On Sun, 15 Apr 2001 18:41:43 -0700 measures <2@vc.net> writes:
>
>>>
>>
>>Hi Rich,
>> And that is for why?
>
>? It doesn't take 500v to operate the gate. Mo' is not always mo'
>betta
>
>--
Hi Rich,
Hmmm, I think there is a bit of a misunderstanding here.
Indeed, the SCR gate may only require as little as a 1/4 volt to
trigger.
Let's see if I can describe a circuit with only 3 (important) nodes in
3 sentences...
The cathode of the zener string and the anode of the SCR are connected
to the tube's screen.
The anode of the zener string is connected to the gate of the SCR and
one end of a resistor; R.
The cathode of the SCR and the other end of the resistor, R, are
connected to ground.
A typical value for R may also be instructive... Let's say that the
zener string is operated at a maximum current of 100mA. Then, assuming
something like a 300V screen supply (and a 2N688 400V SCR), R is selected
to pass the zener current without exceeding the SCR's "must not trigger"
gate voltage specification: .25V. 2 ohms (at a half watt) would be a
good choice in this instance.
The maximum gate voltage will be .2 volts in normal operation.
In the event of an arc, the zener current will rise rapidly, and the
voltage developed across R will trigger the SCR (typically at around
350mA of zener current), protecting the zeners and the tube socket bypass
capacitor.
Incidently, the 2 ohm resistor will not significantly degrade the
already somewhat poor dynamic impedance of 300 volts worth of zeners
(rated for 100mA) which is in the vicinity of 100 ohms.
73 & Good night,
Marv WC6W
*
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