On Sat, 28 Aug 2004 15:51:42 GMT,
(James Lerch) wrote:


Very impressive - what drive system are you using and what
do you use for tank capacitors?

Jim

Jim,

I'm using a TL494 for the frequency and duty cycle generator. The
output of the frequency generator feeds a pair of Mosfet gate drive
chips, thru a 1:1 transformer to the gates on the 4 corners of the
h-bridge.

My first experiments didn't use tank capacitors, but I found I HAD to
have them to heat the graphite efficiently. Currently I'm using 12x
0.1mf caps in parallel as the tack capacitor, but this is a problem.
The capacitors can't handle the REALLY HIGH ripple current (100 amps
or more?) and I keep melting the legs off.

I'm currently on the hunt for a nice water cooled 2mf cap, but not
having much luck. Most of the ones I'm finding or for high voltage,
not high ripple current.


Take Care,
James Lerch


Sounds pretty good but I'd follow your present
route and use multiple paralleled small capacitors - the
extra surface area is useful and, if you're using extended
foil capacitors, a fin on each leadout wire helps to get rid
of the heat.

I expect you've already researched this but the breed of
extended foil polypropylene capacitors used for snubber duty
is ideal for this duty. The extended foil results in a much
lower series R than with metallised film, and the loss
tangent of polypropylene is less than 1/10th of that of
polyester or polycarbonate.

Have a look at
http://www.wima.com/product.htm
http://www.wima.com/navig/menue/prsu...hcr/forhcr.htm

They're a European outfit but I think Intertechnical Group
inc is a USA stockist.

Jim,