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Re: [Amps] 3-500Z glitch resistor

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Subject: Re: [Amps] 3-500Z glitch resistor
From: David G4FTC <>
Date: Mon, 8 Sep 2008 19:16:45 +0000
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> However the purpose of the resistor is not to absorb 100 % of the energy
> BUT to limit the current to a managable level until the primary breaker
> trips.

True - but the arc will continue until the applied voltage has dropped to quite
a low voltage, which without a crowbar, means that the resistor will in effect
dissipate the energy in the capacitor bank, and providing the resistor can
handle the peak current for the fraction of a second while the capacitor
discharges then all will be well. As Steve has already noted - it's vital
that the resistor can handle the voltage across it without arcing.

If the capacitor bank has very high capacity, then a crowbar may be required,
primarily not to protect the resistor, but to ensure that the energy within the 
arc in the tube does not exceed that which will damage the tube.

I recall that there are some good RCA application notes available on the web on
design considerations for high power broadcast transmitter power supplies, and
how to test the protection system by grounding the HV supply via a suitably 
fuse wire and seeing that the fuse wire doesn't melt.

> In a SB-220 for instance there is no room for a 50W or larger enamel
> resistor and a 25W will survive. The fact that it works is that the
> original PS diodes have survived.

Agreed - a 50 ohm 25watt vitreous enamel resistor will do the job nicely.

Vitreous enamel resistors are particularly good (and recommended) for 
glitch protection applications. They can have high peak current capabilities 
with the ability to withstand the applied voltage. 


David G4FTC
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