Too big variac, and 3 phase

[Forman313]

The goal is to have an adjustable powersupply. One day 26-0-26 and 45-0-45 the next day... or 90-0-90 for that matter. Lots of cheap amplifier boards on ebay.

Overkill for a 4xTDA7293 ? Nah...

[thaumaturge]

Variable CENTER TAPPED secondaries.... Only way to do it is to start with an overvoltage model* variac then feed power to wiper and fixed end with a hard stop minimum feed size so your wiper won't feed too small a primary area (or even dead short). Then physically issolate the other half of the windings and divide accordingly. As primary turns are reduced secondary voldage goes up. Minimum feed size determined by heat dissipation ability.

*by overvoltage I mean 240 for 120 mains or 480 for 240 mains.

A more workable scenario is to feed a fixed second transformer off the variac.
Doc

[kevinkr]

Direct connection to the mains is hazardous and discussion of such ideas is grounds for closure on this forum. Your load MUST be completely isolated from the mains for safety, and a suitable transformer with center tap will allow you to get the X-0-X voltages you need.

You must use a conventional power transformer powered by one of your variacs to provide the voltages you need for experimentation.

I would recommend you check out Antek here: Antek - Your reliable source of transformers, power supplies, and more. - even with shipping to the EU their models may be competitive.

[thaumaturge]

I never once advocated direct connection to mains. If you will read my posts I referred st splitting off -issolating- secondaries in all cases. Perhaps I presumed wrong, but common sense should dictate painting over bare copper areas of resultant secondaries.
Doc

[Forman313]

Originally Posted by thaumaturge

Variable CENTER TAPPED secondaries.... Only way to do it is to start with an overvoltage model* variac then feed power to wiper and fixed end with a hard stop minimum feed size so your wiper won't feed too small a primary area (or even dead short). Then physically issolate the other half of the windings and divide accordingly. As primary turns are reduced secondary voldage goes up. Minimum feed size determined by heat dissipation ability.

*by overvoltage I mean 240 for 120 mains or 480 for 240 mains.

A more workable scenario is to feed a fixed second transformer off the variac.
Doc

Ahh.. perfect. That means I can reduce the size and weight drastically by taking it apart and only use one of the variacs.

I have a 1500va 90-0-90 and a 1000va 32-0-32 .. maybe I should make two, cause I assume the transformers current capability stays the same even with lower voltage? E.g. i´ll fry the big one if I use it with a low voltage high current amp, like an Aleph?

Any idea how big of an impact this will have on the sound? Assuming the the exposed windings and roller brush is nice and clean...

Thanks a lot

uhh.. this also means I wont be current limited at lower voltages. Brilliant.

[thaumaturge]

I assume you are referring to driving a second transformer with one of the variacs? Sigh of relief if so! And perhaps keep moderator off my case.

Reducing the primary input voltage will reduce the secondary proportionally, but current capabilities SHOULD actually go up a bit as transformers are rated in heat dissipation ability (dissipation of heat out of windings). VA=power in watts. The VA rating stays the same, so as V dimminishes A (amps) can go up without exceeding the VA rating. This will only be true to an extent because the wire size remains constant and drawing too much current may still burn up a winding. (You can't draw 50 amps through an 18 gauge wire)


Doc

[Forman313]

Yes, I was talking about using a variac (as it is) to feed a normal fixed toroid. I cant see any drawbacks with that solution, only benefits.

About the current capabilities... I have always believed that only current dissipates as heat, not voltage. Assuming the current and resistance stays the same, the heat dissipation stays the same.. or so I thought.

Maybe it would have been easier to wrap my head around this electronics business if I had gone to school and got an education. hehe..

[thaumaturge]

Power in Watts = Voltage x Current in amps. Ohms law 101...

[kevinkr]

Originally Posted by thaumaturge

I never once advocated direct connection to mains. If you will read my posts I referred st splitting off -issolating- secondaries in all cases. Perhaps I presumed wrong, but common sense should dictate painting over bare copper areas of resultant secondaries.
Doc

Comments were directed at the OP..

[Forman313]

Originally Posted by kevinkr

Direct connection to the mains is hazardous and discussion of such ideas is grounds for closure on this forum. Your load MUST be completely isolated from the mains for safety, and a suitable transformer with center tap will allow you to get the X-0-X voltages you need.

You must use a conventional power transformer powered by one of your variacs to provide the voltages you need for experimentation.

I would recommend you check out Antek here: Antek - Your reliable source of transformers, power supplies, and more. - even with shipping to the EU their models may be competitive.

I dont get it...

I was just asking if it was possible to rewire the variac for use with single phase. How is that any different then asking how to wire any other transformer?
I never said anything about removing the isolation transformers