Here's an old topic that's come up many times, but I'm still not sure of the
best answer. In my homebrew amp that runs 4500V with a voltage doubler power
supply, I have two series strings of rectifier diodes. I had a flashover a
couple of days ago which took out several diodes in each string. (It never
should have happened A grounded solder lug was touching the enamel coating
on a 100K/100W bleeder resistor and arced through the coating. Evidently,
when I tightened the screw on the solder lug, it rotated around and touched
the bleeder, and I didn't notice it. It blew the bleeder in half and sounded
like a shotgun blast.)
I built the diode strings in the 1970s. Each one used eight HEP-170 diodes
(remember those?), each shunted by a .01uF cap and a 470K resistor. I'm
replacing the diodes with 1N5553 diodes. These are mil spec glass
encapsulated diodes rated at 3A/800PIV, with 2uA reverse current at rated
PIV. (By comparison, 1N5408s are rated at 500uA reverse current.)
My question is whether I need to shunt these high quality diodes the same
way I did the old HEP-170s? I'm aware of the reasons for shunting, e;g;
using caps to distribute switching transients, resistors to equalize the
reverse voltage distribution. But is it really necessary with modern,
controlled avalanche diodes, presumably made to tight tolerances? For space
reasons, I'd prefer not to shunt them unless it's really necessary.
I've looked at some modern commercial amplifier schematics, and I find no
consistent approach. Some use resistor shunts only, some use capacitor
shunts only. Those that use molded diode blocks don't use any shunts. So I'm
interested in knowing the latest thinking on this subject. Thanks!
73,
Jim W8ZR
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