On Jul 19, 2004, at 9:27 AM, Traian wrote:
"Ian White, G3SEK" wrote:
Oleg Skydan wrote:
OK. Consider I have a 40 uF capacitor in the PS charged to 3000V
(one that uses in the pulse laser PS). Will it be safe for electronic
crowbar to discharge it ?
Good call - you have to think about the maximum current that the
crowbar
will have to handle. The OE5JFL circuit is quite carefully designed to
limit that peak current, using a combination of a small inductor to
limit the rate of rise, and also a 10R sense resistor. (The inductor
is
only 30ft of wire, random-wound on a bobbin, but it makes a very
significant difference. It also means that the 10R sense resistor
doesn't have to be built like a conventional glitch resistor.)
Bottom line: if you're going to copy OE5JFL's circuit, copy *all* of
it.
http://www.qsl.net/oe5jfl/flashover.htm
The Harris RF-110A uses a similar circuit for protecting the screen
circuit
from tube flash over discharge (R series resistor at the bottom of the
screen's
regulating string of zener which act a SCR; all of the tubes screens -
two
8122 drivers and two 4CX1500B finals- are OR connected by diode at
the SCR anode). It saved me the entire power zener string once, when
I had to verify a 4CX1500B (the tube was verified ok at hi pot before,
but the big bang occurred anyway and destroyed the RF124's PS HV fuse).
When a fuse is destroyed during a fault, it failed to do it's job of
interrupting current flow without creating a metal vapor arc. Thus, a
more capable fuse is needed.
AG6K
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