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Re: [Amps] Fault protection

To: <>, <>
Subject: Re: [Amps] Fault protection
From: "Tom W8JI" <>
Date: Thu, 22 Mar 2007 15:29:43 -0400
List-post: <>
Thanks Bob,

> First reference to the .82 Ohm resistor is in the 
> unpacking section each new
> amplifier comes with two spare resistors.  (sounds like 
> they expect to use
> them as fuses for the HV.

Sounds that way.

> The next reference is in the troubleshooting section and 
> it was made pretty
> clear what the device is for.
> There is an HV shorting interlock on the amplifier head. 
> Here's the exact
> quote, "In the event of a short circuit, resistor R12 in 
> the power supply
> will be destroyed.  This resistor protects the diodes in 
> the power supply
> and it should be replaced with an IRC 0.82 Ohm 2 Watt type 
> BWH resistor
> Only."

There we go. When there is a HV fault while the supply is 
up, the electrolytics' stored energy will blast the little 
resistor out of the way with hundreds of amps of dump 
current. As the caps approach a few hundred volts or less 
the arc will quench and the circuit open, so the diodes only 
have to stand high current on the down swing of the 
electrolytic. The sacrificial resistor, if the transformer 
and power line ESR is high enough, saves the rectifier 

Makes perfect sense.

I used to do the same thing with several inches of #28 wire 
between my oil filled caps and the RF deck in my 4-1000A's. 
All those coulombs would vaporize the wire with a big bang.

What the little resistor (and my fuse wire) does not do is 
limit fault current as the power supply caps dump into the 

If we want to protect the system we need a reasonable 
resistance and high enough voltage rated fault current 
limiting resistance, and in that case we probably can just 
forget the little .82 ohm resistor.

73 Tom

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