Re: interrupter

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Mike,

If you're dealing with a high energy system, I'd recommend using a high
power triggered spark gap as the crowbar switching element. If ringing
is a concern, you could add high current damping resistors in series
with the crowbar current path (i.e., paralleled runs of nichrome
resistance wire) to critically damp the system. This approach is
inexpensive, very robust, and the HV switch will have harder "clamping"
action than an ignitron or thyratron. Knowing the expected
operating/standoff voltage, size of the energy storage capacitance (bank
energy), and estimated crowbar loop inductance will allow you to size
the spark gap and determine the minimum damping resistance. 

BTW, if power supply follow-through current is a problem, you can
temporarily interrupt mains power, extinguish the crowbar gap, then
reapply power as part of the crowbar firing control sequence. If your
peak power is at a level that might explode an ignitron, it's likely to
create a similar problem in a Wehnelt Cell from hydrodynamic shock
transmitted from the high energy spark to the container through the
intervening liquid.  

Sounds like an interesting challenge!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Mike Nolley by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<nolleym-at-willamette.edu>
> 
>     This is a question for anyone who might know specifics about the
> Wehnelt interrupter.  The Wehnelt cell was used
> as an interrupter in early spark discharge circuits.  A small platinum
> electrode interrupts the circuit when gas
> formed through electrolysis forms around it.
>     I am in the process of designing a crowbar circuit for a pulse
> discharge application, and I was wondering if the
> Wehnelt cell might be useable in place of the ignitron.  I am concerned
> about finding a reasonable price for the
> ignitron, and the potential for mercury contamination if it explodes.
> Solid state crowbars at this voltage would also
> be too expensive.  My idea was to put a Wehnelt cells in series with the
> discharge path so that it would interrupt the
> pulse after a few 10's of  microseconds.  I need a short pulse that will
> eliminate ringing between the pulse discharge
> capacitors and an inductor.  I am hoping that since the pulse will be large
> the cell won't re-connect before the pulse
> is over.  Also, since the cell acts as sort of a point-contact rectifier,
> no ringing will occur.  My questions are:
> is the Wehnelt cell capable of interrupting a large DC pulse like this,
> (without destroying the electrodes, container,
> and splashing sulfuric acid around) and/or: if it won't work, is there a
> type of crowbar circuit which might work
> instead?
>             --Mike