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Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
I'm using two large electrolytic caps as a voltage doubler, both 12000uf 200V, and I want to parallel resistors or maybe put resistors in series with a light bulb or an LED to indicate when voltage is drained. I've looked around at a few resistors in which their application is "capacitor discharge" or "bleeder". specifically these two bleeders from digikey here and here. Do you guys use resistors like these ever or have any opinion on these bleeders? What kind of resistors or methods do you guys use to bleed/discharge voltage? After searching on google about draining voltage, I've only found very stupid and unsafe ways, mostly just shorting the (+) and (-) together to cause a bang of sparks... not interested in doing that for obvious reasons. Anyways, buying the above bleed resistors would probably be my last resort if I cant make do with something that I already have. I have some 1.0M OHM 3W thick film resistors on hand and I was thinking of trying to series this up with a light or LED. I guess I am just looking for a second opinion on this subject from some more experienced folks out there since I dont often see bleed resistors in tesla coils except for power supply applications here and there.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You have raised two issues;
1) Discharging the capacitors once the ac supply is removed - this is very important for safety, and the convenience of being able to work on the circuit soon after power down. Choose a bleed resistor based on
1.1) time constant = R.C e.g. 12,000uF x 25kOhm = 300 seconds = 5 minutes so if charged to 160 V, 5 min. = 59V, 10 min.=21V, 15 min.= 8V .........
1.2) power dissipation =V^2/R during operation, e.g. at 160V a 25kOhm resistor will dissipate just over 1W, so a 5W or 10W resistor would be adequate for this purpose.
2) indicating when the capacitors have discharged to a 'safe' level, this is most easily achieved with a high value resistor in series with a neon lamp, the lamp will extinguish below about 70V, if across both capacitors that's 35V each ... moderately safe the resistor should limit the maximum current through the lamp to about 0.1mA .. 320V/0.1mA = 3.2 MOhm so 3.3 MOhm would be good.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
My DRSSTCs both have large cap banks that operate at 320V DC. I use 10k, 10 or 25 watt resistors as bleeders, and for warning lights I use the commonly available neon indicators with built-in resistors, intended for running off 240V mains.
For my next one I'm going up to 600V. The cap bank will be two sections in series, so I plan to give each section its own 10k bleeder and neon lamp.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Thank you both! That helped me sort everything out. Last night I ended up stringing together 10 2.2kohms 1/2Watt resistors for a total of 22k and put a LED on the end. I did not make it a permanent part of the circuit because I intend on getting one resistor with the value I require and neon light to try as you both suggested. But I attached alligator clips to both ends and after clipping it to the individual caps it worked just fine at draining them. I thought that the little LED might not be able to handle the power but it did (the caps were not fully charged there was about 50V in each) and it took only about 15 minutes or less before the LED dimmed out. I checked the voltage again and now the caps only had 14 volts in each for a total of 30 across. So this ended up sufficing, allowing me to work on my circuit until I can get a proper resistor. Thanks again for the info!
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bleed resistor question
