How do i test an old power transformer?

[Mikante]

Hi guys, i have an old power transformer there are no informations about it, no manufacture or labels or anything, all i know is that i comes from a little amp, probably a champ clone or something like that.
Could you help me understand how to check it by using a multimeter?
I d like to use it or maybe sell it.

[Phil_S]

There is no definitive way to know the answer without unwinding the transformer or stressing it to the point of overload, at which point you've melted it. There are ways of obtaining a reasonable estimate. You will need to run a/c power into the primary or secondary to find some basic information. You will also need to measure the Ohms of the various windings.

You might give this a try (no experience with it.) https://everycircuit.com/circuit/579849 ... -extended-

I found this... https://www.diyaudio.com/community/thre ... ng.409028/

Here is some other information I've gathered over the years. Not sure how reliable any of this is.

mA Rating of Unknown Transformer
From the Primary Winding
For a 120 volt AC supply the VA rating and primary resistance is as follows.
30 VA = 30 to 40 ohms
50 VA = 13 to 16 ohms
80 VA = 7 to 9 ohms
120 VA = 5 to 6 ohms
160 VA = 2.5 to 3.5 ohms
225 VA = 1.8 to 2.2 ohms
300 VA = 1.0 to 1.3 ohms
500 VA = 0.45 to 0.55 ohms
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply.
Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.

From the Secondary Winding
One very general way, based on a copper loss of say 4%. The ht winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law). Heaters more difficult; same method, but low voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.

One degree further would entail loading of the transformer. Sometimes this is possible with mains globes. Such a load across the ht winding for >1 hour should cause the transformer to get only slightly warm, as you still have no heater load, unless you simulate that too. Very generally heater and h.t. load can be assumed to have similar power figures.

Temperature Method
The operating temperature should not exceed the boiling point of water, 212F. Just load it up until you get close to that on a sustained basis. Remember to consider the VA rating applies to the whole transformer and allow something for the filament windings.

Rule of Thumb (Credit to Stephen Keller at AX84.com)
A good rule of thumb for the current limit of a transformer can be found by measuring the cross-sectional area of its core in inches-squared, mutliplying that value by 5.58 and squaring the result. (Refer to the Radiotron Designers Handbook 4th ed. page 235.) This formula gives a reasonable approximation of the total VA rating of the transformer. You may have to remove the transformer bell-ends to obtain these measurements.

For example, for a transformer that uses E-I laminations to form the core, you would measure the width of the middle leg of the E part across the face of the laminations. If you were looking at the E as if it were a letter, this would the height of the middle leg. Next measure the total thickness of the stack. Multiply those two values to obtain the cross-sectional area. Suppose you have a transformer that uses an EI-76.2 standard core, where the width of the middle leg of the E is 1 inch and suppose the stack is 1.25 inches thick (yes this is a Fender Vibrochamp transformer, part numbers 125P1B or 022772). This gives a cross-sectional area of 1.25 (1.00*1.25) and an approximate VA rating of 48.65 [(1.25*5.58)^2]. That rather handily corresponds to Hammond's rating for their replacement transformer of this type whose secondary windings are: 325-0-325 @ 81 mA, 6.3 @ 2 A and 5 @ 2 A:

(325*.081)+(6.3*2)+(5*2) = 48.92

I should mention that this gives a decent approximation of maximum total current you can expect the core to support. Other factors such as the gauge of wire used in the windings, number of turns, number of windings, insulation thickness (which may drive up core size), etc. place additional limits on the maximum current a given transformer winding can deliver.

[martin manning]

I'd start with this:

Determine how many separate windings there are using a continuity tests or DC resistance measurements.
- If any of the separate windings have three leads, it is likely that it has a center tap.

Measure the DC resistance of each pair of leads on each winding.
- You may be able to identify which leads are center taps from the resistances.
- Windings with the lowest resistances are probably heater windings (there may be two, a 5V and a 6.3V), and the primary.
- You may get an additional clue from the lead wire gauge. Thicker leads are likely to be heater windings.

Guess which winding is the primary and apply some low voltage AC.
- Measure the voltage on each of the other windings.
- Determine which leads are center the taps on the ones that have three leads.
- Revise your guess as to which winding is the primary if you think you guessed wrong the first time.

Calculate voltage ratios for all other windings with respect to the assumed primary.

Multiply the ratios by the nominal mains voltage, and see if the windings can be sorted into primary, heater(s) and HV secondary.
- If it's not making sense try assuming a different winding is the primary and try again.

[R.G.]

Kudos Phil! That's a great collection of info.
But I just can't stop myself from kibitzing on transformers

@Mikante:
You might like to read my series of posts titled "Transformers" for some understanding of winding number, winding resistance, and regulation (actually, resistive voltage loss, but that's what the transformer guys call it). It will help this to make some sense.

