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[AMPS] resistors

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Subject: [AMPS] resistors
From: (Kenneth D. Grimm, K4XL)
Date: Sun, 21 Feb 1999 13:30:30 -0500
Rich Measures wrote:
> Carl said:
> >
> >Which is why 100K is the suggested max in any literature I have seen for
> bleeders.
> ?  References, please.  .  In a typical 8171 anode-supply, a 100k-ohm
> bleeder would dissipate 810 watts.
> >>>>If I wanted a Reliable bleeder for a 4kV C-filter supply, I would
> >>>>series up 8 to 10  Matsushita/Panasonic 100k-ohm, 3w MOF resistors on a
> piece of
> >>>>perfboard and be done with it.
> >>>
> >>>Absolute YUK !  800K to 1M way Jose.
> >>>
> >>Ä  What is to be gained by using a 200k-ohm bleeder on a C-filter, Carl?
> >
> >Quicker HV bleed down time, less chance of a recharge.
> ?  What mechanism recharges the capacitor?
> >  All my concerns are safety related and for others not well versed in PS
> issues.
> >
> >A string of 3W does not offer much of a voltage breakdown safety margin.
> >Matsushita not a common distributor item and similar wattage Panasonic,
> >etc are only 350V.
> ?  The manufacturer's (Panasonic/Matsushita)  rating at 70 deg. C ambient
> is 500v or 3w .  For a 100k-ohm unit, the realizable dissipation is 2.5w
> at 500v.  .
> Rich...

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 450k
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 safe
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
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