How to discharge a capacitor

[Jim McCarty]

As some of you know from an earlier question, I'm having to replace the arbor bracket, arbor and bearings on my 30 year old Unisaw. It was suggested that I replace the motor bearings also while I was this deep into the saw. Sounds like a good idea. I've done a little research and it seems like a simple enough job if I'm careful and don't pinch any wires. One comment I read concerns me though. It had to do with discharging the capacitor. Is this necessary? It's a 3 hp rockwell motor with one capacitor and hasn't be run for about 5 days. I build houses, but this is really out of my league. Any advice or tips would be really appreciated. Thanks to all. Jim

[Cary Falk]

Jim,
If there is any charge on the capacitor then if you touch the ends you will get a good zap. If your are worried about it, disconnect the cap and and set it aside till you are done. You can discharge the cap by putting a screwdriver across the terminals. Make sure you are holding onto the plastic and not in contact with the metal part.

[Mike Henderson]

After several days, it's highly unlikly that the cap would have a charge on it - if it had a charge on it to start with.

If you're worried about it, just use anything metal and touch the two contacts. That could be a piece of wire (strip the ends and hold the insulation), or a screwdriver, or anything else.

But even in the absolute worse case where you would receive a shock from the cap, it would mostly surprise you and not hurt you.

Mike

[Brian Dormer]

I'll have to respectfully disagree with Mike on this one. That cap most certainly COULD be holding a SIGNIFICANT charge. Don't try to disconnect or move it without discharging it.

The proper way to discharge a cap (or a TV picture tube) is to short the positive and negative terminals together with some HEAVY wire (heavy enough that the current flowwon't vaporize it). If you have some 12 or 14 gauge wire, that should be adequate.

Take a length (1-2 feet should be plenty) of insulated wire and strip a 1/2 inch or so of the insulation one end. Securely attach an alegator clip to the end (screw it or, even better solder it on). Strip 4-6 inches off the other end of the wire and wrap it around the shaft of a BIG screwdriver with an INSULATED HANDLE (like a big Craftsman with a plastic handle). Wrap the wire/screwdriver shaft with electrical tape. Leave the last inch or so of the screwdriver shaft un-taped.

Put on some heavy gloves (at least on the hand holding the screwdriver).

CAREFULLY Clip the alegator clip to ONE terminal of the cap. Touch the end of the screwdriver to the other terminal of the cap. You may or may not get a spark - so be prepared (it can be a pretty big pop - but not hearing or seeing anything isn't unusual either).

Keep the end of the driver on the terminal for a 3 count. Then pull it off. Wait about 20 seconds and repeat this procedure at least 2 more times. After that - the cap should be discharged completely.

If the cap has more than two terminals - do this procedure starting with the clip on each terminal - touching every other terminals at least 3 times.

Big caps can easily store enough juice (for a very long time) to kill you - so don't mess around with this unless you are reasonably sure of what you are doing.

I've been doing this stuff for over 30 years and I still get sweaty palms when I deal with circuits like this. I've only been zapped a couple times - but thats enough to give me a good deal of respect for what electricity can do.

[mark page]

I second what Brian stated. It reminds me of the good ole days of charging distributor condensors and laying them around the shop. People always seem to grab them by the wire. lol. Or lighting small lightbulbs on your nose from high voltage electric motor armature testers.

[Fred Woodward]

There's a huge difference between a picture tube and the starter cap on your saw. The TV tube will hold a far greater charge for a longer time than the saw. Still, you could get a little zap from the starter cap after a few days.
Unplug the saw. Short the terminals of the starter cap to each other or one at a time to the frame of the saw with either a length of wire or a screwdriver tip. If there are multiple connections coming from a common post on the capacitor, shorting one of the connections is good for all that are common. On a large capacitor or high voltage circuit (your color picture tube or microwave) you might be better to put a resistor in line with the wirer you are using to short the cap to reduce the amount of current drawn on the discharge. Protect you and the component. Most high voltage capacitors like in a microwave have a built in resistor to automatically discharge the cap when the voltage is removed from them but to be on the safe side, always short them to the chassis or frame to be sure.
The injury you would most likely receive if you discharged the start cap through your finger would be the pain in your elbow when you jerked back and hit it on something. Don't ask me how I know that.
You'll be OK but make sure the saw is unplugged before you mess with any of the electrical circuits.

