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[AMPS] SB-1000 mods?

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Subject: [AMPS] SB-1000 mods?
From: (Johan Nordin)
Date: Sat, 8 Sep 2001 16:14:01 +0200
Ian White wrote:

> The glitch resistor must be large enough to handle all the stored energy
> in the smoothing capacitor, 
Yes but not if I want it to act like a fuse....

>and then hang in there until *something else*
> switches off the HV.
Thats the problem, if I have a "heavy duty PSU" and I have 
 *something else* that swiches of the HT 
there will most likely be a "couple" of  "bangs/tics" (hopefully tics)
b4 it swiches the whole thing off. (this might happen during only 1/10 sec or 
 If the string blows,(hopefully
at the first tic/bang) there´s a better break dwn system.

> 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. 
The Idea was that this shouldn´t be a fuse on it´s own but a string
to blow during failure and still have the sufficient R like a glitch...
One thing that might worry, is that if say three (or more) of them blows/flashes
at the same time, there is just 20 Ohm (or less) left at that moment :-)

This was just a thought, I have lots of them hi

73 de Johan

> 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)
> --
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