I know this probably doesn't apply to the 10-50 Ohm series R in the plate
lead of a PA, but we recently redesigned our series R from our stored
energy banks for the 3 MW amplifiers. Used to use a 5 Ohm series wirewound
(about 50 watter) in series with every 4.6 uF 40 KV capacitor. That's about
50 caps in parallel OK?
Then, in 1992 a cap shorted and took out every stinking resistor. The wire
literally exploded off the cores, and there was nothing but broken porcelan
and melted wire, after the smoke cleared. Of course the cap took a slug of
energy from the rest, and sort of exploded, taking out it's nearby
neighbors. I believe that the cases are rated for about 3KJ of energy fault
without rupture. The resistors, well the windings all arced over, and
tracked. They did not blow open quick enough to stop the tremendous fault
current in the network.
Last year, we redesigned the bank to use Cesiwid "globars", long ones, to
each C, and the common end of all the resistors go to a big copper bus,
well insulated. This goes through a few Ohms made in a NON INDUCTIVE
wirewound thing made from Nichrome wire. It uses an "Artron-Perry" winding,
where one helix is wound over the other, in opposite sense. The winding can
be found in most old copies of Terman's books. We also incorporated a
special fusable link, that blows out, and has a large spring wire to clear
the arc. THis is on every capacitor Hot post, between it and the resistor.
You can read about this installation in the proceedings of the 1997
Particle Accelerator Conference, IEEE Nuclear Transactions "Development of
a High Speed Crowbar for LANSCE", by C. Friedrichs, J. T. M. Lyles, J. M.
Doub of Los Alamos National Laboratory
This technique has worked so far, in simulated events and one real failure.
The thing to watch for, is that if you DO use a wirewound series R in the
plate lead of your amp, be aware of the change in dampening of the
resonance in that supply. L and C work to make a resonant circuit at some
low frequency. Also, during a fault, an inductive R may cause a damped
ringing with overshoot on the plate V. Something to consider in your
design.
ALso, the wirewounds tend to arc over and become a shorted mass of melted
metal in higher voltage systems such as this one. So we go with the
Carborundum (Cesiwid) types usually.
Oh, by the way, the original series R from the commmon point of the R's
from the caps was a thing called an Ohmweve. They are inductive, but are
used in MW transmitter dummy loads up to a few MHz. WE call them toaster
elements, or bug zappers. But in reality, a large power low value R can be
made in a compact space, and immersed in oil for HV rating and cooling. We
use them a lot.
John
K5PRO
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