RE: Resistor sizing for primary capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Dirk Stubbs" <dirk@xxxxxxxxxxxxxx>

Thanks for your help, Jim. I realize that you shouldn't trust the resistors
solely. Everything should be double checked.

I may use 2 strings of resistors incase one string blew.

I also plan to use some sort of shorting device that I can engage safely.

Thanks again,
Dirk



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Wednesday, October 26, 2005 6:34 PM
To: tesla@xxxxxxxxxx
Subject: 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
>




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