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Unread postAuthor: rp181 » Tue Apr 29, 2008 9:19 pm

by bleeder resistor i mean a resistor shorted across the cap terminals to prevent harmfull charge build up and keep them safe.
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Unread postAuthor: jimmy101 » Wed Apr 30, 2008 1:54 pm

rp181 wrote:by bleeder resistor i mean a resistor shorted across the cap terminals to prevent harmfull charge build up and keep them safe.

Sorry, I figured you wanted a practical method to "safe" the cap. So what do you want to do, keep it at a safe voltage or slowly discharge the cap when the circuitry is off?

It is usually pretty difficult to overcharge a cap, when you exceed the voltage rating of the cap it'll start to leak like crazy. So, if you power supply will do 400V and the cap is only rated to 300 the cap won't go much above 300V.

I can only think of two situations when you would need a bleed resistor "to prevent harmful charge build up and keep them safe". (1) If your power supply can generate much higher voltages than your cap is rated for and, the supply has a pretty beefy output current rating that can overwhelm the internal leakage of a typical big-ass cap. This might happen if you did something like hooked up a 25V cap to rectified 120VAC mains voltage. (2) You are using several caps in series and you are concerned about having just one of the caps charged. (see <a href="http://www.pupman.com/listarchives/2004/June/msg00202.html">here for more info</a> on this situation).

The usual function of a capacitor bleed resistor is to slowly discharge the cap when the circuit is turned off so you don't have a potentially lethal power source present in a device that is "turned off and unplugged".

So, what do you want to do?
1. Provide a method to slowly bleed the voltage off the cap(s) when the circuit is turned off. The caps "safe" themselves after some length of time after main power is removed. Typically, the length of times is measured in tens of minutes, or hours, or perhaps even days.
2. Provide overpower protection to the caps so your ginormously powerful HV supply doesn't cook'm.
3. Provide a method to prevent excess charge build up on a single cap in a series of caps. (This seems unlikely, coil guns rarely use caps in series, they are almost always used in parallel.)
4. Provide a method that allows you to safe the cap(s), when you want to, in a short period of time (couple seconds).

What I posted using light bulbs does (4). It'll also do (1) if you connect it when you store your circuit.

If you really want a traditional bleed resistor (item 1 above) the design goes something like this:

1. What is the current rating of your HV supply? You probably want your bleed resistor to dump no more than about 10% of the current the HV supply will provide. More than 10% and too much of the energy of the supply immediately gets dumped through the bypass resistor. (If you don't know what the current rating is of the supply I'll give a way to estimate it later on.)
2. Given the final charge voltage on the cap, the desired bleed current from (1) you can use ohm's law to calculate the needed resistance. For example,
Bleed current = 1mA (power supply can source ~10mA)
Cap voltage = 300V
R<sub>bleed</sub>=(300V)/(0.001A)=300Kohm
Check to make sure you aren't dissipating too much power in the resistor;
Power<sub>initial</sub>=VI=(300V)(0.001A)=0.3Watt
Too much for a 1/4 watt resistor, need at least a 1/2 watter.
Time to discharge the cap to ~37% of it's initial voltage is the "RC time constant";
RC=(300V)(0.001F)=300 seconds
(I've assumed your big-ass cap is 1,000uF)
Figure it takes 4 RC periods to reasonably safe the cap (the cap voltage will be about 6V at 4RC if it started at 300V), 20 minutes to safe the cap.

If you want to safe the cap quicker you can use a lower resistance and higher wattage resistor. Or, several smaller resistors in series. But, if you sink to much current through the bleed the HV source won't be able to charge the cap up fully.

What do you do if you don't know what the output current of the HV supply is? Hook it up to your cap along with a voltmeter. Measure about how long it takes to charge the cap up to ~63% of it's final voltage. Now do the math for a bleed resistor that has an RC that is ~10X the time you measured. Check that the resistor won't be overpowered by the cap.

Personally, I would prefer and "on demand" safeing circuit that safes the cap(s) right now, not 20 minutes from now. Perhaps use a DPDT On-Off-On switch and a couple 25W 120VA light bulbs. Flip the switch to "charge" and the cap(s) is charged without any bleed. Flip the switch to the off (center) position and the cap is isolated from the charger and the safer circuit. (This is also handy for controlling the voltage on the cap.) Flip the switch to the "safe" position and the cap(s) are safed within a couple seconds.
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Safer circuit for big-ass capictor
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Unread postAuthor: rp181 » Wed Apr 30, 2008 6:11 pm

yes, i did mean (1). Thanks for the equations, i like your method, with the switch. I think ile do that, and see if i can get actuall resistors (more proffesional looking). For charging, i will use a MOT, so the current will be from between 200-500ma. To moniter voltage, ile use a cheap analog multimeter.

And again thanks =) some great information.
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