[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: Report - Dimmer as a Variac.



Original poster: "David Sharpe by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sccr4us-at-erols-dot-com>

Hi Malcolm!

I've been discussing ideas off-line with Terry F.  You are correct in basic
concept.  I have a circuit that I'm kicking around, only sketches at this
point...
but looks very good on paper.  Will model and build a relatively low power
unit (1kVA) to see if it will work.  A full bridge is a good way to get
high power
but I think saturation effects will eat your (and the transformers) lunch
at high
power.  Plus IDK how robust a FB topology will be to phase swings, kickback
etc. etc.  Although Marco D. on the  THOR project had good success; in fact
the IGBT drive circuitry will be useful in the circuit outline below...

The topology I'm contemplating can best be described as a
bi-directional buck regulator (BDBR), where the transformer primary is in
fact the
inductive storage element, and circuit operation approaches that of a forward
converter. Modulation frequency in the 10-20kHz range, then
I can use almost anything (BJT, IGBT's, FET's).  Input sine wave is simply
PWM chopped and the transformer gleefully rebuilds in an adjustable amplitude
AC power voltage with no windings, brushes or motors to deal with...  :^)
Add some high speed over current protection, soft start and voltage
adjustment and
you have something a variac could NEVER approach.  Controls for positive
and negative alternation synchronization, and PWM controls are tricky and
there is not a commercial IC out there to play this game.  May be a
"Junk Yard Wars" combo of 555's, Op-Amps, SCR's and discrete components.

I've got about 80% of a 40A/120V power stage built on a 14"x14" sub plate.
All I need is a few more parts and breadboard the control stages and I'm set.
Will power through a variac at low power first while monitoring happenings with
a scope, then stand on it and see if anything goes "POOF".

Regards

Dave Sharpe, TCBOR
Chesterfield, VA. USA

Tesla list wrote:

> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
>
> Hi Dave,
>          The sort of approach I'd use would be offline active PFC
> followed by a PWM H-bridge directly feeding the load. Cycle by cycle
> current sensing/control would be a must. The PWM would of course be
> set by finger tip control (a crummy old carbon pot :). The above
> approach (together with the appropriate common-mode filters) should
> address the issues you raise below. Some differential filtering of
> the H-bridge output may be necessary - mandatory for motors to avoid
> micro-machining the shaft, but probably not necessary for running a
> NST (self-filtering). Am I reading your post correctly?
>
> Regards,
> Malcolm
>

<<SNIP>>