[Top] [All Lists]

[Amps] Re: diode and glitch resistor

To: <>
Subject: [Amps] Re: diode and glitch resistor
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Sun, 25 Aug 2002 10:55:25 +0100
2 wrote:
>2, 10-ohm, 10Wers in series would be enough for glitch protection,

In the Europe the "best buy" are the 14W Welwyn series, as shown in

10-ohm resistors of this type are very comfortable at <1A intermittent 
service - but they do run hot! Two or three in series make a nice glitch 
resistor for amps of the 1-1.5kW class.

These resistors have a nominal voltage rating of 750V, but that's the 
steady-state voltage limit for the higher-resistance values. In practice 
the 10-ohm types will withstand their share of 3-3.5kV in a "crowbar" 
glitch test, for as long as it takes to turn the power supply off.

> but a huskier, 3A diode would still be needed.

This diode only functions during a glitch - in other words, any severe 
current surge (and let's not ask why :-)   The maximum possible surge 
current is limited by the glitch resistor, but it's still about 10x the 
normal current rating of a 3A diode, so we're relying on the surge 
current rating of the diode.

What makes me slightly uneasy is that the surge current rating comes 
with a specified time limit. If the surge is larger and/or longer, the 
diode will fail completely. And then we can only hope that diode will 
fail short-circuit... which it usually will, but obviously this isn't a 
property that the manufacturer guarantees.

The 6A diodes in the Triode Board may seem over-engineered, but 200+ 
users have invented some interesting practical tests! Two of the best 
reported were a direct short from B+ to chassis, *before* the glitch 
resistor; and a glitch in a YC156 amp where the resistor couldn't stand 
the sudden 5kV and arced from end to end. In both cases it was like the 
glitch resistor wasn't there  at all, so the surge must have been 
heading for 100A... but the 6A diodes came through OK, and so of course 
did the meters.

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

<Prev in Thread] Current Thread [Next in Thread>