There is no definitive way to know the answer without unwinding the transformer or stressing it to the point of overload, at which point you've melted it. There are ways of obtaining a reasonable estimate. You will need to run a/c power into the primary or secondary to find some basic information. You will also need to measure the Ohms of the various windings.

Dead right! The first step is to measure the ohms of each winding. This is a little problematic for low voltage windings and the typical multimeter, because they get down in the low ohms range, and typical multimeters are of highly questionable accuracy under maybe 10 ohms. Still worth doing the best you can, because this also tells you which wire is which.

One step further while the transformer is unconnected and the ohmmeter is working is to measure resistance from every wire to every other wire. This not only tells you the resistances, but it tells you which are center tapped or maybe bias tapped windings. And it tells you if two windings you would otherwise expect to be isolated are internally shorted together. While you're at it, if the trannie is a cardboard tube bobbin, do a quick scratch with a probe across the laminations to each winding to quickly see if there's an obvious short to the core. Rare, but it happens sometimes and can make a chassis live to the AC line.

My own personal second step is to run the quickie test for internal shorted turns. This is a little abstract, but it's easy to do and has literally amazed a few grizzled amp techs. The test is easy; get an NE2 neon bulb and a 6V lantern battery. Make sure all the wire on the trannie are open and disconnected. Hook the neon across the (probably...) primary winding and then momentarily hook another winding across the battery for a second or two and then release them from the battery. If the neon flashes, there are no shorted turns.
This works because any single winding makes an inductor in there. Hooking the battery across one flow DC current in and charges up the magnetic field. When the battery is removed, there is an inductor flyback pulse, and all the turns experience the same flyback pulse voltage per turn. With the neon on the primary winding, it will have a high voltage, and the neon will conduct, eating the flyback energy. If any turn on any winding is shorted, it clamps the volts per turn to nearly zero and the neon won't get enough voltage to flash.
There are some special cases on very high or low-resistance windings, but this is a good, fast test for most shorted turns. It mostly works on any two windings, even on the same winding, but using the neon on the primary makes it more general. If you don't know the primary, it's either the highest or second highest resistance winding.

Right here, I would assume that the transformer is probably safe to measure voltage on, and apply an AC voltage to the highest resistance winding with all other windings open and measure all the voltages, then get out the calculator and compute which windings have which voltages if you were to put AC mains voltage on one of them. It's easy to guess which is the primary then, and also the resulting open circuit voltages.

mA Rating of Unknown Transformer
From the Primary Winding
For a 120 volt AC supply the VA rating and primary resistance is as follows.
[...]
Simply multiply all ohmage values by four (4) for a 230 / 240 volt supply.
Derate to 65%, which is probably reasonable and to allow 15VA for the filament windings.

That was a new one on me, but it does make a kind of sense. There is a certain amount of area in the winding window for primary turns (generally about 1/2 the area for wire) and the VA rating of a transformer is a function of the area product of the laminations. The area product is the wire window (i.e. hole space for winding in the stack) area and the cross sectional area of the tongue in the full stack. It's nonlinear, but many core configs can be assigned a VA by computing the area product alone. And the number of primary turns is approximately inverse to the core area, so yeah, it makes sense that the primary resistance is kinda an estimator of VA.

From the Secondary Winding
One very general way, based on a copper loss of say 4%. The ht winding rating is probably the most important. Find the secondary voltage, e.g. 300Vac. Take 4%, giving 12V. Measure the winding resistance; one half if 300-0-300V (i.e. a 300V winding), if bridge then the whole winding. The current will now be that which causes a 12V drop across the winding d.c. resistance. Thus dividing 300V by the resistance would give a ball-park figure for current (Ohms Law). Heaters more difficult; same method, but low voltage winding resistance is usually impossible to measure accurately. The heater current could be expected to be in line for the output stage that would require the previously calculated anode current.

Good one, ought to work. The 4% number would actually be the "regulation" voltage drop in different sized trannies, and that varies, as you might guess from the resistance->VA list. The smaller the VA, the bigger the voltage loss under load, and 4-5% is probably an OK number for many guitar amp transformers. That would lead to about an 8-10 overall regulation/drop, and that's reasonable for 40-100W amplifier transformers in class AB. A Champ might be on the low end of that. A really small transformer of a few VA might have no-load to full load losses of up to 20%!

One degree further would entail loading of the transformer. Sometimes this is possible with mains globes. Such a load across the ht winding for >1 hour should cause the transformer to get only slightly warm, as you still have no heater load, unless you simulate that too. Very generally heater and h.t. load can be assumed to have similar power figures.

Temperature Method
The operating temperature should not exceed the boiling point of water, 212F. Just load it up until you get close to that on a sustained basis. Remember to consider the VA rating applies to the whole transformer and allow something for the filament windings.

