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[AMPS] Discharge energy

To: <amps@contesting.com>
Subject: [AMPS] Discharge energy
From: Ian White, G3SEK" <g3sek@ifwtech.com (Ian White, G3SEK)
Date: Mon, 8 Oct 2001 10:49:45 +0100
Steve Thompson wrote:
>
>From Phil, K5PC:
>>The value of the glitch resistor is
>>selected to limit energy discharge from the filter caps to 50 joules,
>>and the wattage of the glitch resistor is selected to keep it from
>>self-destructing while the breaker is in transit.
>
>First of all, please can I stress that my comments are NOT intended as
>specific to Phil - his posting contains a comment I've seen a number of
>times, which might appear in some manufacturer's info and which puzzles me a
>bit. Phil's posting gets used purely because I've chosen this moment to make
>my comment.
>
>The energy in the filter caps is defined as 1/2*C*V^2. I'm puzzled as to how
>a glitch resistor allows the *energy* in a discharge to be controlled. To my
>mind, it can only set the peak current and the time constant.
>

When the capacitor is discharged through the series combination of the
resistor and the tube, the stored energy is divided between those two
components. Strictly speaking the energy going into the tube is not
limited, but shared.  

If there is an arc, the voltage across the tube drops to only a few tens
of volts, so almost all the energy from a HV supply goes into the series
resistor. In that sense the resistor limits the energy available to go
into the tube.

Eimac Bulletin #17 recommends an energy limit of about 4J for
indirectly-heated tubes up to 1500W dissipation (as a rough way of
classifying tubes according to size) and suggests that this can be
achieved by limiting the peak current to 40A. Directly-heated tubes are
less subject to arc damage, but Eimac point out that damage to the
control grid of a smaller triode such as the 3-500Z may also be a
limiting factor, since the grid is at the receiving end of an arc from
the anode.

All this is pretty hand-waving stuff because controlled experiments are
very hard to do. The best we can hope for is guidelines, and they come
with no guarantees. 

There are obviously other resistive elements in the loop, notably the
internal resistance of a string of electrolytics, which explains why
tubes do survive with smaller glitch resistors than Eimac would
recommend, or indeed with none at all.

The important thing, as Phil said, is that the resistor is big enough to
hold together while the fuses or breakers are operating, because you
can't guarantee that it will fail open-circuit.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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