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Re: ignitron tubes
>From bigr-at-teleport-dot-comTue Jun 25 22:01:57 1996
>Date: Tue, 25 Jun 1996 13:29:26 -0700
>From: bigr-at-teleport-dot-com
>To: tesla-at-pupman-dot-com
>Subject: Re: ignitron tubes
>Someone had asked several weeks ago about the identity of some mystery
>tubes. They were found to be thyratrons, but the normal mercury vapor
>type. However, as I said, I understand that there are "hydrogen"
>thyratrons that are very fast and would be suitable for Tesla coil gap
>replacement. (although I wouldn't want to use one that had 02 leaking
>into it, ha) But, I've never seen any on the surplus market- nor do I
>have any specs for any- were probably only used for exotic gov
>projects back during the Cold War. Rob.
Right you are.
I (along with quite a few other people) have used small ones of
these devices for triggering 5KV pulses to a conductance cell for
pulse radiolysis experiments on chemicals, doing basic research on
electrons and electrical conduction in liquids under X-ray
radiation. In fact I designed the circuit (back in the last 60's)
for the experiment. You need one tube to fire the circuit on and
another to fire it off. The "off" thyratron (bottom end goes to
ground) is capacitively coupled via the "off" capacitor to the DC
supply downstream of a ballast resistor or inductor. The "on"
thyratron is in series from this point to the load. This "off"
capacitor has as discharge resistor around it which would only pass
a *smaller* DC current than the "keep alive" current for the "off"
thyratron. So when the "off" thyratron fires, the "off" capacitor
charges up sucking all the current away from the "on" thyratron
(remember the ballast in series with the supply HV), producing a
temporary reverse-voltage spike across the "on" thyratron thus
shutting if off quickly. Then the "off" thyratron eventually turns
itself off as the "off" capacitor reaches the supply voltage and
its charging current goes to zero. The leakage current caused by
the discharge resistor around this "off" capacitor is not enough
current to keep the "off" thyratron conducting, and so the "off"
thyratron eventually shuts off and the total current draw goes
right to zero. The load sees the HV snap on and snap off. The
duty cycle is anything we want it to be - the HV can be held on for
several seconds or just a fraction of a second. It all depends on
the timing between the pulse to turn the "on" thyratron on and the
pulse to turn the "off' thyratron off. What we have here is a
thyrotron bistable circuit. The circuit is quite reliable and the
rep rate depends on the discharging time of the "off" capacitor.
For really fast repetition rates, a more positive circuit has to be
designed to discharge the "off" capacitor quickly after the "off"
thyratron has stopped conducting. I imagine you could get a few
kilohertz out of such a circuit. The peak current handled by the
thyratrons (depending on the rep rate) at turn on may be several
amps (foggy memory here) but the DC max current is definitely NOT
very high, probably only milliamps.
25 years later, on TV I recognized these same tubes as being part
of the body of evidence that the USA used to prove that IRAQ (?)
was trying to build a nuclear weapon. Apparently these little
thyratrons are used in the trigger circuit. The TV program
suggested there was no other use for these little devices, but I
certainly know that's not true. But I also doubt Iraq was doing
conductance-cell research.
Fred W. Bach , Operations Group | Internet: music-at-triumf.ca
TRIUMF (TRI-University Meson Facility) | Voice: 604-222-1047 loc 6327/7333
4004 WESBROOK MALL, UBC CAMPUS | FAX: 604-222-1074
University of British Columbia, Vancouver, B.C., CANADA V6T 2A3
"Accuracy is important. Details can mean the difference between life & death."
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