Re: Vacuum tube tank cap vs. transistor fast diode

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

Tesla list wrote:
> 
> Original poster: "Craig Fuller by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <cbfull-at-hotmail-dot-com>
> 
> I came across an electical engineer's website the other day which said that
> you should always put a fast recovery diode across the primary in a flyback
> circuit, to reduce the stress on the switching device (for single transistor
> feedback circuits only, I think).  I have always wondered myself why people
> have not been doing this.

	If I understand what you're saying, that won't work at all!  The high
voltage comes from the voltage induced (L x di/dt) by the rapidly
collapsing magnetic field when the primary circuit is opened by the
switch.  A reversed shunt diode would short all that current and the
voltage wouldn't rise at all.  If the diode were in the other polarity
all of the current from the switch would try to go through it and
something would probably fry pretty quickly; certainly not much current
would flow through the primary and not much would happen when the switch
opened.

> Also, the vacuum tube coil circuits are not that different from the flyback
> circuits, and I am hoping (when I can get a workshop) to get my VTTC working
> with a fast diode instead of a tank cap.  Anyone have any experience with
> this or already tried it?  It sure would be nice to do away with that
> de-tuning problem in VTTC's whenever you put any kind of load on it(i.e.
> drawing a spark).

	Same comment.  A diode isn't a capacitor!

Ed