On Dec 28, 2005, at 9:33 AM, Vic K2VCO wrote:
> Bill Turner wrote:
>> I have a question for all the amplifier experts out there.
>>
>> In this schematic:
>>
>> http://members.ispwest.com/dezrat1242/cathode.jpg
>>
>> there is a 250 ohm resistor connected across the fuse/zener. What is
>> the purpose of that resistor, and why is that particular value
>> chosen? (I have also seen 200 ohms used.)
>
> My non-expert guess: if the fuse blows, it prevents the filament
> transformer secondary from rising to a very high positive voltage,
The problem is negative potential.
> possibly exceeding it's insulation rating.
The fly in the pie is that 250v fuses don't simply open in 3kV circuit,
during a glitch they create a metal vapour arc that does not limit
current until after they explode. Thus, during a glitch, the cathode
can rise to c. 2900v negative. Using a component at 10-times its max V
rating is probably not good engineering practice.
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
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>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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