How do I test Transformers to see what their value is ?

[Jamforthelamb]

Hey everyone,

The title pretty much says it all. I have a couple of transformers that I've taken out of an old telephone amplifier, and I want to find out what kind they are. I'm assuming that they are for audio considering what kind of device they were in. Any procedures I can use to find out ?

Thanks,
JFTL

[Seljer]

I think you'd go and measure the resistances on each of the windings and then get the ratio between the windings

[Jamforthelamb]

Looks like one side is about 80 ohms, and the other is about 300 ohms.

This would likely be useless for guitar stuff huh ?

Thanks,
JFTL

[geertjacobs]

Since transformers only work with AC voltages, you should test them with AC voltages.
- first find out which windings belong together by doing a DC resistance measurement. This shows which are the primaries and secondaries (perhaps multiple secondaries, perhaps separate secondaries,...)
- the voltage ratio can be found by putting a small AC signal voltage on one end (a few tens of millivolts...) and reading the "transformed" AC voltage at the other end.
This will give you the voltage ratio. You can deduce the impedance ratio from the voltage ratio:
link to formula
- the size of the transfomer and the thickness of the leads of the transformer may give some indication on how much power it can handle.

[jmusser]

Thanks for the link and information. That's always been one of my questions, since I have several small transformers I've stripped out of old PC boards.