[Amps] Surge resistor

[Peter Chadwick]

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