Blew another damn transformer on my Trane XB80

"Don Klipstein"

** You missed the point entirely.

A voltage spike ( or a series of them) can easily cause insulation failure in the enamel winding wire of the primary - then the energy to explode the lead in and lead out wires comes from the 120 volt AC supply.

.... Phil

With AC, volts times amps is not necessarily watts.

If determination of volts and amps are both "true RMS", (non-"true RMS" meters are usually OK for line voltage and for current through a cap across an AC line),

then volts times amps is "volt-amps". Ratio of power consumption (in watts) to VA is power factor. Capacitors have very low, ideally zero, power factor.

Digi-Key has 3 easy enough to find 1uF ones with X1 or X2 rating, all $3-plus plus shipping with minimum order requirement to avoid a surcharge.

Among those 3, my favorite is FC233820105, which is by Vishay/BC Components. Its datasheet refers to 2 UL standards and 2 CSA ones, and has a link to an application note making a claim that the referenced UL ones are sufficient for achieving the "UL Recognized" ("backwards UR") mark.

That one needed me to delve into its datasheet to see that it claims to meet the two relevant UL standards and that it has X2 rating. Its AC voltage rating is 275 volts. 91 cents each plus shipping, with a minimum order requirement to avoid a surcharge.

Any of the 4 capacitors above (2 mentioned specifically) look good, in the unlikely event what is needed is a capacitor across the line.

--
 - Don Klipstein (don@misty.com)

PF of a capacitor suitable to connect across an AC line is usually less than .01.

Phil Allison is well known in sci.electronics.design to be quick to get brash, even calling people names and sometimes a little worse. If only he would avoid that, a lot more people would think a lot more highly of him than they do now, since he does fairly well know what he is talking about when it's electrical.

--
 - Don Klipstein (don@misty.com)

Is the O.P. having other things in his house blowing from voltage spikes severe enough to blow transformer primary winding insulation?

I have seen lots of things blow from line voltage surges, but no transformers indoors blowing when line voltage spikes blow other things.

My experience is that usual 120V-primary step-down transformers can produce pulses of 2 kilovolts when used on pulses in reverse. In fact, I have done that with about 8 different transformers dozens of times each, and none of them lost their ability to do that. (I am aware of line voltage spikes being noted to get even higher.)

There is also the issue of line voltage surges not easily being loaded down by capacitors for whatever reason. I have experience with them blowing things that had capacitors, including a CFL that had probably a 22 or 47 uF capacitor across the output of its internal bridge rectifier as they usually do. No transformer failures in the same house from the same event, though there were electronics failures.

The usual solutions to absorb line voltage spikes are MOVs and other devices that absorb voltage surges by becoming conductive in response to excessive voltage.

--
 - Don Klipstein (don@misty.com)

** Totally irrelevant.

The furnace unit and the transformer in question are all we are discussing.

You have clearly not bothered to read my first or my other posts in this thread.

Eg:

" 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."

Whenever AC power to that furnace is disconnected, the blower fan will deliver a back emf spike - meanwhile that poor, little tranny is wired in parallel with it. Other devices in the house are NOT involved.

Most AC supply transformers can tolerate repeated 2kV spikes on the primary till the cows come home - but a badly wound one cannot. This is a specific and fairly recent problem with small transformers made in China and elsewhere in Asia where the makers are not fully aware of the issue of insulation failure in the enamel windings.

Once there is a layer to layer insulation failure ( between adjacent wires) in the primary of that tranny - the AC current draw will jump up to many amps and may cause the feed in and out wires to explode - as seen in the pics.

If the spike voltage is suppressed, the tranny will likely survive.

A 1uF cap provides a low impedance path for such a back emf spike, virtually shorting it out.

OTOH a varistor provides no conduction path until its breakdown voltage is exceeded, but is also a means of suppressing the spike voltage to a safe value.

.... Phil

So you are agreeing with what I said before - most likely problem is cheap crap component.

