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240v AC Transformer

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Burnt

New Member
Hi All,

I have a step down Transformer 240v AC to 12v 100amp output.

My question is, can i use this as a step up transformer as well by connecting to the other side of it?

My supply voltage is of course 240v AC, this can be controlled with a 240v 10amp Variac to take it easy on things.

Not sure so i thought i had better ask first.

Thank You
 

JimB

Super Moderator
Most Helpful Member
Transformers are bi-lateral, they operate both ways.

However, when using your transformer as a step-up, you must not apply more than 12 volts to the primary winding, which will result in no more than 240 volts on the secondary.

If you connect the 12v winding to a 240v supply, there will be a flash, bang and a lot of sparks.
At best the supply fuses will blow, at worst you will burn the place down.

JimB
 

Burnt

New Member
Transformers are bi-lateral, they operate both ways.

However, when using your transformer as a step-up, you must not apply more than 12 volts to the primary winding, which will result in no more than 240 volts on the secondary.

If you connect the 12v winding to a 240v supply, there will be a flash, bang and a lot of sparks.
At best the supply fuses will blow, at worst you will burn the place down.

JimB

Thanks JimB,

Yes your comments are noted.

So if i carefully apply the voltage via the Variac with no more than the mentioned 12v it should be ok?

Thanks again
 

JimB

Super Moderator
Most Helpful Member
So if i carefully apply the voltage via the Variac with no more than the mentioned 12v it should be ok?

Yes, no problem.

JimB
 

Hero999

Banned
Where did you get the 100A 12V transformer from?

It sounds like an oddd rating, normally 1.2kVA transformer have a higher voltage and lower current than that.
 

Willbe

New Member
You'll probably have lousy regulation on the 240V side.
 

Hero999

Banned
I don't think I'd agree with that, I don't see how the load regulation shouldn't be any worse than if it were used the right way round.

The only thing is to get 240V on the secondary, you'll need to put 14V on the secondary.
 

Willbe

New Member
I don't think I'd agree with that, I don't see how the load regulation shouldn't be any worse than if it were used the right way round.

The only thing is to get 240V on the secondary, you'll need to put 14V on the secondary.

I think I tried it; a step down hooked to a step up to make an isolation transformer. It worked terribly.
Maybe it's the impedance being multiplied by N^2, plus the wire resistance.

Dunno' fer sure. . .
 
Last edited:

Diver300

Well-Known Member
Most Helpful Member
Your turns ratio is about 20:1, so 1 amp on the 12V side gives 50 mA on the 240 V side

With a 10 A variac, the best you can hope for is 1/2 amp at 240 V.

That is a lot of metal for that much power. If you could find a 110V output transformer you could get much more current out with that run backwards.

BTW, 12V 100A isn't a very odd transformer rating. There is a 350 A, 5V transformer in use at my work.
 

grim

New Member
what are you actually trying to achieve?

taking 240 mains, turning it down to 12v with a variac, sticking it into the 12v secondary of a tranny and getting 240v back out seems an unusual and as mentioned above very wasteful exercise :(
 

Willbe

New Member
My question is, can i use this as a step up transformer as well by connecting to the other side of it?

You can put the primary in series with a 240v 10w bulb and the source and see what comes out of the secondary. Cautiously increase the bulb wattage for more output. This is more of a current source feed than the voltage source presented by the Variac.
 
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