Conrad Writes ...
>
>
> Rich wrote
>
> >>Transistors that C/E short offer zero protection.
>
> Ok so sense the voltage drop across a resistor across the screen supply,
> feed the transistor via an opto isolator,make the transistor trip a fast
> relay in the primary of the supply transformer.This is better and more
> reliable than a fuse surely.
>
Make that trip a fast zero crossing solid state relay in the primary
of the screen supply. The SSR should "open" at the zero voltage point
and, with the proper MOV to limit the inductive kick-back spike, should
be the quickest, most reliable protection. Frankly, I would sense screen
current, screen voltage, as well as plate current and trip both the
plate and screen supplies in the event of a fault. I might even use
a big SCR (screen) or triggered spark gap (plate) to discharge the
supplies to limit the fault (if I could justify the CO$T).
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