Saturday, September 23, 2017

Electronic "variac" / Variable AC power supply

To be fair I was looking for a real autotransformer, since I could not find one at a decent price or in stock on the local electric material shop, decided to build one electronic version that would do the job.
The need is just a way of testing old mains powered electronics devices with increasing input voltage. Another way of doing the same would be with a serial light bulb, in any case that's not "elegant" enough.

Anyway, here's the outcome, basically a shunt regulator between two AC transformers. Circuit is not mine (original from here) I only gave a little adaptation based on experimenting and what I had available.

 Some output values:

Here's the box:
I ended up not placing the voltmeter in the pictures since it would be a very close fit to the box so placed just a small battery indicator, 300uA with rectifier diode and 180K series resistor. The dashes mark 50, 100 and 150V AC.

The transformers were from salvaged electronics. Still visible leftovers from experimenting with other circuit.

Some more testing:

Based on the design and the components used, on a 15W load I cannot output more than 170V, that is enough for now, if I need the real "deal" then would for sure buy an autotransformer with more capacity.

The original schematic from I2VIU didn't gave full regulation from 0 to 220 so I made some small changes (also due to the fact of using different transformer values:
My schematic: (changes are with white background)

On the original page there is one schematic with regulation by feedback and with a comparator, I tried that design but was not having full voltage swing and used an extra transformer.
Here's the one that was tried but didn't suit.

One fact, there is not many AC variable power supplies schematics available on the Internet.

Hope this is useful to someone.

 Have a great day!

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