how to id transformers ?

i have a couple of mystery transformer pulls {microwave, vcr, clock radio, etc} i try to google codes on trans with no proper hits ...

how to id these transformers ? is there a way ? do the numbers have significance on there own ?

i want to figure/estimate the ratings they might be useful for ? thinking of using in power supply ?

eg, one T has

9199-207-029 41B512 DY 217 B

2 sets of windings around standard type metal core - B

1 winding has 2 white, 1015 - 22 AWG wires connected other winding has two yellow and two red 1007 - 22 awg wires coming out when i apply 115 Vac to the white wires i get 15 Vac out of red and 3 Vac out of yellow wires

thanks for any ideas / info, robb

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None whatever.

The part numbers are usually unique to the manufacturer. The best you can hope for is a label with the voltages.


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If you're taking them out of things, then don't merely take them out of things. Mark them with what you took them out of. Mark which side connects to the AC line. Even trace out some of the secondary to see what's there.

What something is used in originaly gives a clue to what it's good for. A microwave oven transformer is pretty specialized, high voltage and with limitations for other uses. A transformer in a clock radio will be relatively low voltage, and relatively low current, but often useful for powering various simply projects.

Knowing which is the primary (there is sort of a color code but it's not always used), then you can at leats hook it up to the AC line and measure the secondary voltages. Be careful, since if you don't know where it comes from, you can't rely on the voltages being safe.

You can do this without knowing which is the primary, but it's risky since if you connect the wrong winding to the AC line, then the step up from a low voltage winding to the real primary can result in a quite high voltage on that real primary. SOme would suggest if you don't know the primary, use a low voltage transformer to feed the unknown winding until you figure out the primary by looking at the relative voltages out of the rest of the windings.

Tracing out the general circuit will give you an idea of the voltage coming out, and maybe more important the current. Current isn't easy to judge merely by looking at the transformer, though presumably the bigger the transformer for a given voltage output the higher its current capacity. But if you trace the circuitry a bit, you may see clues to what its expected to supply current wise. If there's only one winding and it goes to a low current regulator, that says something as does going to a to-220 or to-3 packaged regulator. It won't give you exact, but it will give you clues.


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Michael Black

As said, scope out everything. I would assume you allready have a understanding of transformers. if ou look at the wire, you might get a rought idea of its capabiliries, but it could also throw you off. As said in the sci repair faq, I gave a simple way to estimate output current ratings. You simply load the output untill it drops 15% in voltage and montor voltage and current. A very good transformer can have a 10% drop rating. A very poor one can go up to 30%. 15% is an average figure. Let it run for a length of time and also monitor temperature. Back off and you should be OK.


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them out







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Thanks Michael,

I was hoping the numbers on the transformers were some sort of encrypted code or P/N that i could decipher /use to find specs. when i pulled them i did look at what was connected but months go by then all i can remember is what they came out of.

Is there a way to make an experimental survey or educated gues of a transformer's limits/specs without frying the transformer in the process ?

ideas like the size of wires or the weight of the transformer , maybe some simple stress and stability measures , feed a variety of inputs Hz/Vac deltas and plot input vs outputs then find some ideal function/curve that defines upper/lower bound limits of a transformer ?

this is more for eductional instruction/experimentation than for practical use

thanks for help , robb

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montor voltage


Thanks greg,

i just posted a reply question regarding what you have just pointed out.

so i get a variable current load and monitor current, voltage and temperature until i see drop of 15% in output (or temperature of ???) as a average target for upper bound current limit.

regarding temperature, is there a typical operational rating value for most basic average transformers ?

thanks again, i will go visit the repair faq, robb c

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We all like the transformers to not get hotter than what you can easily hold your hand on, but they can get hotter. When it starts to smell, your probably going too far !! There are transformers that have a temperature cut out, and it will usually make the unit unopperative unless, you can cut away the insulation and expose and replace it or bypass it.


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I expect that power transformers in most consumer products will be custom-designed for the particular application. Any numbers on them will be the equipment manufacturer's part number. Specifications are unlikely to be available.

I suggest you look at a transformer manufacturer's catalog (see

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of example) and compare the size and weight of your transformers to their products to get an estimate of the power ratings.

Peter Bennett, VE7CEI  
peterbb4 (at)  
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Peter Bennett

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