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 Post subject: Identifying Unknown Power Transformers
PostPosted: Jul Sun 18, 2021 6:17 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Burbank, CA and Thailand
Note: these suggestions are meant for people who are moderately experienced. Most of this will be old news to the more experienced folks here. Beginners probably shouldn't mess with this stuff.

Lately, as a busywork project I've been pawing through boxes of old power transformers to figure out specs on the unlabeled ones. There are a lot of tricks and methods one can use, though there's no one size fits all procedure. The main thing is to be safe -- don't shock yourself, damage the part, or create an over-current condition.

With leaded transformers the most common wire scheme is: black (primary), yellow (5V), green (6.3 and/or 12.6V) and red (high voltage). Exceptions are common, however. Striped leads are usually center taps.

You first need to ID the primary winding (usually 110-125 volts), or at least take an educated guess. For multiple region transformers there may be two sets of windings, or a single winding with one or more taps.

If you have no idea which wires or terminals are the primary, you can try measuring all the winding resistances, which will help you make an educated guess. This step is a process of elimination. Heater windings will be very low resistance, often 2 ohms or less, and they'll usually have thicker wires, so it's easy to locate those. The remaining windings will be primary and high voltage. The primary will be lower resistance than the HV windings, so knowing this, you can usually determine which leads are the primary.

Sometimes digital meters will give over limit or flashing resistance readings when measuring transformer windings. This is because induced hum in the coils generates a noise voltage that confuses the meter. Sometimes you can prevent this by connecting the transformer case to ground, or grounding one of the DVM leads. Or, even better, use an analog meter, which won't have that problem.

If there are leads that show infinite resistance against all the other leads, they may be ground or shield windings. Those are usually unique colors and not paired. Sometimes they are not insulated.

Once you have ID'd (or think you have ID'd) the primary leads, you can safely energize the transformer by connecting them to a 60 Hz. sine wave oscillator set for 1.2 volt output. This allows you to estimate secondary voltages simply by measuring and multiplying by 100, with no risk of shock or damage.

Of course, your meter readings will be 10-15% higher than the actual design rating, because you're measuring with no load. Also, older transformers may have 110 or 117 volt primaries, so the secondaries will be 5-10% higher with today's typical 120-125 volt service. So you'll need to calculate for these things accordingly.

Heater windings are pretty easy to ID and spec because they will almost always be 5, 6.3, or 12.6 volts.

It's harder to precisely measure high voltage windings because there are no standard voltages. But you can usually estimate them closely. And with power transformers in tube circuits there are many variables that affect operational voltage anyway, and generally, exact voltages aren't important or expected.

For transformers with multiple windings meant to be jumpered in series or parallel, you may need to identify phase. Sometimes I do this by using an asymmetrical 60 Hz. signal and looking at the other windings with a scope, which is fast. But I think most people just do it by trial and error, using a meter to look for voltage addition or subtraction.

After doing low voltage tests you may wish to do real world tests with line voltage. (You should only attempt this if you are experienced, and know how to safely work around high voltage!) You can estimate the heater winding current ratings by powering up the transformer and connecting a large load resistor to the winding under test. Most heater windings will be at least 1 amp, so for a 6.3V winding you could start with a large 8 ohm resistor or for 12.6V, 2 of them in series. A large variable resistor is easiest. Gradually reduce the load resistance until the measured voltage drops to 5, 6.3, or 12.6 volts. You can then use ohms law to determine current at that voltage, or measure it. Your power (cheater) cord must include a fuse, perhaps 1 amp fast blow. If the transformer gets warm or buzzes, power down immediately.

When doing these tests I rarely measure current on high voltage windings with full voltage applied, and don't recommend doing it. You'd need a lot of high value high power resistors, and the temporary connections might be dangerous. If you really want to do it you'd need to select your load resistors very carefully, and maybe power up slowly using a variac.

Of course there are lots of exceptions to the above guidelines and these methods won’t always work, but you should be able to identify most power transformers with reasonable accuracy.

Again, please be extra careful and extra safe!

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Last edited by David Kulka on Jul Tue 20, 2021 12:34 am, edited 2 times in total.

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 Post subject: Re: Identifying Unknown Power Transformers
PostPosted: Jul Mon 19, 2021 10:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1754
Location: Wayside, NJ Monmouth
Nice write up Dave. Thanks. I copied it to a word file and have added it to my Work Shop Hints & kinks note book.


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 Post subject: Re: Identifying Unknown Power Transformers
PostPosted: Jul Tue 20, 2021 2:07 am 
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Joined: Sep Mon 16, 2013 2:42 am
Posts: 5101
Location: Tucson, Arizona U.S.A.
Quote:
Sometimes digital meters will give over limit or flashing resistance readings when measuring transformer windings. This is because induced hum in the coils generates a noise voltage that confuses the meter. Sometimes you can prevent this by connecting the transformer case to ground, or grounding one of the DVM leads. Or, even better, use an analog meter, which won't have that problem.

More often, this is caused by the inductance of the transformer confusing the autoranging circuit in the meter. Transformers, especially large ones, can have a lot of inductance and develop significant voltage when the current is changed. The meter applies a current and measures the voltage. If it isn't in range, it changes the current. This causes the transformer to develop a voltage. If the new voltage isn't within range, the meter changes current again. And so on. It frequently happens that the generated voltage is too high for one range and too low (or reverses polarity) on the next range. So the meter flips back and forth between ranges looking for an in-range voltage which it never finds. The obvious solution is to turn off autoranging (consult the instruction manual if you don't know how) and select the range manually. The transformer will still develop the same voltages as the ranges are switched, but they will die away before a human operator can change the range.

Old transformers are likely to have 2.5 volt windings; some have 1.5 volt for #26 tubes.

You can keep the transformer leads out of trouble (if they are long enough) by connecting them into a Euro-style terminal strip. https://www.altechcorp.com/PDFS/Splits/tba78.pdf. This also provides a safe way to attach a line cord and voltages can be probed through the screw holes. Even though these are rated at some low voltage, 300 volts for the example, there is plenty of insulation to handle receiver type transformer voltages. It's safer than having loose wires hanging around waiting for you to touch them.

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