[AMPS] Zener screen supplies

[measures]

>
>
>On Sun, 15 Apr 2001 18:41:43 -0700 measures <2@vc.net> writes:
>>
>>>>
>>>
>>>Hi Rich,
>>>   And that is for why?   
>>
>>Ä  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.

ok

>
>   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.
>
ok

>  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.
>
When the screen arcs to the anode, and the SCR has not yet fired, we have 
several kV on one end of the 2-ohm R.    My thought is that c. 1500A 
might damage the gate.  

cheers, Marv
\
thanks
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-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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