7
\$\begingroup\$

I have a high voltage (400V) electrolytic capacitor, and I sometimes discharge it by shorting the terminals with a screwdriver. I get a fat spark and a bang. However, apart from minor damage to the terminals/connections, does doing this cause any damage to the capacitor?

\$\endgroup\$
  • 6
    \$\begingroup\$ If capacitor has enough pulse current capability, flying bits of molten metal from vaporized screwdriver tip is a hazard to your eyes. \$\endgroup\$ – peufeu Dec 17 '18 at 13:39
  • 1
    \$\begingroup\$ Yes, easy to do (I've done it). The point where the pin is crimped to the foil is a weak spot. But if it hasn't happened yet, you may be OK. \$\endgroup\$ – Robert Endl Dec 17 '18 at 16:08
12
\$\begingroup\$

The possibility certainly exists, depending on how the current distributes within the cap and how much energy is suddenly converted to heat inside the cap- and there are capacitors designed for pulse discharge applications, such as photoflash, but even there there is some series resistance and inductance to limit the current.

The place to look would be in the application data (probably not the datasheet) for your model of capacitor. For example, the Nichicon Application Guideline document cautions (emphasis added):

  1. In the equipment ( 1 ) Do not directly touch terminal by hand. ( 2 ) Do not short between terminals with conductor, nor spill conductible liquid such as alkaline or acidic solution on or near the capacitor.

And also

5) For a circuit that repeats rapid charging / discharging of electricity, an appropriate capacitor that is capable of enduring such a condition must be used. Welding machines and photo flash are a few examples of products that contain such a circuit. In addition, rapid charging / discharging may be repeated in control circuits for servomotors, In which the circuit voltage fluctuates substantially. For appropriate choice of capacitors for circuit that repeat rapid charging / discharging, please consult Nichicon. If excess a rush current due to drastic charge/dis-charge was applied to conductive polymer aluminum solid electrolytic capacitors, and conductive polymer hybrid aluminum electrolytic capacitors, it may cause a short circuit or an increase in leakage current. Therefore, Please do not apply a rush current that is larger than 10A.

So if you're using the conductive polymer type, that's a clear danger. Tantalums also have a well-known propensity to self-immolate if subjected to excessive transient current.

\$\endgroup\$
5
\$\begingroup\$

This is clearly a bad practice considering the potential damage to the capacitor, to the board and to the screw driver. Not talking about the stress when the spark occurs : ) In our lab, we have built cheap "dischargers" made of a 3-W/1-k resistor isolated with some heat-shrink tubing as the below picture shows. It is easy to handle and you painlessly reduce the bulk capacitor voltage to a safe value in 1 second or so. Highly recommended if you deal with high-voltage boards!

enter image description here

\$\endgroup\$
  • 2
    \$\begingroup\$ Be careful using those cement wirewound resistors in high pulse power applications, I've been rather successful in getting them to explode. The shrink wrap might contain it, but I wouldn't want fingers there. If you keep peak power within a small multiple of the steady state power rating it ought to be fine. \$\endgroup\$ – Spehro Pefhany Dec 17 '18 at 17:04
  • \$\begingroup\$ Hello, well, we use this principle for 20+ years on ac-dc power supplies with 400-V charged bulk caps in PFC stages and never had a problem. It can of course be different in some of the applications you mentioned but I don't have the experience with those then. \$\endgroup\$ – Verbal Kint Dec 17 '18 at 17:09
  • 2
    \$\begingroup\$ You probably used the 10K shown, and not say 10 ohms. ;-) \$\endgroup\$ – Spehro Pefhany Dec 17 '18 at 17:10
  • 1
    \$\begingroup\$ Yes, 1 to 10 kohms is fine but certainly not 10 ohms : ) \$\endgroup\$ – Verbal Kint Dec 17 '18 at 17:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.