Putting 120V AC on 12V AC Winding

[kurtlives]

Have a 120V - 12V transformer...

What would happen if I put 120V AC on the 12V side? What voltage would come out of the 120V side?

Now same thing but what if I put 120V AC on a side that was 24V? What would come out of the 120V side?

Thanks

[ACS]

Might be wrong on this, but my logic says that 120>12V is a 10:1 ratio.  Reverse that by putting 120V into the 12V side and I reckon you'd get 1200V out of the other...

...but maybe only briefly!

because of course the real question here is would the windings handle it without frying?

[alanlan]

You should never connect anything to a voltage above the rated value, especially where mains voltages are concerned.  At best you might damage the part - at worst you might damage yourself.

Please don't try this unless you know exactly what you are doing.

In theory, you would get a step up, but the primary is only rated for 120VAC so it may well breakdown on the secondary or primary.  Also, with the smaller number of turns on the secondary, you may see a lot of current flowing which may fuse the windings if the mains fuse itself doesn't blow.

[petemoore]

  Just because a DMM has a voltage limit which it can measure  doesn't mean voltages can't go any higher than that.
 

[German]

And your secondary will blow..
Its resistance could be about 10 ohms, or smaller..
10/120 - 12A..
Fuses in your home will disconnect the power.

[kurtlives]

Figured it was something like that....

Thanks

[R.G.]

That's actually an interesting question. What you'd get is probably about 180Vac, and a rounded-corner square wave, at least until the primary insulation overheated and went up in smoke.

Setting aside for a moment the smoke and flames issue, if you could do this long enough to measure, you would see that the primary is saturating. Iron will only take so much magnetizing force before all of the leetle, teensy, tee-niney magnetic domains all get aligned, then it can't hold any more. This is saturation. When that happens, any change in primary flux will be lost, and so cannot be transferred to the secondary. So the primary saturates and when that happens the secondary voltage flattens out.

This is *exactly* what happens in a ferroresonant transformer - also called a constant voltage transformer or regulating transformer. The primary does not saturate, but part of the secondary is set up to saturate, and the output is magnetically coupled to the saturating part of the secondary. At a single frequency, a mag core will saturate at a specific voltage, so the output of a simple ferro transformer looks like a square wave with rounded corners.

Given that the 120:12 transformer is of normal construction, the "12V" side is probably wound to about 14V. The primary is wound to have the right inductance to withstand 120V, plus some surge. So call it 180V on the primary to good, hard saturation. The secondary is going to be following this. It's a quirk of transformer design that all magnetic conditions are symmetrical per winding, so if you can drive a 120:12 transformer with 12V on the 12V side, you'll get something like 120V on the 120V side. You won't get exactly that because secondaries are typically wound a little higher than the nominal full load voltage to get full voltage at full current. So the 120:12 is really a 120:14 (or so) that sags. So putting 12 on the "12" side give you (12/14)*120 = 103V on the primary.

But because the primary has saturated, the incremental inductance no longer limits the magnetizing current on the primary. So putting 120Vac on the "12" side causes the current from the AC line to be UN limited ever half cycle for everything over about (guessing) 18V, and so (120-19)/(wire resistance) current flows. Smoke the boils out of the transformer for the fraction of a second it takes for the circuit breaker to blow.

[km-r]

i was expecting the "ideal" transformer from mr RG! 

[R.G.]

i was expecting the "ideal" transformer from mr RG! 

I tried for years to design one of those ideal transformers. Never worked. That's where all that "Mother Nature" stuff came from.  No matter how much I tried to convince Her to make it work, She always ignored me.

[David]

Might be wrong on this, but my logic says that 120>12V is a 10:1 ratio.  Reverse that by putting 120V into the 12V side and I reckon you'd get 1200V out of the other...

...but maybe only briefly!

because of course the real question here is would the windings handle it without frying?

THEY WON'T!  I accidentally did this when I was a teenager.  Holy Cow!  What a mess!  The thing basically exploded.  PEE-YEW!!!!!!!!!!!