One point about the spark gap surge arrestors is that the breakdown voltage is
dependent upon the rise time of the applied voltage. For example, the 90volt
Siemens types used for protecting telephone lines (and receiver front ends!) do
break over at 90 volts on a slowly rising waveform, while with a 1kV/microsecond
ramp, they fire at 1kV.
How long does the excess voltage have to exist across the in-socket capacitor
before it goes? Intuitively, nanoseconds seems unlikely (and across 1000pF or
so, it takes a lot of doing to get nanosecond rise times) while tens of
milliseconds appears pretty certain to cause breakdown.
Another way of using the SCR is to have a somewhat larger sense resistance in
the zener string, a resistance in series with the gate, and a capacitor between
gate and anode. This allows the rapidly rising anode voltage to pull the gate
positive, before the zener current has increased very far. Of course, SCRs can
turn on with no gate drive at all if the dV/dt of the voltage applied across
anode - cathode is high enough.
73
Peter G3RZP
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