simple transformer use question

[jrenkin]

I have a power supply transformer that can be used for 115 or 230 V input. So it has 2 primary coils. Since I am at 115V (120V actually) can I safely use the other 115V winding as a 1:1 secondary? Or is there something intrinsically different about this winding that I can't.

I measure no continuity between the two 115V windings, so it seems that it is just another winding to use if needed.

I figure I could use the winding for a bias circuit, or simply use it like a 1:1 isolation transformer when testing.

[Minion]

Dual Primaries are generally wired in Paralell for 120v and in series for 220v so there should not be an un-used winding , though if you did just use a single winding you would only get half the rated current .... it would only be 1:1 if it was 120v input 120v output .....

[fftulip]

You should be able to get voltage out of the other primary, but it's not a good idea. There needs to be good isolation (insulation) between the primary and secondaries for safety, and there may not be between the dual primaries. Best to use it only as a primary as intended.

[jrenkin]

Originally Posted by Minion

Dual Primaries are generally wired in Paralell for 120v and in series for 220v so there should not be an un-used winding , though if you did just use a single winding you would only get half the rated current .... it would only be 1:1 if it was 120v input 120v output .....

So to use it as intended, I need to apply mains voltage to both primaries.

In regard to insulation, used as a 1:1 isolation transformer, since there isn't a dead short, wouldn't there be enough insulation as long as I am working with trivial currents? (I just am trying not to buy an isolation transformer...).

[Minion]

Yes , You will need to use Both primaries to get the full current and they need to be in phaze with each other , To test that they are in phaze , that is if the wires aren"t color coded , Connect them in Paralell and test the output voltage , if it is in phaze (from what I remember) you will read the expected AC Voltage on the Secondary winding and all is good , if it is Out of Phaze you will get no AC Voltage or a considerably lower output Voltage , If this happens, quickly disconnect the power and reverse one set of Primary wires ....

Do all the testing quickly cuz when it is out of Phaze the Transformer will start to heat up , I never had a Problem with Big Transformers but wouldn"t want to try it on a really small one ......


Cheers

[jcx]

you really shouldn't even think about considering the dual primaries as isolated - treat them both as "live"

mains-sec isolation requires separation which must be rated for 2-4 KV depending on application and meet creepage and clearance construction standards for the pri-sec insulation