Voltage on transformers query

[paddy ryan]

Might seem a simplistic question, but am confused about how you check what the primary voltage on a transformer is when there are no markings at all to identify the values. I don't want to test it with say 240 volts from the mains if its only 120 volts, in case I damage the transformer. I know there is a formula via the windings, but too complicated for me! Advice would be appreciated. Thanks

 

[Nigel Goodwin]

Apply a lower voltage, use a variac if you have access to one. However, first question is where did the transformer come from, as that's a huge clue as to what it might be.

 

[paddy ryan]

Thanks Nigel, I have a varian so thats good. The transformer is out of an old 16mm projector, from the 50s, quite a small thing, possibly from an amplifier . Don't know whether its 120 or 240 though, as I said. no markings on it whatsoever.

 

[MaxHeadRoom78]

Do you know the voltage rating for the secondary?
If so, feed the secondary with its rated voltage and measure the primary.
Otherwise, feed it with 120v and see of the secondary is realistic for the application.
Was it for the lamp?

 

[Diver300]

Put a 240V incandescent light in series with it and connect it to 240 V mains, with no load. If there's only about 120 V across the transformer, then it's probably saturating under those conditions and it's a 120 V transformer. If there's over 200 V across the transformer, it's likely to be a 240 V transformer.

 

[Reloadron]

The transformer is out of an old 16mm projector, from the 50s, quite a small thing, possibly from an amplifier .

I wouldn't rule out an audio transformer possibility.

Ron

 

[paddy ryan]

HI Max, Sorry have been away from my computer, yes the transformer was for the lamp, so will o ahead with your advice, thanks

 

[Lax Luthier]

If the projector is still around, it should have a nameplate attached to it stating the mains voltage it was intended to used at. However sometimes old turntables have their motors in series with power transformers so that's something to keep in mind. Are the transformer's leads color coded?