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Re: [Amps] 3-500Z glitch resistor

Subject: Re: [Amps] 3-500Z glitch resistor
From: Paul Whatton <>
Date: Mon, 08 Sep 2008 19:08:35 +0100
List-post: <">>
For Vic who originally asked this question, I've seen the figure of 
about 40A suggested as the current limit so for a 3-500Z with 3kV, 75R 
should do. It isn't the normal power rating of the glitch resistor that 
is the problem but as David points out, it's the resistor's ability to 
survive the energy pulse from the power supply during flashover.

In my 2m amplifier I use a modest 2kV supply with about 50uF. When I 
built the amp I used a 47R 250W thick-film as the glitch resistor (5 
times bigger than the 50W RCH series). It "felt" and certainly looked 
like it should be more than big enough. But it failed, fortunately open 
circuit, on a flashover. Hindsight is a wonderful thing and checking the 
datasheet I'd exceeded the overload ratings. BTW such a thick-film 
resistor costs about GBP45 new from RS in the UK, that's an expensive 
fuse. Fortunately I'd only paid a couple of quid at a hamfest.

IMHO a 50W+ ceramic wirewound is a much better and cheaper bet than the 
biggest thick-film in this application.

73 Paul G4DCV

David G4FTC wrote:
>>> Everyone seems to agree that a glitch resistor between the plate choke
>>> and the HV supply is a good idea, to limit huge current surges due to
>>> arcs caused by gas, etc. But recommendations for the value seem to vary
>>> all over the map.
>> I like the look of Vishay RCH series or similar (Tyco do an
>> equivalent). The 50W version is rated at 5.5W without heatsink and
>> is specified to handle 2500Vrms (that's 3500V peak) short term
>> overload. If you want to bolt it to a panel for heatsinking, the
>> insulation is rated for 3500Vrms (5kV peak).
> Checking page 4 of the datasheet for the RCH50 
> shows a maximum overload capability of about 50 Joules.
> A capacitor bank for a linear of 50uF charged to 3kV will deliver about 225 
> Joules and
> under glitch conditions the resistor will be absorbing most of this power, 
> more if
> the power supply doesn't have glitch detection/protection and remains 
> connected
> to the mains.
> In order words to be safe you'll need about five RCH50 resistors in series.
> I don't know what energy the tube can absorb under flash over conditions 
> without 
> damage and this will also influence the selection of the resistor.
> Regards
> David G4FTC
> __

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