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[AMPS] Screen protection WHAAAATTTTTT!!!!

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Subject: [AMPS] Screen protection WHAAAATTTTTT!!!!
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Tue, 17 Oct 2000 17:20:43 +0100
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 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.

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

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