Re: Scopes Away! - mosfet

To: tesla-at-pupman-dot-com
Subject: Re: Scopes Away! - mosfet
From: Richard Hull <hullr-at-whitlock-dot-com>
Date: Tue, 17 Sep 1996 13:17:53 -0700
Tesla List wrote:
> 
> >From 100624.504-at-compuserve-dot-comMon Sep 16 22:13:36 1996
> Date: 14 Sep 96 17:01:55 EDT
> From: Alan Sharp <100624.504-at-compuserve-dot-com>
> To: Tesla List <tesla-at-poodle.pupman-dot-com>
> Subject: Re: Scopes Away! - mosfet
> 
> Greetings all,
> 
> >When all the smoke clears and theoretical stuff is
> >pushed asside, when the wishes, hopes, and dreams for the future are
> >relegated to that future,  FETs, IGBTs and all the other stuff of this
> >century is pretty pathetic when it comes to big power at high voltages.
> >The voilence contained a flaming air arc is still the best, low cost,
> >power switch on the planet over 2500 volts and 600 amps.
> 
> Agreed, for raw power - nothing beats a spark gap. So what have mosfets
> got going for them?
> 
> First of all the power gap is not a great as you might expect - with about
> 500watts
> of power I can get a 14" brilliant white arc - I can reasonably expect to double
> this when
> I get to 1200W. 

I would be amzed if this happens.  See a recent post to Malcolm.  R. Hull

The FET's are only switching 140v at 10A (peak) but they
> do it at the resonance frequency. Mosfet drivers are very efficent in turning
> electricity
> into spark output. But they cannot match the peak power of a spark gap - to give
> the very
> high peaks and long sparks I wonder how close I can get though - I could
> probably double
> the peak output - my MOSFET's hardly get warm at all just now.

You have disocvered the secret of using mosfets.  They must be slammed on 
hard and spend zero time in their linear mode of operation. (not too easy 
to do in some cases.  R. Hull
> 
> 2 mosfets can do it all day - I limit my runs to a few minutes because of the
> ozone
> build up.
> 
> 3 they offer very precise control and measurement.

This is why they are so tempting to me as well.  R. Hull

> 
> 4 it should be possible to experiment with arrays of coils -
> all operating at a different phase of the same frequency -
> or at harmonic frequencies.
> 
> I've got some surplus transformers to give me 180v at 1200W.
> I'll use six IRF740's on each side of my push pull design (I
> know a bridge is more efficent but I'll stick with what I have
> got working), and see where I get to.

The IGBTS are a much better switch (being transistors) but I can't find 
anyone who can actually deliver the big power jobs.  These will probably 
be better for switching in the distant future than FETs.  I use power 
MOSFETS in a lot of robotics applications and find that they are the best 
all around semiconducting device as they can be made to do almost 
anything you want, like and old vacuum tube.  R. Hull
> 
> But having said all that I reckon that one of these days -
> I'll get a pole pig, some beer bottles and a rotary gap and
> try for the 10' arcs!
> 
> Have fun,
> 
> Alan Sharp (UK)