Let me offer a possible explanation from my limited knowedge of vacuum
tubes and mercury vapor rectifiers in particular.
In a mercury vapor rectifier, the voltage drop is fairly constant (10 to 15
volts) no matter what the current. The reason is that it takes an electron
of 10 to 15 eV to ionize the mercury atoms. The positively charged mercury
atoms then head towards the (negative) cathode, neutralizing the space
charge, making many more electrons immediatly available for conduction.
My guess is that something similar is happenning at the relay contacts -
the electron flow is space charge limited, the contact material that is
released by localized heating is then ionized (albeit by higher eV
electrons than with mercury, due to the properties of the contact
material), the space charge is neutralized by the ions, and constant
voltage conduction is sustained.
Don Borowski
Schweitzer Engineering Labs
Pullman, WA
"Pommerenke, David" <[email protected]>@majordomo.ieee.org on 05/15/2003
04:59:35 AM
Please respond to "Pommerenke, David" <[email protected]>
Sent by: [email protected]
To: <[email protected]>, <[email protected]>,
<[email protected]>
cc:
Subject: RE: HV relay
[snip]
If anyone knows a theory why there is about 25 V drop for a small distance,
medium current spark gap, please let me know.
David Pommerenke
University Missouri Rolla
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Wed 5/14/2003 5:38 PM
To: Pommerenke, David; [email protected]; [email protected]
Cc:
Subject: RE: HV relay
David,
thanks for the great information! This is the theory we're subscribing to.
The arc mechanism takes place at the negative electrode. I just tested a
mercury wetted coaxial relay, from 0 to 500 volts and it exhibits a 10
volt drop across the arc. It is not polarity sensitive, which surprised me
a bit. I expected a difference between electron emission from the reed
(tungsten?) and from the mercury pool. I guess that in this case the
mercury arc dominates things. I'll see what happens if I heat the relay to
raise the mercury pressure and ion mobility.
I will order the SF-6 filled relay that you recommend and give it a try. I
might experiment with contacts immersed in FC-40. The runt pulses are not
a show stopper but are an annoyance.
Dave Cuthbert
Micron Technology
-----Original Message-----
From: Pommerenke, David [mailto:[email protected]]
Sent: Wednesday, May 14, 2003 3:48 PM
To: [email protected]; [email protected]
Subject: RE: HV relay
Dear Dave,
You are working on a difficult issue.
Every HV relay will breakdown before the contacts touch. The reason is
as follows:
When the contacts come closer the surface field strength
Increases. The field strength will increase so much, such
that the field emission current reaches current densities
sufficient to melt the surface material. This melting
surface material will create a conductive path. The process
is called Explosive Electron Emission and takes place
in Vacuum and in gas filled relays.
Now there are some particularities for gas filled relays.
i the pressure of the gas is not that high, the field
eission or other processes might initiate a gas breakdown.
At least for high pressure relays and for Vacuum relays,
nrmally the breakdown is caused by surface explosions.
Now, after the initial breakdown and some current the arc might
extinguish again. This will show up as "bouncing" in the current pulse
and times of 50 to a few hundred nanoseconds are typically. There are
many influencing factors, some of them are:
- Speed of approach of the electrodes
- External current
- Gas filling
- Surface materials
After the contact meet, the contacts might bounce, leading to
interruptions or changes in the current in the us or ms range.
Regarding the "Vacuum breakdown strength". There is no
'Vacuum breakdown", it is all a surface process.
In general, I suggest to think about using N2-SF6 filled relays. The
HC-5 from Kilovac is a good choice up to about 10 kV, if mounted in
epoxy. You can reduce the drive current or shape the drive current to
reduce the problem of bouncing.
David Pommerenke
University Missouri Rolla
-----Original Message-----
From: [email protected] [mailto:[email protected]]
Sent: Wednesday, May 14, 2003 11:09 AM
To: [email protected]
Subject: HV relay
I'm designing some ESD test equipment for in-house use and need some
suggestions on relays. I am experimenting with a 5 kV COTO vacuum relay
and notice that I get some "runt" pulses. It appears that the contacts
arc before they touch and then the arc extinguishes after 50 ns and then
reignites. About half the time it the contacts appear to make physical
contact before arcing. If I remember correctly, the strength of a hard
vacuum is 200 kV/cm. So, at 5 kV the contacts can arc when they are 25
microns apart (actually further apart due to field enhancement). Are
there any vacuum relays that do not exhibit runt pulses?
Dave Cuthbert
Micron Technology
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