using a transformer in reverse ??

[Minion]

Hi , My local Salvation Army thrift store has a couple 230v to 115v stepdown transformers , it says on them that they put out 100w at 115v.....

I"m pretty sure I can use them backwards and apply 115v on the secondary to get 230v on the primary but was wondering what the output power would be ?? 50w at 230v ??

I was hopeing it would be a good and cheap (probably about $5 each)candidate for a tube amp PSU ......

Thanx guys .....

[CanAm Man]

Yes..you can use the transformer as a step-up transformer (i.e., in reverse).

No..... your Volt-Amp capacity would be the about the same: 100 watts at 230 VAC..... but keep in mind, this is only half the current at double the voltage, or about 435 ma.

But if you run the transformer in reverse....wouldn't the music come out backwards???

[Minion]

Thanx for the answer ..... I"m pretty sure that 435ma will be plenty for a tube amp .....

Thanx again .....

[amptramp]

Depending on what you want to do with it, you should determine whether the windings are separate or if this is an autotransformer. If you have separate windings, you can feed 115 into the 115 side and get 230 out the other side, which you can rectify with a diode bridge. If the windings are not separate, this gives you a permanent shock hazard and a ground problem.

[Richard Ellis]

Almost all the 220/230- 110/120 transformers are of the 'Autotransformer types. Every single one available down south here in South America are the 'Auto' type. Take a DVOM to your Salvation Army...& test it to be sure.

___________________________________________Rick... ............

[Elvee]

Originally Posted by Richard Ellis

Almost all the 220/230- 110/120 transformers are of the 'Autotransformer types. Every single one available down south here in South America are the 'Auto' type. Take a DVOM to your Salvation Army...& test it to be sure.

___________________________________________Rick... ............

Yes, and even if they are full (isolating) transformers, you must take into account the fact that there is normally a built-in asymetry, to compensate for losses etc.
The transformers would in fact be 230V to ~140V, and if you use them in reverse, you suffer from a double penalty: absence of compensation + original compensation working in reverse.

This means that you will end with a loaded output voltage of 200V or so.

[AndrewT]

Originally Posted by Elvee

Yes, and even if they are full (isolating) transformers, you must take into account the fact that there is normally a built-in asymetry, to compensate for losses etc.
The transformers would in fact be 230V to ~140V, and if you use them in reverse, you suffer from a double penalty: absence of compensation + original compensation working in reverse.

This means that you will end with a loaded output voltage of 200V or so.

yes, the turns ratio predicts the open circuit voltage.
The regulation gives the predicted fully loaded voltage.
The stepped up voltage is likely to be ~ 2times the regulation low.

It is very easy to find if the windings are separate or tapped.
Not so easy is the isolation standard that each winding has been prepared to.

[Minion]

I wish I could test it at the thrift store but I can"t , the transformers are in a chassis that has a female 3 pronged USA plug in it and a male 2 pronged european plug ...... I"ll get one of them and test it and if it isolated I will go back and get the other one ...... Thanx