Dimmer instead of variac?

[Doug Kim]

Hi all

I started my new SET project, and as I always did before, I want to use variac when I test the amp.
But I couldn't find any local store that sells variacs. Instead, Home Depot or Lowes sells dimmer that works up to 300-500 watts.
Has anyone tried using a dimmer to slowly power up their electronics (not lights)?

Thanks

Doug

[jrockhead]

Hi Doug, do not under any circumstances use a light dimmer to input voltage to a device with a transformer. They will overheat badly and possibly burn.

Dimmers do not output a sine wave, it chops the sine wave into pieces that light bulbs work fine with.

Try the old light bulb in series with one of the AC lines trick to fire up a new amp.

[star882]

A motor speed control designed for induction motors (such as ceiling fan speed controls) or a dimmer designed for transformered halogen lighting should work. The problem is not so much the chopped waveform but rather waveform asymmetry causing saturation.

I once used a UPS transformer connected through a ceiling fan speed control as an adjustable high current, low voltage power supply (forgot what I used it for, probably scoring/cutting clear plastic with a hot wire). Worked just fine.

[grayson]

better yet, search ebay for "variac". handy tool to have. lots of used ones out there for cheap.

[Doug Kim]

Thanks all for inputs.

The light bulb idea sounds very interesting. So is it connecting a few 115V incandescent light bulbs in series with an amp so that only a fraction of AC volts can be fed into the amp?

Doug

[sreten]

Originally posted by Doug Kim
Thanks all for inputs.

The light bulb idea sounds very interesting. So is it connecting a few 115V incandescent light bulbs in series with an amp so that only a fraction of AC volts can be fed into the amp?

Doug

Hi, Yes. The lower the bulb wattage, the lower the voltage fed through, /sreten.

[richie00boy]

You only need one bulb.

What happens is under normal idle conditions not much current is drawn from the supply so the bulb is cold and consequently not much voltage is dropped across the bulb. When a fault occurs it usually pulls a high current, which makes the bulb heat up and increase it's resistance thereby strangling the voltage supplied to the connected device and protecting it against excessive current draw. It also provides a handy visual indication of a fault so you can shut off the supply quickly.