Peter Chadwick wrote:
>
>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.
>
Assuming that the arc strikes extremely rapidly, the rise time is going
to be of the order of RC, where R is the glitch resistor and C is the
bypass capacitance. That comes out to a few tens of nanoseconds.
>
>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.
>
That raises the interesting question of how to turn the screen supply
on...
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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