to measure the current and voltage on the secondary? If it were me and I saw transformers burning up, I wouldn't leave it on until I had some confidence as to what was going on.
>But of course, I'm repeating myself
Yes, by putting in each new transformer and watching it fail.
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On 4/9/2011 11:49 AM, snipped-for-privacy@optonline.net wrote:

I said no such thing. Your advice is perfectly sound, but your suggestions were just some of many, and you seem to think I didn't follow *yours* (not true), and yes you were kind of a jerk in the way you voiced it. It wasn't really necessary, was it? You apparently don't have the patience to read everything I wrote, otherwise you wouldn't claim that I'm just "replacing transformers and watching them blow".

More evidence that you've chosen not to read everything that was written.

I'm not an idiot. Not completely, anyway.

I know how to measure voltages (and I did; I mentioned that already) and resistance to test for open circuits (continuity - See? I know some of the terms. I even know the difference between A/C and D/C). But measuring and understanding the ramifications of current (amperage) is where I get a little fuzzy. Would you help me out? That's why I came here.

I agree completely. I had my reservations about following such advice (and I only went a *little* higher on the replacements; far less than what some suggested), but I'm not an expert and many people were singing the same song:
"Discount the first failure, things simply get old and fail" "The second failure could likely just be an inferior replacement part."
Clearly the third failure proves those two theories false. Live and learn.

I already explained what happened. I'm sorry you feel the ongoing need to point out the stupidity of my approach. It is what it is. <Shrug>

By golly, I think I'll do that again, it was such great fun.
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On Sat, 09 Apr 2011 14:41:05 -0500, Steve Turner wrote:

I advised you the last time to measure the 24 volt circuit amperage draw and convert that to the specs (volt-amps) of the transformer. If the draw is out of bounds of the specs then the 24 volt circuit is the problem. You can't put a band-aid on a bullet wound.
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On 4/9/2011 2:58 PM, A. Baum wrote:

Yes, you did, and thanks for the suggestion. I've never used my meter to measure amperage before, and I don't know how to do that conversion, but I will study up on it.
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On 4/9/2011 2:04 PM Steve Turner spake thus:

It's not a "conversion".
To measure current (which, properly speaking, is what you're measuring, not "amperage"), you have to break the circuit and put the ammeter in series with the circuit, so that all the current goes through the meter. (As opposed to measuring voltage, where you put the meter *across*, or in parallel with, the thing whose voltage you want to know.)
In your case, since you want to see how much current is being drawn from the transformer, you'd put the ammeter between one of the transformer secondary leads (doesn't matter which one) and whatever wire from your unit that's supposed to connect to that lead.
Since you're measuring AC current, you'll need an AC ammeter, which rules out most digital multimeters, which only are designed to measure DC current. Not sure where you'd (quickly, easily) get an AC meter. Maybe others can suggest? But that's how you do it.
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On 4/9/2011 4:28 PM, David Nebenzahl wrote:

Interesting; thank you. My meter is an Extech MN26T:
http://www.extech.com/instruments/resources/manuals/MN26_UM.pdf
According to the operating instructions, my meter supports "AC or DC Current Measurement"
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On 4/9/2011 3:11 PM Steve Turner spake thus:

Yep. You've got a better meter than I, and it's up to this task. Just connect it as I described (be sure to use the correct connectors on the meter, the ones marked "10A" and "COM"), make sure it's on the highest current range (10A), and you're good to go.
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On Sat, 09 Apr 2011 17:11:31 -0500, Steve Turner

leads in the common and 10a jacks.
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On Sat, 09 Apr 2011 14:28:28 -0700, David Nebenzahl

Pardon? every one of my digital meters has an ac current range.. The one on my desk right now has a 200ma and a 20 amp scale, while for DC it has 2ma,20ma, 200ma, and 20 amp.
My "pocket" meter has 2 amp and 10 amp AC and dc
My "bench" meter has 300ma and 10 amp, both AC and DC.
The first digital meter I ever owned (and I still have it) has 2ma, 200ma and 10 amp ranges (and it is 30 years old) And my"amp clamp" reads 200 or 1000 amps
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On Sun, 10 Apr 2011 09:30:50 -0400, "Stormin Mormon"

a current transformer or an "amp clamp" - which is either a current transformer or a hall effect device (generally) depending on whether it is AC or DC..
With DC there are also "magnetic" poximity amp meters that just sit against or over the conductor. - and of course there is always the "shunt" - which is technically part of the "ammeter" in series with the load and the "meter" is a voltmeter reading the voltage drop across the "shunt" which is just a very low resistance.
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