Ian, is there any preference for the type of resistor used for the glitch
resistor?
I have some 25 ohm/25 watt Dale RER type metal body resistors, but I am
concerned that they might be prone to flash over due to the relatively small
gap between the leads and the metal case. Presumeably a ceramic body wire
wound resistor is the way to go. Should I be concerned about inductance
(L*di/dt kick) - some wirewounds seem to be classified as "non-inductive"
whereas some aren't?
Thanks,
Mike, W4EF...........
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK@ifwtech.com>
To: <amps@contesting.com>
Sent: Saturday, September 08, 2001 6:10 AM
Subject: Re: [AMPS] SB-1000 mods?
>
> Thanks, everybody, for all the suggestions about the SB-1000. I'm still
> in "repair" mode at present, but will think carefully about them all.
>
> Meanwhile...
>
> Johan Nordin wrote:
> >
> >>
> >>>> Fuses are bad enough because of time delay and internal arcing,
> >>>> and resistors make very poor fuses.
> >>
> >How about inserting 5x10 Ohm 1/4Watt in series as a glitch R
> >Would that be a combined glich/fuse??? Good or bad?
> >
> Bad... let me explain.
>
> The glitch resistor has two purposes: 1. To absorb most of the energy
> from a current surge that might otherwise go into the tube; 2. To set a
> limit on the peak current, which protects both the tube and the power
> supply.
>
> The glitch resistor must be large enough to handle all the stored energy
> in the smoothing capacitor, and then hang in there until *something else*
> switches off the HV. You may use a fuse in the HV line, but a fuse on its
> own is *not* the answer - because it doesn't blow quickly enough.
>
> A fuse has a reaction time, and while it's heating up it will pass very
> high currents... you don't know how high, or for how long. A resistor has
> zero reaction time, and you know for sure how it will limit the current
> (simple Ohm's law).
>
> So the problems with the idea of a string of small resistors are:
> 1. Resistors are very slow fuses, and if you spread the energy between 5
> resistors, it will take even longer for one of them to blow; 2. You then
> have to open the power supply... make sure the HV is all gone... sweep
> out the blasted pieces of resistors... get out the soldering iron... find
> some more resistors... you get the idea!
>
> I much prefer a system where a big glitch resistor handles the fast
> surge, and a current sensor switches off the mains within a few
> milliseconds. It's reliable and there's nothing to replace or repair.
>
>
> Just one other point: if you have a separate HV supply, the glitch
> resistor *must* be in the power supply, not the RF deck. This is because
> one of the most likely failures is a short to chassis in the HV cable or
> its connectors. If the resistor is in the amp, it's downstream of the
> short, and no use at all!
>
> 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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>
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