Calculating Discharge Resistor for High Voltage Capacitors

Found in the first hit by googling "capacitor discharge"

http://hyperphysics.phy-astr.gsu.edu/HBASE/electric/capdis.html

This has also been partially covered in a previous thread

http://cr4.globalspec.com/thread/3090/How-to-Calculate-Capacitor-Discharge-Resistor-Values

As for choosing the time constant, the time depends on how long you are willing to wait, the power dissipation of the resistors and the running losses that are acceptable due to the resistors (which are connected across the capacitors).

You could pick a fairly high value for the fixed resistors across the capacitors with a TC of a few minutes (for safety reasons and to keep the capacitors from recharging due to memory effect) and have an additional discharge circuit that switches in a low resistance high power rating resistor bank to safely discharge the capacitors in a few seconds. This is a common practice and maximises safety while minimising resistive losses.

What's the application and charging voltage?

Thank you for your reply, but unfortunatly you don't seem to have read the question carefully enough.

I did not ask for information on how to calculate the discharge time and so on, I can do that good enough. I ask for clues how to determine what time constat is considered safe AND still feasible.Also application and charging voltage have already been pointed out.

The hint with the additional discharge circuit is a rather good one though. Thank you.

I included a brief overview in the first part of my answer for completeness before answering the question asked in much more detail.

Also application and charging voltage have already been pointed out.

'Discharge experiments' doesn't tell me much. Depending on the type of application the capacitor bleed resistance could be quite critical (in that too low a resistance would discharge the capacitors too quickly), especially if the capacitor charging power supply is disconnected from the capacitors before the experiment.

If this is a general experiment like a coil gun or HV pulse experiment (where the time between disconnection of the capacitor charging power supply is short, and a small amount of voltage drop due to capacitor discharge thru the capacitor bleed resistors is acceptable) then that is a different story.

Oh, forgot to mention additional safety features such as using voltmeter, LEDs or similar visual indications across the capacitors to monitor voltage levels. As a plus the LEDs and there series connected resistor make a good capacitor bleed resistor with built-in indication located right next to those potentially live capacitor terminals (although TC will be high due to low LED current so would advise if using them to put in parallel with a lower resistance bleed resistor).

At least one voltmeter across the capacitors is a must (preferably permanently connected to reduce the possibility of measurement error or assumption).

What's the application?

I figured so much for myself already. I understand the theory pretty good, but I need actual advice on how to determine what a acceptable time constant is.

I can't be more specific about the application, because the units are used as demonstration devices and actual application will vary from time to time. Mostly they are pulsed with a trigatron into thin wires (for vaporisation), various coils for demonstration of induction or spark gaps to sustain high power arcs.

The bleed resistor wont only create a voltage drop but will also dampen the discharge current quite a bit, which is my actuall major concern. Best bet will be a secondary discharge circuit a guess.

If that is the case then stick with a TC of say 5-10 minutes as well as incorporating a a secondary discharge circuit.

If still unsure or if a TC of 5-10 minutes results in a resistance that is still too low, choose the bleed resistors based on the minimum resistance that is acceptable for your output. Note that even a high value of resistance will be better than nothing at all (from a safety point of view).

Remember you should have the bleed resistors as an additional safeguard and to prevent the capacitor recharging effect over time (which the secondary discharge circuit wont protect against).

If there is a chance of anyone getting into the works who is not 100% trained in such areas, maybe you'd better go with Jack's 5 - 10 minutes, but put the whole thing in a cage with a solenoid-release lock on the door. You can use a Safety rated run-down timer to make sure the 10 minutes has elapsed before the guard unlocks.

Won't be cheap.

Yeah, I understand the thought. Actually the units will always be used by trained personnel but apparently some people pointed out to us that there is slight possibility that the audience might overcome the urge to storm the demonstration area and lick everything with a danger sticker on it.

But after I spoke with the people that actually matter, the whole thing should be fine with bleeding resistors and a "touch-proof" casing.

Hate to be boring, but you're really going to have to do the whole risk assessment thing & follow your local codes with this. I don't envy you.

Word of warning - if you're involved in designing or setting up this thing, you can't rely on "the people that actually matter" if (god forbid) the shit hits the fan.

Everyone will be throwing the blame a everyone else - you may end up not working in your field again, or you may end up behind bars.

Be safe!!!

I worked on a MJ (5kV) capacitor bank many years ago. Several safety interlocks AND a manual shorting bar were used before anyone could even enter the room. Special resistors, capable of withstanding high voltage and high energy impulses, were used throughout.

I used a large HV mechanical relay for charge control.

Default relay position (through gravity and in case of power loss) connected the discharge resistor across the bank. Safe discharge was measured at about 2 minutes.

Active charging energized the HV relay which connected the charging power supply and disconnected the discharge resistor. Charging took about 8 minutes. This configuration was the only way we could get fast and efficient charging and discharging at that time.

Your energy and time constants are considerably smaller and you are actually in the better position to decide how fast a safe discharge should be for your application.

Be SAFE!

Yes the the relay realy good as a solution. I will look into it. Thank you.

How was the manual shorting bar operated? What did the mechanics consist of? Must have been quite big, in case the bank is accidently shorted while being fully loaded.

Whatever is considered "safe" before the accident will not be considered "safe" after the accident. Be sure to comply with whatever is legal. Then, on top of that give consideration to incorporating every reasonable suggestion. It should take at least two mistakes (rule violations) to get shocked, more if possible.

Rule I'd use is a compromize between two obvious factors. Not too much of a current to cause significant unnesecary losses and not too big time window for somebody to have the chance to come close to the hazardous area after power shut off and before voltage dropped to a safe level. The time usally advertized at this voltage range (used in most switching mains supplies) is 20 sec for voltage to drop to 20V but I guess it's a little exaggerated. And don't have to remind you that these resistors must be well oversized (in watts). No failures accepted

Regards.

It depends on certain considerations:

1. Max time of discharge to safe level say 10 ~ 20 V or else

2. Power consumption if connected permanently

3. If connected on the disconnec of power eg through a relay / contactor.

4. Too fast a dischage may damage the bank [only possile in case 3. above.

To compromise all the factors is at your end.

hope of some help

It used to common practice to have two safeties. The first was a high value resistor that gave a 1-2minute time constant. In parallel with the resistor would be a gravity or spring loaded shorting bar that would become engaged whenever the capacitor cabinet was opened. If qualified personel wanted to work in the capacitor cabinet, they would have to disengage the automatic shorting bar. This was considered safe enough.

Your capacitor bank stores about 12 times the muzzle enrgy of a 45 ACP round. So it is good practice to treat it with respect. Think about using solid cermet discharge resistors instead of film or wire wounds and the use of automatic tamperproof interlocks on the cabinet to discharge the capacitors in the event of entry into the cabinet.