secondary voltage vs. primary voltage

[Stocker]

Power transformer.

there are 100V and 120V primary wires. Ostensibly (by the label on the transformer) hooking up the 120 to my 120 mains will yield ~20V on the secondary. 6:1 yes? also, if 100V mains are used with the 100V primary, ~20V out. 5:1 yes?

So: will using the 100V primary winding get me ~25V on the same secondary, or just a big ~ZAP~ or somewhere in between? or have I got this all wrong?

 

[Sch3mat1c]

Yup. Mind that the transformer may be wound close to limits - that is, too much voltage lets out the magic smoke. I hesitate to say 20% shouldn't be a problem. (If it's a 50/60Hz unit ran at 60Hz I'll put in a bit more confidence. )

Tim

 

[Stocker]

Thanks. Now if only the current capacity would stay the same...

 

[Sch3mat1c]

It does! In fact, current capacity is entirely dependent on the wire; you could wind a 1 cubic inch transformer with superconductor and pass hundreds of amperes through it!

Tim

 

[sreten]

It does! In fact, current capacity is entirely dependent on the wire; you could wind a 1 cubic inch transformer with superconductor and pass hundreds of amperes through it!
Tim

Err..... I don't think so, the core must be able to maintain the
flux required for the secondary output, so presuming the core
saturates the 25V output will have less current than the 20V.

sreten.

 

[Sch3mat1c]

Actually, the primary's field cancels with the secondary's field (because they have an equal and opposite number of amp-turns) leaving the core only to deal with its inductance. That's why a transformer's primary only draws as much power as the secondary, plus a small idle magnetizing current equal to the primary's inductance.

Tim