On Thu, 12 Apr 2001 09:13:55 +0100 "Steve Thompson" <firstname.lastname@example.org>
Hello Steve & the group,
I finally got a moment to read all this screen mail...
The original query was about how to protect a "string of zeners" shunt
One answer is with a SCR crowbar across the zeners, to which Ian
mentioned a reference (which I haven't seen) & then alluded to other
Triggering may be simply implemented by connecting the gate of the
crowbar SCR to a small value R in series with the bottom of the zener
string. So, in this case, the other circuitry is a single R.
Incidently, the rate of rise of the fault current should be in the
several microsecond range, allowing plenty of time for the SCR to
function and protect things, since, except for the small energy
contribution of the plate coupling cap., the risetime will be limited by
the L/R time constant of the plate choke and whatever is in series with
This will save the zeners, and (if present) the reallllllly hard to
change in-socket screen capacitor. To complete the protection,you'll
need to add a couple of (back to back) diodes across the screen meter and
shunt before your next ..."event".
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