[Amps] HV fuse

[R.Measures]

On Jul 19, 2004, at 9:27 AM, Traian wrote:

>
>
> "Ian White, G3SEK" wrote:
>
>> Oleg Skydan wrote:
>>> OK. Consider I have a 40 uF capacitor in the PS charged to 3000V
>>> (one that uses in the pulse laser PS). Will it be safe for electronic
>>> crowbar to discharge it ?
>>>
>> Good call - you have to think about the maximum current that the 
>> crowbar
>> will have to handle. The OE5JFL circuit is quite carefully designed to
>> limit that peak current, using a combination of a small inductor to
>> limit the rate of rise, and also a 10R sense resistor. (The inductor 
>> is
>> only 30ft of wire, random-wound on a bobbin, but it makes a very
>> significant difference. It also means that the 10R sense resistor
>> doesn't have to be built like a conventional glitch resistor.)
>>
>> Bottom line: if you're going to copy OE5JFL's circuit, copy *all* of 
>> it.
>>
>> http://www.qsl.net/oe5jfl/flashover.htm
>
> The Harris RF-110A uses a similar circuit for protecting the screen 
> circuit
> from tube flash over discharge (R series resistor at the bottom of the
> screen's
> regulating string of zener which act a SCR; all of the tubes screens - 
> two
> 8122 drivers and two 4CX1500B finals- are OR connected by diode at
> the SCR anode). It saved me the entire power zener string once, when
> I had to verify a 4CX1500B (the tube was verified ok at hi pot before,
> but the big bang occurred anyway and destroyed the RF124's PS HV fuse).
>
When a fuse is destroyed during a fault, it failed to do it's job of 
interrupting current flow without creating a metal vapor arc.  Thus, a 
more capable fuse is needed.

AG6K