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Re: Resistor sizing for primary capacitors

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 10:05 PM 10/25/2005, Tesla list wrote: 
Original poster: Ray von Postel <vonpostel@xxxxxxxxxxx>


On Oct 24, 2005, at 17:23, Tesla list wrote:

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 09:35 AM 10/24/2005, Tesla list wrote: 
Original poster: "Dirk Stubbs" <dirk@xxxxxxxxxxxxxx>

I have a question regarding the use of resistors to bleed capacitors. How do you do about mathematically figuring the size of
resistor needed? What I am trying to do is bleed my primary capacitors (doorknob style).


The basic question is how long do you want to wait to get to a "safe" voltage.

Please, unless your life insurance is paid up, do not consider resistors across capacitors a "safety" device.

Perhaps one of several safety devices. (Or else, why would the NEC require them). As you say, one shuouldn't trust the resistors either. perhaps not so much for personnel safety, but also for
equipment safety, so that the equipment doesn't sit energized for long periods of time.

Resistors open up and the charge remains on the capacitor. This can and does happen even with high wattage wire wound ceramic coated resistors.

"Bleeder" resistors are placed across the output of high voltage power supplies for the purpose of improving the filtering and the regulation of the d.c. output voltage. They are there as part of the "filter" circuit.

Hmm.. only if the bleeder is drawing a large amount of current relative to the load, so the load variations are small. Bleeders increase the ripple


Resistors are sometimes used across the capacitors in a series string of capacitors so as to "equalize" the voltage cross each capacitor.
This usually done when capacitors are connected in series to increase the breakdown voltage or to obtain a particular capacitance. Either reason is not the bestpractice. Best practice is to use the correct capacitor for the job. Yes, I realize it is not always financially possible to follow best practice and we use what we have on hand rather than spend hard earned money.

Always, always short the capacitors in a circuit before even considering putting your hands into equipment. High voltage capacitors are often stored on the shelf with a jumper between their terminals.
They can even become charged just sitting.

I have worked with this stuff for 65 years and seen two people killed by open "bleeder" resistors I have seen several instances where 1/4 " brass "shorting rods" have been burnt in half by supposedly dead capacitors.

Ray