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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. 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. 

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.

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.

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)