RE>vacuum switchs (fwd)

---------- Forwarded message ----------
Date: 31 Jul 1998 13:37:20 -0700
From: Dale Hall <Dale.Hall-at-trw-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: RE>vacuum switchs (fwd)

RE>vacuum switchs (fwd)


A vacuum is a perfect insulator, good for HV, bad for heat removal. 
However here on Earth it needs housed in imperfect glass, ceramic, etc.

I make extensive use of Jennings, Kilovac, others, Vacuum Switches (VS).
I use them for low break rate application only usually but not exclusively DC.
Break rate = ~2 per sec or less.

The worst are those whose glass path between the electrodes is short.
They quickly become resistive due to deposited metal (tungsten) on glass.
At hi current if the electrodes are switched so they make contact, they'll weld.
If you can vary the voltage until spontaneous firing occurs they work best.
In this mode you'll need to operate them well above their rated voltage unless
some stimulation is used .
Expect erratic firing (not much emission in vacuum - though a radioactive
stimulus built inside is one solution used by EG&G vacuum switches).
As they heat thermionic emission encourages firing to become more regular.

Best are those with long glass paths with a metal cup surrounding the gap.

Stimulated firing:
To avoid welding the electrodes at the high peak discharge currents I provide
a narrow pulse to the coil to merely jog the armature to encourage arc-over. 
(physical contact is not made) 
I typically use .1uF Cpri and scope Ipri ~600A.
Those with a metal diaphragm armature I am able to mechanically position
varying the breakdown voltage required.

VS's are not optimum for high break rates due to the metal deposition problem
and heat removal and they are very expensive though often OK -at-swapmeets.
Advantages:  excellent quench, and they are very quiet.

What model do you have ?  
Life expectancy is related to length of the glass path of a particular model
and the break rate. Vacuum switches are designed for resistive or "power
off" HV switching not the harshness of high capacitance discharge.

Sealing gaps does bind up oxygen but then heat removal and cleaning 
becomes more difficult.

Dale   Redondo Beach, Calif
Date: 7/30/98 8:49 PM
To: Hall, Dale
From: List, Tesla

Date: Wed, 29 Jul 1998 22:05:35 -0500
From: Paul and Stephanie Miller <smiller-at-rconnect-dot-com>
Subject: vacuum switchs

I have a vacuum type high voltage switch that I beleive was made by
Jennings Radio. Has anyone tried useing one of these for a spark gap ? 
and if so what is the life expectancy of one?
 another thought, has any one tried putting their spark gap in a housing 
and presureizing it with some type of gas like nitrogen or sf-6?

  Paul Miller