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[AMPS] resistors

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Subject: [AMPS] resistors
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Date: Sun, 21 Feb 1999 16:02:37 -0500

On Sun, 21 Feb 1999 13:30:30 -0500 "Kenneth D. Grimm, K4XL"
<> writes:
>The primary purpose of a bleeder in the power supply for my typical
>amateur amplifier is *not* for voltage regulation but to allow the
>voltage to bleed off of the filter capacitor.  A good rule of thumb is
>100 ohms per volt, ie. for a 4500 volt supply, the bleeder would be 

Excellent suggestion Ken but I would consider that up to about a 2KV
supply. A 200K total bleeder takes less time to discharge and there are
those of us who are impatient!

>ohms or thereabouts.  Using ohms law, the current through the bleeder
>would be 4500/450000 = 0.01 amps.  The power rating of the resistor
>would then be 4500 x 0.01 = 45 watts.  Since safety is the primary
>concern with the bleeder, I would be inclined to use a power rating of
>100 watts or more.  From a practical standpoint I would be inclined to
>use 5 series connected 100k 25w resistors. 

 However, the only truly 
>bleeder is a big shorting bar on the end of a very long non-conductive
>stick, with ear-plugs inserted in your lug-holes, just in case...
>Ken K4XL

I still have my "Mil-Sec" shorting bar. However due to the common
metering schemes and a B- rail raised above ground with a diode, damage
can result with a dead short discharge. Back to back 1N54nn series diodes
can usually protect the metering and should be a first step upgrade in
most commercial ham amps.
I prefer a shorting bar with a series 10K 10W resistor and use it when
the voltage gets down to 1KV or so. ( which is why I prefer lower value
bleeder R.) Then a direct short jumper lead while I have my fingers in
the hardware.  Just remember to remove the jumper before firing up !

Above all, safety first.

73  Carl  KM1H

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