[AMPS] SB-220 Glitch Resistor ???

Ian White, G3SEK Ian White, G3SEK" <g3sek@ifwtech.com
Tue, 28 Nov 2000 07:44:54 +0000

Bob Johnson wrote:
>So my last mod before shooting the juice to the finals is to add the
>"Glitch Resistor" !!!
>It has been recommended on this reflector that RFC-2 be replaced with a 15
>to 20 Ohm real enamel wirewound resistor.
>My questions are:
>Is 15 to 20 ohms "Cast in Stone" ?
>Would 25 ohms be OK ?

Not cast in stone. 

Eimac recommend a resistor that would limit the peak short-circuit
current out of the power supply to 40A (for tubes up to 1500W
dissipation). So for example, if the B+ is 2.5kV, at first sight  the
resistor would need to be 2500/40 = about 60 ohms.

However, you can also "take credit" for the internal resistance of the
smoothing capacitors, which allows you to reduce the calculated value of
the glitch resistor. In practice 25 ohms will take most of the sting out
of a current surge, so that's a typical value that people aim for. Some
would go as low as 10 ohms, others a bit higher.

>What wattage should be used ?
Enough to handle the full-time normal anode current with something in
reserve, so I^2R would say a minimum of about 25W... but that isn't the
whole story.

>I have checked the Mouser catalog and they stock the OHMITE Series 270
>Power Resistors in 15 and 25 ohms for the 25 Watt version (L25J).
I don't have that catalog, but the rule is to use a long-bodied resistor
to avoid a direct arc from end to end. Having decided on the resistance
and wattage, choose the longest resistor you can find.

>If this resistor is suitable, would it be better to go with the 25 ohm
>value on the basis that the peak discharge current would be lower or would
>the higher resistance cause other unforeseen problems ?
Higher values give less peak current and more protection. Lower values
waste less DC power and less anode voltage. It's a judgement call.

Something often overlooked is that even if you fit a glitch resistor to
handle current surges, there also needs to be some way of interrupting
the power input from the mains. I've heard of situations where someone
dutifully added a glitch resistor, but when the B+ was shorted the power
supply kept right on going because the fuse/breaker ratings were set too

Usually a step-start solves this problem, because it allows the mains
fuse/breaker to be changed down to a rating only just above the normal
operating current.
You might also consider adding a high-voltage fuse, in series with the
glitch resistor. In the past few years they have become easily
available as microwave oven spares, complete in a shrouded holder with
wire ends. Unfortunately the ratings only seem to go up to 1.0A, but
that should be OK for a pair of 3-500Zs.

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

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