# Transformers

#### Blaise

Joined Jun 26, 2004
2
When you step down the voltage from say 230v to 12v what happens to the current? Transformers can be up to 99% efficient so presumably power out should nearly equal power in. To simplify calculations say we put 100v @5A into a 10:1 stepdown transformer theoretically the output should be 10v @50A if P=VI. This is clearly not the case. So if both voltage and current is reduced power out does not equal power in and efficiency is greatly reduced. How does it work?

I relise this is a pretty basic question but please just humour me.

#### mozikluv

Joined Jan 22, 2004
1,437
hi blaise,

here's the humor you were asking,

When you step down the voltage from say 230v to 12v what happens to the current? nothing

Transformers can be up to 99% efficient so presumably power out should nearly equal power in. theoritically yes, in reality no

theoretically the output should be 10v @50A if P=VI. This is clearly not the case.
you are right

So if both voltage and current is reduced power out does not equal power in and efficiency is greatly reduced. How does it work?

the difference is filled up by the core loss, copper wire loss, power factor & coil resistance