Re: Bleed resistor question.

Hugh,

You've made some good points here, but I think you'll agree that bleeder
resistors will run quite warm. Of course they must be kept within their
operating temperature limits, they certainly should NOT be BURNING UP!

Large HIV capacitor banks have an additional requirement, that of not
putting an excessive voltage drop across a single resistor. HV cap
banks may also have diodes across each resistor/cap pair. Together the
resistors and diodes help to equalize the voltage drops across each
capacitor, insuring that an individual cap/resistor does not develop a
disproportionate voltage drop across it's terminals, which would lead to
the components' failure.

Bleeder resistors are rated in several "watts" of dissipation (often 10+
W), they WILL dissipate heat! I.E., they will get HOT!

Often resistor values are calculated to discharge a filter cap within a
specified time (previous thread?). The voltage and resistance of the
bleeders can be used to calculate the dissipation, and then a healthy
safety margin can be applied to insure long life.

Hope this additional information is useful!

Alan (Loves 4-1000 "TUBE" Ham transmitters with HV power supplies)
KM6VV


Hugh Prescott wrote:

>
> Just a thought here
>
> I believe that if you bleed resistor is getting HOT that you have selected
> too small a resistance / wattage.
>
> There is an additional hazard in that a resistor running HOT has a higher
> possibility of failing because of overload. If it fails open (most likely)
> you are left with no bleeder and charged caps.
>
> Large cap banks that I have worked with (1000 VDC 10,000 uf) made up of 10
> 1,000 uf caps in parallel had bleed resistors on each cap. Of course each
> resistor was 10 times larger that that required for the whole bank. The
> failure of any one or few of the resistors was not as critical as the
> single failure of one resistor discharging the whole bank.
>
> Also place the resistors right at the cap and do not put heat shrink over
> them, makes it easier to tell at a glance if the resistor has blown or leads
> are broken.
>
> Then never trust the bleeders, measure the voltage first to be sure the
> bleeders are working before you reach in to work.
>
> Probably not encountered in machine shop cap banks but some cap banks come
> with a Thunder Stick designed to safely discharge the cap bank when
> required.
>
> Hugh
> There are old electricians and there are bold electrictians but there are
> few old bold electrictians.
>
> ----- Original Message -----
> From: Alan Marconett KM6VV <KM6VV@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Thursday, March 08, 2001 6:24 PM
> Subject: Re: [CAD_CAM_EDM_DRO] Bleed resistor question.
>
> > Hi Jason,
> >
> > If the two caps are in parallel, you can put the bleeder resistor across
> > either cap. Just make sure it won't touch anything, as it will get HOT!
> >
> > Alan KM6VV
> >
> >
> > andesign2@... wrote:
> > >
> > > In the power supply I am building I have used two capacitors in
> > > parallel to get the rating I need. I have purchased a resistor to
> > > use as a bleed, but I am not quite sure how to wire it up. Can I
> > > simply wire it across one of the capacitors? Any information would be
> > > appreciated.
> > >
> > > Thank You,
> > >
> > > Jason