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Thoughts on Variac Power Supply

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Hi,

conceptually what would you guys do?

I have recently acquired a three phase variac for free that I wish to turn into a variable bench power supply. The phases are mechanically coupled and cannot be uncoupled. Each winding is capable of 240V @ 2A. The power here is 220V.

I was looking at options like to switch in as many phases as required in order to increase the output current. Or perhaps to use each phase with independent circuits.

I don't make many circuits and they are mostly low voltage low current applications. The pain is always to find a suitable power source when required. As the variac is free it would be nice to make full use of it and to put it to good use.

Any thoughts or am I beating a dead horse pursuing this one.

Cheers
Andrew
 

ericgibbs

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hi,
I expect you know already, but just to be sure,, a standard variac is NOT mains isolated.:)

I would use a 220V to 24V step down transformer on the output of the variac.
 

ericgibbs

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Hi Eric,

the 24V transformer, would that be to provide the isolation or would you have another reason?

Thanks
Andrew

hi,
As well as providing the required isolation.

Each phase of the variac is say 240V at 2A, 480VA
So using a 240/24Vac transformer that had a primary rating of 2Amps would mean the secondary could be rated at 20Amps at 24Vac.

If the variac was adjusted down to say 120Vac out, the secondary would drop to 12Vac with a 20A capability.

Rectify the secondary output with 20A diodes and a large electrolytic, it would ideal for a variable/unregulated bench supply.
Or battery charger.
 
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crutschow

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If you use a transformer with a center tapped output you can use a bridge rectifier to give a tracking plus and minus output voltage. You ground the center tap for the circuit common. The two bridge rectifiers with the common cathode provide the positive output and the two rectifiers with the common anode provide the negative output.
 

crutschow

Well-Known Member
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I have seen a commercial power supply which used a variac unregulated supply to feed a tracking linear regulator. This keeps the voltage drop across the linear regulator constant with output voltage, minimizing the dissipation in the linear regulator series pass transistor. This gave an efficiency comparable to a switching supply. It had a pot ganged to the variac shaft to adjust the linear regulator so its output tracked a few volts below the variac rectified output.
 
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