backwards transformers

[theChris]

ok, i'm no expert on transformers, but if i took a transformer that was a 120V primary 48V secondary and wired it backwards, i would get a 120V primary, 300V secondary right? and if i had a 240V primary and a 120V secondary and wired it backwards i would get a 120V primary and 240V secondary, right?

[hacknet]

you might saturate the core. becareful

[tiroth]

What I've done before is to take two such transformers and wire them back-to-back. e.g. if you have dual primaries then you could do 120:48 -> 48:240 without any fears.

[Sch3mat1c]

Transformers are typically wound near their limits, so increasing supply voltage even by 120% may cause the transformer to smoke - let alone 200 or 300%!

Putting them back-to-back as Tiroth mentions is a good way of getting around it, but you double the transformer losses and regulation.

Tim

[theChris]

well, o hadn't planned on running the transformer to it's limits. i wanted to run a headphone tube amp. the only problem i was having was finding suitable transformers for cheap. i have access to things like 120V - 48V, so i figured i'd use that for the high voltage and use a smaller transformer for the heaters

[Positron]

Hi Chris,

"but if i took a transformer that was a 120V primary 48V secondary and wired it backwards, i would get a 120V primary, 300V secondary right? and if i had a 240V primary and a 120V secondary and wired it backwards i would get a 120V primary and 240V secondary, right?"

Interesting question. Even if you had No load at all, connecting the 48 volt winding to 120 vac will destroy it instantly. The primary inductance won't be nearly high enough.

You are correct on the second example as the 120 vac secondary winding has enough inductance.

Good luck Chris.

[Sch3mat1c]

It's not the inductance, it's the saturation. Related but different.
(Someone may argue, well saturation occurs at a certain flux level, which is dependent on current and turns right? Well that inductive current through the winding depends on frequency and voltage.)

Current load has no effect, as current cancels out of the equations.

Tim

[PassFan]

Even if you could get it to work you cannot change the balance between voltage and current. If you step up the voltage the current will go way down. A transformer is a near perfect thing, most are high 90's when it comes to efficiency percentage. 100 va's in will get you just about 100 va's out. You can't change that.

VA's = E x I

[PRR]

> connecting the 48 volt winding to 120 vac will destroy it instantly.

Not quite instantly. Been there, done that. It may take 30-60 seconds to get burning, although with a large transformer you may just blow house-fuses first. An over-volted transformer is much like a short, whether you compute inductance or flux.

They don't give you much leeway on transformer rated voltages: why pay more if you aren't going to use it? A 120V winding fed 130V won't burn-up, or not for a very long time. But a 48V winding fed 120V will saturate, act like a near-short, suck a lot of current, and something will die.

You can do it the other way: put 120V on a 230V winding. The current rating is hardly changed, so you paid for twice as much core as you needed, and the resistance is not as low as a winding optimized for the working voltage, but nothing will blow-up.

[richwalters]

You didn't mention if it was a toroid! If so, manufacturers usually wind for 1.6 Teslas i.e limit of Bmax mag sat. Playing about back to front on this type could cause interesting black outs.
I am also a qualified welder. Better to get the right part for the job.