We can be relatively sure it wasn't lightning 3 times. If the back emf from the motor is causing the problem, the bad connection to the motor should have made itself VERY evident by now. Not saying it is NOT part of the problem - like you, I wasn't there watching it fail - and neither you nor I heard the noise you speak of.

And IF the problem is what you say it is (and I'm not saying it is not), then probing the old primary and getting to the winding beyond where the solder joint "exploded" you should be able to measure a significantly lower than normal primary resistance.

That measurement has not, as far as I know, been made and reported.

If it was mine, or if I had the transformer at hand, I'd have it apart and analysed in no time. If the primary is shorted, I'd know, within an hour or two of the failure.. And with that second primary, it would not be hard to determine if the

110 volt primary is shorted without even dissassembling the transformer..

At this point no-one has actually posted FOR SURE what the primary configuration is. Is it a mult-tapped primary, set for 115, 208, and

230 volts, or is it 2 independent primaries, or is it 2 primaries that need to be connected either in series or parallel depending on the voltage (115 or 230)

I don't know this, and you don't know either unless you are clairvoyant, because it has not, to this point, been established and reported.

So we are all guilty of the same thing - making ASSumptions.

>

I have to agree with Don. DC power component on secondary OR primary, or not connected right are the 2 most likely problems after "cheap crap component"

If the back emf from the motor is causing the problem, the bad

** Not true at all. ** What is clear is that the primary has suddenly drawn a very large current and that means there are SHORTED turns inside the primary.

Go away wanker.

..... Phil

** You are both wrong. ** Which is no help to the OP.

.... Phil

Why would that be, in your words, "totally irrelevant" to whether or not the O.P. had any electrical/electronic failures elsewhere in his home attributable to voltage surges?

As opposed to 3 transformers blowing in the same appliance with nothing else anywhere in the home running into trouble from voltage surges?

While I have experienced roughly 8 transformers surviving repeated abusive pulse-in-reverse-direction developing about 2 KV across the primary without any degradation against ability to do so?

Why would the solution be deploying a capacitor, rather than repairing the poor connection or deploying a voltage-dependent spike-absorbing device such as an MOV?

So, why should we hear about problems about that from only one customer blowing 3 of them?

Why should 1 uF protect the trannies in question while 22-47 uF fails to protect a compact fluorescent lamp from a line voltage surge that blows even other electronics in 2 houses but did not blow any in-home trannies?

Certainly protects against applying for even a microsecond more than roughly 250-300 volts across transformer primaries, where it appears to me that we agree that transformer primaries usually survive 2,000 volts pulse voltage?

I seem to think that the trannies are probably blowing from either improper wiring (connecting only 1 of the 2 primary winding sections possibly noted for 120V usage), or from secondary load malfunction including bridge rectifier failure in manner of a diode "failing open".

Or, extreme-oddball trouble such as sticking a magnet to the tranny. But that's grasping-at-straws, like line voltage irregularities that blow

3 trannies in 1 piece of equipment but draw no other complaints such as blowing of electronics downstream of the tranny in question, or elsewhere in the house where one appliance blew 3 trannies.

--
 - Don (don@misty.com)

Please add helpfulness, as opposed to adding nothing but saying who is wrong.

I chimed in explaining stuff every time. At this point that I respond to, you are doing nothing but claiming who is wrong.

It appears to me, posting nothing but who is wrong is even lower than my grasping-at-straws bit of any magnets on the transformer in question.

--
 - Don Klipstein (don@misty.com)

Yes, many moons ago I was playing with adding a mylar cap in series with the primary of a neon sign transformer. It worked great for making the neon dimmer which was my attempt, but I recall measuring a much higher voltage across the cap due to the reactence and impedence of them in series. I'm guessing but I think it was up in the 400vac or so range maybe higher. Not good for the transformer winding either. I didn't blow them but stopped using them after reading the voltages.