[Greg Funk]

If you're worried about it measure the voltage across the terminals. It's highly unlikely there is any voltage on it. The starter cap on a motor is not charged up like the caps in an old TV.

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

[Jack Ganssle]

No, there's no charge on it, especially after 3 days.

The cap in a TV is in the TV's DC circuit. A rectified AC waveform is applied to the cap, creating a clean DC signal. That DC charge stays there for a time after the TV is turned off, and can be dangerous.

The cap on a motor has an AC signal applied to it. The charge cancels out 60 times a second.

Besides, motor caps are quite leaky. Even if there were a charge, it would self-discharge very quickly.

[Rick Levine]

I had to replace some capacitors in a high powered electronic flash many years ago and I was told to use a large resistor to do it. If you use a screw driver you could possibly damage the capacitor. At least that what was I was told by the company that manufactured the unit. You'll have to determine the size of the resistor needed. I used a 60 ohm resistor if I recall correctly. At least that is what I remember. I'd check with an electrician first though.

[Rob Russell]

Ummm - a capacitor discharge is what defibrillators use.

The start cap in a motor could most certainly have a charge. The proper way to discharge it is with a heat resistor. You could use an old stove element and alligator clip that to the cap.

[Gene O. Carpenter]

Yes, some capacitors will in fact be toasted if you short across the terminals! When I was still "Techin appliance's" we were each issued a 3/4"X2"X2" block of Particle board with a high end cap with 2 leads with insulated alligators AND strict instructions to cease shorting cap's with a screwdriver!
I had always used my volt meter set on 440V to drain them and the Co. said NO to that practice too!
Old habit's are hard to break!
Gene

[Greg Peterson]

No harm in correctly discharging these caps. If they have no stored energy, no harm no foul. If they do in fact still have any energy still stored in them, correctly discharging them is well worth the effort.

[David Epperson]

I had always used my volt meter set on 440V to drain them and the Co. said NO to that practice too!
Old habit's are hard to break!
Gene

Using a large enough resistor to lower the outrush current would reduce the effect. But since these motor caps are used to alter the phase angle on AC motors and not to store rectified DC there really should not be much of a charge left. Simply attempting to turn the motor on with it unpluged from the power source would be the easiest way to assure that it is discharged.

[Jim O'Dell]

I second what Brian stated. It reminds me of the good ole days of charging distributor condensors and laying them around the shop. People always seem to grab them by the wire. lol. Or lighting small lightbulbs on your nose from high voltage electric motor armature testers.

Or charging one of those ignition caps up and dropping it in someones pocket!
In automotive class in Jr College when studying electrical circuits, the teacher made all of us get in a circle and hold hands. He held one leg of a charged cap, and made the next one to him grab the other end. You could feel the charge go in one hand up the shoulders and out the other hand all around the circle. Didn't hurt at all, but sure taught you about being careful when you opened up a distrubutor!! Jim.

[Anthony Whitesell]

There should not be any change remaining, not because they will discharge on their own (which is a load of @#$%, I am an electrical engineer and deal with them hourly), but because the two sides of the capacitor are connected via the internal windings of the motor. If you can find a quick schematic of wiring a motor you'll see that capacitor is connected across the windings and back to the wires from the power switch. But if you want to be sure, a resistance across the terminals would be perferred but a screwdriver will work. Don't jump too high when if it makes a little spark which most likely it because it shouldn't be charged.

P.S. Don't forget to unplug the saw first.