On Wed, 30 Jan 2002 17:40:50 -0000 "Steve Thompson" <g8gsq@qsl.net>
writes:
<snip>
Hi Steve,
> I'm going to have to build this up and hook the storage 'scope on!
> The way I worked it was to assume 2kVish arcing to screen limited to
about 40A
> by a glitch resistor. This gives a time constant in the region of a
couple of ms.
> At the screen, the source impedance is quite low if there's 40A flowing
and,
> given that we want to keep the screen voltage stable at audio
frequencies,
> we should have a low impedance path to the regulator for a pulse of 2ms
> upwards so I was expecting a lot of the 600V to arrive there with
plenty of
> current capacity, in which case the screen supply cap could end up with
a
> significant top up. If there's any significant capacitance hanging on
the
> gate/base of the series device, or if the control circuit tries to pull
the
> gate/base down to try and regain regulation even 10's V could be enough
to
> cause breakdown.
The screen bypass caps would necessarily have to be rated to survive
whatever your MOV's will allow surgewise.
The pass FET should be OK in this instance. You might want to add a
small R in series with the FET drain to make the time constant long
enough back into the screen supply cap (presumably 10-20uF) to side-step
the surge.
Incidentally, I typically solder a small 10V zener directly on my
FET's (gate-source) to protect them when handling / prototyping. This is
good safety measure in any instance.
> Maybe I'm being pessimistic, and these problems don't occur in
practice.
Frequently, the unforeseen occurs in practice! :-)
> I'd be delighted to know I'm being over paranoid but I don't know as
I've not
> come across any reports that such circuits did (or didn't) survive
> anode/screen flashovers. Shunt circuits by G3SEK and GW4FRX et al
> are known to survive repeatedly.
When in doubt, add a SCR crowbar -- protects your screen caps,
regulator, etc.
73,
Marv WC6W
*
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