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
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.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
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