Re: switching options for huge cap discharges (fwd) (Copied from Tesla List) (fwd)

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Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 14 Jul 2007 15:58:52 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: switching options for huge cap discharges (fwd) (Copied from Tesla
    List)

Hi Dave,

Per Schaefer and Kristiansen ("Gas Discharge Closing Switches"), the 
best performing materials for low rep-rate, ultra-high current spark 
gaps are brass, machinable copper-tungsten (Elkonite or similar), 
tungsten, or even pure copper. At 100 kA+ levels, brass or 
copper-tungsten are preferred. By using large area, relatively flat 
electrode surfaces, electrode life can be significantly extended. Large 
thermal mass and good thermal capacity are best for low duty cycle high 
current switches. Keeping discharge times relatively short and avoiding 
full damped ringing discharges (such as can crushing) will also help to 
extend electrode life.

I have personally used 2" dia. x 3/4" thick brass electrodes on my older 
trigatron switch, and also on a newer solenoid-driven switch. The former 
has worked in coin crushing service for over 6 years, while the new 
switch has been in service for over 2 years. The trigatron was retired 
after switching over 5,000 shots. Although the main electrodes showed 
relatively little wear wear, the small triggering pin (the center 
electrode from a modified spark plug) had worn down to the point where 
it could no longer reliably trigger the switch. This is the weak link 
and the usual failure mode for trigatrons. I converted to the 
electromechanical switch since it has a much larger operating range and 
it doesn't suffer from trigger failures or, worse yet, self-trigger for 
a 6.5 kJ "surprise"... :^)  The solenoid switch is very reliable and has 
required virtually no maintenance since it was put into service.

One downside of using brass electrodes is that the hot arc apparently 
evaporates small amounts of zinc from the surface of the brass alloy. 
This leaves an electrode surface that, under magnification, looks 
similar to a cracked riverbed, with networks of thin cracks surrounding 
little islands of conductive material. The vaporized zinc then oxidizes, 
slowly depositing a whitish film on the interior of the spark gap 
chamber. Although it detracts cosmetically, it doesn't seem to cause any 
other problems.

BTW, I used a hairpin current path between the bus bars and gap on both 
types of gaps in order to create a strong magnetic field that "sweeps" 
the plasma across the electrodes when the trigatron is fired. This helps 
spread the wear across more of the electrode surface to reduce melting 
and extend life. It's not clear if this helped to extend trigger 
electrode lifetime. In any event, here's what the solenoid-driven switch 
looks like:

http://www.capturedlightning.com/frames/gallery/newgap5a.jpg
http://www.capturedlightning.com/frames/Newgap2a.jpg

I'm also including an image of one of the electrodes from the retired 
trigatron. The center of this electrode is actually the end of a 1/2" - 
13 silicon bronze mounting bolt. While the tip of the bolt shows 
evidence of melting, the main brass electrode does not. You can easily 
see the dry riverbed pattern on the brass electrode.

http://www.capturedlightning.com/photos/HVStuff/Trigatron_Electrode.jpg

Unless you happen to already have some large pieces of tungsten or 
Elkonite laying around, I'd really recommend using plain old brass, 
since the performance seems to be perfectly fine for this application, 
it's considerably less expensive, much easier to work with, and 2" brass 
rounds are easy to locate.

Good luck and best regards,

Bert
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***************************************************
We specialize in UNIQUE items! Coins shrunk by huge
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Tesla list wrote:
> Hi all,
> 
> This may start to get a little border line OT but I was hoping to get some input
> on this. I am now in the process of trying to reconstruct my quarter shrinker
> cap assembly and the only thing that is really giving me a fit is how to safely 
> and reliably switch the 10s of thousands of amps from the discharge of a 15
> to 20 kJ capacitor bank. I've done this some before in the past but the huge
> currents systematically melted and destroyed the 1 1/4" square brass con-
> tact blocks after a few dozen shots. I noticed Ed Wingate used a drop switch 
> mechanism that utilized what looked like about 1/2" diameter tungsten rod con-
> tact points to switch the huge currents. http://www.tb3.com/tesla/ratcb2005/pages/IMG_3997.html
> What about triggered spark gap firing for this purpose? I'm sure that just
> as in a Tesla coil SG, tungsten is the superior metal of choice for quarter
> shrinker switch contacts as well. If anyone has experince along these lines
> and would like to share their experiences with me, I would be most grateful ;^)
> 
> David Rieben
> 
> 
>