>
>Joe Subich, K4IK wrote:
>>
>>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).
>>
>
>The SCRs don't need to be very large - the B+ surge resistor limits the
>possible current to 40-60A, which even a TO-220 SCR can handle.
>
>One SCR can handle the screen, if the rest of the screen supply can than
>take care of itself.
>
>For the B+, OE5JFL has shown how to string ordinary 1000-1200V TO-220
>SCRs in series and fire them all using opto-couplers. The SCR crowbar
>handles the energy out of the reservoir cap, and a zero-crossing SSR
>turns off the mains.
>
>Circuits are at http://www.mrs.bt.co.uk/dubus/9002-7.pdf and I've seen
>it working.
>
>FYI, Siemens require for their 1500W TV transmitting tubes that if the
>B+ supply is shorted with a few inches of 3A fuse wire, it shuts down
>without melting the wire. Likewise RCA used to require that the supply
>can be shorted on to a cigarette foil without burning a hole in it.
>An SCR crowbar will do that.
>
guilding the lilly. Meanwhile, in the land of reality, one fast acting
fuse in the primary of the screen supply transformer would have
adequately protected the 275w screen.
>
cheers
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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