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
less)
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)
> http://www.ifwtech.com/g3sek
>
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