How to discharge a capacitor

[Mike Henderson]

A TV picture tube was often charged up to 30kV and was capable of holding the charge for days.

I can verify that! I worked as a TV technician afternoons in high school and have been zapped quite a few times on a picture tube. But as the boss told me when I started, "You'll hurt yourself more jerking your hand out than from the shock." He was right.

Mike

[Tom Veatch]

What Jack and Anthony (and maybe others) said. The start/run capacitors in an AC electric motor are there only to induce a phase shift in some of the windings in the motor. There is little or no DC component to the voltage applied to the capacitor and there will be little or no residual charge in that capacitor after the motor is shut down.

It would be a totally different situation if the capacitor was charged from a DC source, but that's not the case here.

[Jim McCarty]

My sincere thanks to all who responded. I took the cover off the capacitor and shorted across both poles with a big screwdriver and there was now spark of any kind. I was expecting something but nothing happened. Got the inside of the motor all blown out and I'll pick up the new bearings tomorrow and should be good as new. Thanks to this great site- you're the best.

[Christopher Pine]

You triggered memories of auto shop... Hint: if someone says your name and tosses you somthing, small, silver, cylinderical don't catch it!

LOL....

Funny!

[Sam Yerardi]

As an electrical engineer, I can tell you that capacitors can be very dangerous. Extremely dangerous depending on the circumstances. In the example here, my first thought is you probably won't have that problem, but as with all things electrical, you can't see electricity (sparks and lightning are the RESULTS of electricity), you can't smell it (you may smell ozone as a result of electrical discharge, but that's still not electricity), but you can feel it (that IS electricity). Before I became an engineer I worked on TVs, electronics, for many years and I can attest from first hand experience to electrical theory that there is no good way to say whether or not something is safe for touch or doesn't have enough energy to harm you. Where did you touch it? How did you touch it? Were you sweating? Where you wet for any reason when you touched it? All of these things have a role. Too many factors involved for it to be something cut and dried. As someone pointed out on the picture tube, it will store a charge and if you are what's providing a path to ground for it to discharge you could recieve anything from a harmless startling unpleasant shock that might cause you to hurt yourself physically by responding to the shock, to a shock with enough charge to knock you back very painfully (or worse). Picture tube operate with in excess of 60,000 to 80,000 volts DC. The shock that could be delivered is similar to the discharge a defibrillator delivers, but the waveforms are completely different. But there can be enough energy there to stop your heart if it is delivered the right (actually the WRONG way) way. As far as capacitors, one of the safest ways (for you as well as for the cap) is to use a resistor, and not a screwdriver or wire. For the following, let's assume that there is a charge on the cap. You can use just about any size resistor, but the keep the following in mind: A smaller ohmmage resistor will discharge a cap much quicker, and the closer the resistance is to a plain copper wire, the greater the likelihood you will draw a spark. If there is a significant charge remaining on a cap, placing a copper wire across it will discharge it but it could respond similar to putting a fork in an electrical socket. Very dramatic, loud, and may lead you to get hurt just from being startled. A very large surge of current will try to pass through the wire (or better 'short') very quickly. I typically use something on the order of 200-1000 ohms at about a couple of watts size or more. It doesn't have to be that, but if you have it available it just makes sense (unless you liked the Frankenstien movies like me and love to see lightning ). And as far whether it is AC or DC we're talking about, don't ever trust that ANY electrical component will work as claimed. Or trust that just because it is being used one way, it will be safe. Always assume the worst. Don't be afraid of electricity, but RESPECT it. Yes, we trust GFI receptacles, but even they fail. In all my years as a TV repairman up through being an electrical engineer I've learned to ALWAYS take the careful route when it comes to electricity. ALWAYS. Don't assume ANYTHING no matter what anyone tells you. Always check it yourself. I once saw two experienced electricians get injured when they took the advice of their supervisor that a 13,800 Volt bus was dead. It wasn't and they didn't check for themselves. They placed an ohmmeter across a couple phases and the meter litterally exploded. The leads vaporized. Obviously they suffered some injury. Never take ANYTHING for granted when it comes to electricity.