[AMPS] New Triode Board from G3SEK

[Ian White, G3SEK]

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.

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. In
conjunction with a HV fuse, this provides a high degree of protection. 
A thyristor crowbar circuit will provide even more protection if you
want it. 

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, and the Triode Board has a different circuit to monitor and
detect it.


73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.demon.co.uk/g3sek

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