And temperature is really where it's at. This is what the pros use, but they have much better abilities to measure temperatures and much better ways to load the trannie.
The real limit on power rating in a transformer is the temperature where the wire and layer insulation degrades and lets shorts form. The iron and copper will work fine (albeit with some voltage loss/drop) at temperatures up near the Curie temp of the iron, 1390F, 770C. Insulation gives up long before this. Most commercial transformers use 105C/ rated insulation or better, and that's 221F, so the boiling point of water is safe. However that's for the hottest spot inside the windings. The outside is cooler as it is what the outside air is cooling. A rule of thumb (literally!) is that most humans will not keep their fingertips on a surface that's hotter than about 130F. And crudely, if a run-of-the-mill transformer has a 130F outside lamination temperature, the hottest spot inside is under 105C and all is well. If you can keep your fingertip on the laminations when it's fully hot, it's fine.

That begs the question - when is it fully hot?
Here's where the pros and their testing setups come in. They calculate a thermal time constant estimate for how fast the internal temperature might rise. For 100VA-500VA transformers, this can be hours. For huge distribution transformers it might be days. They load the trannie up to an estimated near-max load and measure the temperatures accurately as it heats up. They do this by using the rise in the copper resistance as a thermometer. Copper resistance rises by 0.393% per degree C, so they measure a winding accurately, then let it heat for a while. They quickly remove power, then measure the resistance of the winding they're using as a thermometer, write that down, and reconnect power. The percent rise in resistance lets the temperature rise be calculated as 0.393% per C. If you have a high resistance winding, over maybe 100 ohms, this gets reasonable for even many home multimeters.
A funny fact is that if you chart the temperature rise versus time, you can compute not only the time constant of the temperature rise, but estimate the final temperature it will get to, as the temp will quit rising after maybe five time constants. This is a direct analogy to RC time constants, with temperature as the "voltage"; it works because thermal resistance is approximately linear over small ranges.

Rule of Thumb (Credit to Stephen Keller at AX84.com)
A good rule of thumb for the current limit of a transformer can be found by measuring the cross-sectional area of its core in inches-squared, mutliplying that value by 5.58 and squaring the result. (Refer to the Radiotron Designers Handbook 4th ed. page 235.) This formula gives a reasonable approximation of the total VA rating of the transformer. You may have to remove the transformer bell-ends to obtain these measurements.
[...]
I should mention that this gives a decent approximation of maximum total current you can expect the core to support. Other factors such as the gauge of wire used in the windings, number of turns, number of windings, insulation thickness (which may drive up core size), etc. place additional limits on the maximum current a given transformer winding can deliver.

Yep, that works as an estimator. It's one variation/consequence of the area product thing.

[Phil_S]

Kudos Phil! That's a great collection of info.

There is no definitive way to know the answer without unwinding the transformer or stressing it to the point of overload, at which point you've melted it. There are ways of obtaining a reasonable estimate. You will need to run a/c power into the primary or secondary to find some basic information. You will also need to measure the Ohms of the various windings.

Dead right! The first step is to measure the ohms of each winding. This is a little problematic for low voltage windings and the typical multimeter, because they get down in the low ohms range, and typical multimeters are of highly questionable accuracy under maybe 10 ohms. Still worth doing the best you can, because this also tells you which wire is which.

Well, there was a time when I was buying used inexpensive iron on eBay to use in one-off amp builds. I needed to figure it out! Necessity is the mother. No one wants to let the smoke out! Thank you for the kudos.

As to the meter resolution problem, just add a resistor in series and then subtract. Something on the order of 10Ω to 100Ω should mitigate that problem.

[trobbins]

One of the basic tests I do for any transformer I want to use, but don't know its origin, is for insulation resistance at say 1kVdc between windings, and between windings and core. That test is a quick form of 'hi-pot' test that would be done by a manufacturer prior to sale, and similar to a sparky's megger test on new or altered house mains wiring. The point being that you don't know if the transformer has been in a damp garage, or subject to rain, or metal dust, or been zapped beyond its limits, etc.

[pdf64]

Regarding winding resistances, especially HT if there's a CT, I think transformers intended for use with valve rectifiers, or just designed when they were often used, will tend to have at least 50 ohms per anode, eg total HT winding >100 ohms.
As a valve rectifier needs at least that much 'protecting resistance' when used with a reservoir cap.
If it wasn't incorporated into the winding, external resistors should really be used.

[Mikante]

Wow, you guys really gave me some to read.
Thank you
My first goal is to understand how much juice it has, i mean, milliamps and then the secondary winding volts, is it a 270, a 300 or even more.
The secondary does not have a 5 volts out that is for sure and no center taps too.

[Roe]

apply a low voltage and measure, with a fuse in line.

If it works well, increase the voltage. be careful

[Phil_S]

My first goal is to understand how much juice it has, i mean, milliamps...

Yes, that is the task! You can apply full mains voltage to the primary (be careful, use an inline fuse, screw the PT to a board, etc.) to determine the unloaded secondary voltages. Voltage is easy. Current is more difficult and more important to know.