# How do I safely discharge a 3900 pF 3 kV (and 9 kV) capacitor?

I am planning to built a circuit with 3 polypropylene film 3900 pF 5% 3 kV DC radial capacitors connected in series. Before I start this, I want to make sure the capacitors are safely discharged. I am also thinking that I will need to safely discharge them after they are soldered together. I tried researching this and found a site on Digikey to calculate what size resistor should be used, but found that it would allow any ohm value based on time:

What would be an appropriate resistor for safety discharging these capacitors? I am not concerned if it takes a while for it to discharge, I just want to make sure I do not choose one that is to small for capacitors of this size.

Thank you.

• Think about why the resistor value depends on time... Mar 7, 2021 at 19:59
• So what did the Digikey calculator answer when you selected Solve for Resistance? Did it ask for Discharge Time? Faster means a larger power resistor. Mar 7, 2021 at 22:12
• I fifth grade it was a funny game- charhe a film capacitor in 220VAC mains, then discharge on a friend :D
– user76844
Mar 8, 2021 at 3:33

If you were using 250V,350,500V,600V 1/4W rated resistors, you would need a safe string (in tubing) of 9kV/0.25kV= 36x 10Meg resistors with a value to determine the current and power limit.

There are also 1kV~10kV resistors which might be a better choice. There may be lossy materials that can withstand 10kV to function as discrete resistors that are mainly lossy insulators handled with neoprene gloves rated for this.

Always consider the datasheet safety sheet for a dielectric breakdown.

• on second thought a 10M 350V resistor will bleed the cap before air will ionize over the surface of the part and arc across zzttt, It would sound like touching an old hotel doorknob with nylon carpet and neoprene soles. not exactly lethal. 10M*3900pF=39 nanoseconds Mar 8, 2021 at 0:48

You should use a discharge stick (sometimes called grounding sticks or shorting probes) when working with high voltage. Many times, it's not practical to have capacitor bleed resistors in place while a circuit is in use. Using a discharge stick allows you to safely discharge high voltage devices for service while not having to increase the power required to drive the circuit because of the wasted power in the bleed resistors.

One more thing, a direct discharge from > 1 kV circuit can be fatal. You should exercise extreme caution when working with high voltage circuits.