>
>Rich Measures wrote:
>>? In order to protect an 8877 grid from gold sputtering during an
>>intermittent UHF oscillation condition, the grid trip circuit should
>>probably be capable of interrupting 40 amperes. Can the Triode Board do
>>this, Ian?
>
>I don't want to get into the debate about what causes this gold
>sputtering - for the purposes of power supply design, it's enough to
>know that large current surges can and do occur.
>
>We also agree that a "glitch" resistor in the HV+ supply is a MUST, to
>limit the maximum possible surge current to 40A as recommended by Eimac.
>
>So here's our starting-point: there's a big current surge coming along,
>maybe peaking up to 40A - and what is the control circuit going to do
>about it?
>
>It isn't the grid trip circuit's job to interrupt that 40A current surge
>(or even to detect it). The surge comes from the HV supply, and that's
>where it has to be detected and interrupted.
>
^This would require a 100w / 100-ohm glitch resistor.
>The Triode Board continuously monitors the current in the HV-minus
>return line, and it provides a control output that switches within <1ms
>of going over the pre-set current limit. The limit is typically set to
>2A for an 8877, but you can adjust it lower or higher.
>
>You can use that output to control the HV supply, and as a minimum I
>recommend that you switch the mains supply to the transformer.
The engine of gold sputtering is the stored charge on the filter
capacitor.
> Inconjunction with a HV fuse, this provides a high degree of protection.
>A thyristor crowbar circuit will provide even more protection if you
>want it.
A thyristor with a withstanding potential of 4kV?
>
>So what is the grid trip for? To detect excess current due to RF drive.
>This "normal" grid current flows in the OPPOSITE DIRECTION to an HV
>surge,
The only source of grid current is emitted electrons from the cathode -
barring a leaky ceramic/metal seal.
- cheers, Ian
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>
- Rich..., 805.386.3734, www.vcnet.com/measures.
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