# How can I calculate the sparks when I discharging a big capacitor?

Several years ago I've tried to find answer to this question but I couldn't find the answers. I hope you help me.

Because my job I have to discharging constantly "big" capacitors ( 400 vdc, 300 vdc, and so on). Ok really they aren't so big but problem is sometimes when I shortcircuit the capacitors , they to make huge sparks.

I have to avoid sparks mostly because several time I've damages IC's very importants into the circuit, and second I'm afraid of the sparks every time I shortcircuit a "big" capacitor Jaja.

So my idea was to conect a parallel resistor to capacitors . Problem is How to calculate the resistor to descharging quickly the capacitors but also avoiding the sparks.

For example:

$$C=100uf -160vdc; R=470 ohm$$

$$Vc = vo*e^{-t/T}$$

$$vo=160 v$$ $$T=r*c$$ $$T=100uF *470 ohm$$ $$T=47mS$$

$$vc = 160*e^{-t/0.047}$$

Wao, this discharge very fast : five tao = 5*0.047 = 235mS to discharge the capacitor

This is result: NO sparks

But I need more quickly so I try a 100 ohm resistor

$$C=100uf -160vdc; R=100 ohm$$

$$Vc = vo*e^{-t/T}$$

$$vo=160 v$$ $$T=r*c$$ $$T=100uF *100 ohm$$ $$T=0.01mS$$

$$vc = 160*e^{-t/0.01}$$

Wao, this discharge is faster : five tao = 5*0.01 = 50mS to discharge the capacitor

But this is problem : sparks This is a litle spark. Problem is when I discharge bigger capacitors 300 vdc, 400 vdc, etc.

Are they some formule for trade-off quickly discharge and minimun sparks ?

Thank you.

## 1 Answer

You could use an NTC resistor, the same kind used to limit inrush current on power supplies (hint: they're used to limit the charging current to the input caps.)

An NTC will start at a high resistance, then heat up and lower its resistance as it discharges the cap. This is kind of what you want to avoid sparks.

• Thank you. Now I know the NTC resistors – NIN Jul 13 '20 at 3:17
• Thank you. Now I know the NTC resistors – NIN Jul 13 '20 at 3:17