Re: electronic PWM variac

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> A few months back, someone mentioned that they had
> built an
> "electronic variac" using a bridge rectifier and a
> single PWM FET.

If this circuit works with inductive loads, then it's
only by sheer luck. There is no path for the current
to free-wheel when the FET is off, so you wind up
either destroying it, or having to use an oversized
snubber that wastes a lot of power to keep it alive.
It's better to use four FETs and make a synchronous
buck converter with bidirectional switches.

> A 555 can swing the
> voltage to turn
> most FETs on, but does it have the drive capability,
> and the 0-100%
> duty cycle you'd want?

The drive capability is a few hundred milliamps,
enough for small FETs. You can get 0-100% duty cycle
using the circuit with steering diodes. (See "Nomad's
555 Circuit" on the link below.)

http://www.cpemma.co.uk/pwm.html


> Has anyone tried to turn one of these into an
> "electronic ballast"
> that synthesizes the effect of series inductance
> into a SG coil?

I started down this road. But I soon realised that the
electronic ballast will never behave the same as an
iron one because it has nowhere to store energy.

Steve Conner