Over 40 years ago, as an apprentice, I worked on a TWT ground station
transmitter where we had a crowbar on the high voltage supply. By K5PRO
standards, it would be pretty small, being only (!) 22kV at 3 Amps. The power
supply was a three phase bridge, with an input choke which weighed 2000 lbs,
and a 24 microfarad 32kV capacitor. That itself was quite large - 3 feet high 2
feet wide and 1 foot deep. The glitch resistor feeding the crowbar was a 10 ohm
carbon resistor, 3 inches in diameter and 15 inches long, weighing something
like 3 pounds, and supported on a 1/4 inch thick piece of fibreglass by 14SWG
(about 12AWG), 1.5 inch wide copper strap.
Firing the crowbar got the volts down in under a millisecond. After about 100
firings, sparks could be seen running along the surface of the resistor. At
firing 352 (I was doing these tests, so I well remember), there was an enormous
bang. The resistor shattered, pulled part of the fibreglass away, and shot out
of the enclosure, coming straight through the hardboard (Presspahn?) and over
the top of the Tektronix 545 scope in front of me. It nearly hit the training
officer who had come to see me, and he was convinced I'd done it on purpose.
(Who, me? Would I?) However, the point of this is that glitch resistors have a
hard life when they have to do their duty, and I'm not totally convinced that
they can be over-rated . A lot depends on how often they have to limit glitch
current.
73
Peter G3RZP
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