Rich says;
>... length helps because it lower the V gradient. Two 10w resistors in
>series are better than one 25w resistor.
Very true - I use one resistor in the present PSU, but it's about 6 inches long
and 1-1/2 inch diameter, though. (Over the years, I've collected physically big
vitreous enamelled resistors of 50 watts and up in various values) An important
point is the pulse rating, as there's a lot of energy to dissipate. For
instance, a 5 watt resistor 6 inches long may well not flash over because of
voltage gradient, but explode with the amount of energy involved.
About 35 years ago, when I was an apprentice (sound of violins playing!) I was
working on a 10kW output linear at 6.2GHz. (2 TWTs - 1mw in, 10watts out for the
first, and 10kW out the second) The PSU ran 18kV at 3 amps, and we had a crowbar
on it. I ran a lot of the tests on the crowbar; we dumped the energy into a 10
ohm carbon resistor about 15 inches long and 3 inches in diameter. After about
500 firings, the resistor exploded! After about 300 firings, you could see
sparks running up and down the resistor every time you fired the crowbar.
By the way, when I say exploded, I mean it. It was clamped with 1/8 inch thick
copper straps each end to some 1/2 inch thick fibreglas board. The explosion
tore one end, complete with strap, from the fibreglas board, and propelled it
straight through the side of the power supply cubicle, which was 1/8inch thick
hardboard (Presspahn in American, I think) and half way across the lab. That
supply only had a 24microfarad capacitor and choke input.....
My new linear supply will use a 50 ohm, 100 watt wirewound resistor about
1-1/2 inches in diameter and 12 inches long - I guess that should be OK for
voltage gradient.
73
Peter G3RZP
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