The bigger the better up to the point of no improvement.
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
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.
----- Original Message -----
From: "Paul Whatton" <firstname.lastname@example.org>
Sent: Monday, September 08, 2008 2:08 PM
Subject: Re: [Amps] 3-500Z glitch resistor
> 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
> 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
> circuit, on a flashover. Hindsight is a wonderful thing and checking
> 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
> biggest thick-film in this application.
> 73 Paul G4DCV
> David G4FTC wrote:
>>>> Everyone seems to agree that a glitch resistor between the plate
>>>> and the HV supply is a good idea, to limit huge current surges due
>>>> arcs caused by gas, etc. But recommendations for the value seem to
>>>> 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
>> to the mains.
>> In order words to be safe you'll need about five RCH50 resistors in
>> 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.
>> David G4FTC
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