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My electric lawn mower won't start, upon inspection some wires got loose and damaged by the motor. broken wire

It's a 20µF 500V capacitor. Normally I would try to short the pins with a screwdriver, but as you can see one of the pins is not connected anymore.

I don't have a power resistor. Can I try to put the leads of a light bulb socket with an incandescent light bulb to the connector + wire?

I'm used to dealing with low voltages, but I'm a bit hesitant to "mess" with a (probably still charged) high voltage capacitor.

After discharging, can I just strip the wire, heatshrink & solder the wire to fix it?

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  • \$\begingroup\$ Just strip the ends of an insulated wire and short it. \$\endgroup\$
    – Spehro Pefhany
    Aug 10, 2015 at 19:00
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    \$\begingroup\$ The repair issue is secondary to the valid issue of how to safely discharge the capacitor. This is ON-TOPIC. \$\endgroup\$
    – Passerby
    Aug 10, 2015 at 20:53
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    \$\begingroup\$ The fact is that this type of capacitor probably has enough leakage current that it self-discharged to a safe level (assuming it had a charge in the first place) in the time that it took to type up the question. \$\endgroup\$
    – Dave Tweed
    Aug 11, 2015 at 11:31

3 Answers 3

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I would take a 0.25W 10k ohm resistor, hold the ceramic body gently with a pair of pliers, bend the legs so that they will fit between one connector contact and the nick in the wire and hold it between the two. Keep in in place for a second or two, and once done use a voltmeter to check the voltage across the same two points. When it is properly discharged it should read less than 1 volt. If it reads more, repeat the above.

If you use a piece of wire / screwdriver / other short circuit you will pull a huge current (for a short amount of time) from the capacitor - you will see this as a momentary arc (spark). This could damage it - it is certainly not designed for arc discharge given the application you have pulled it from.

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  • \$\begingroup\$ You seem to have missed the fact that one of the capacitor wires is no longer attached to the connector because of the huge nick taken out of the wire. It will be necessary to make contact with the exposed copper in the nick itself. \$\endgroup\$
    – Dave Tweed
    Aug 11, 2015 at 11:26
  • \$\begingroup\$ Thanks Dave - I just thought it was a partial nick - didn't look closely enough at the picture. Will edit. \$\endgroup\$
    – stefandz
    Aug 11, 2015 at 11:27
  • \$\begingroup\$ LOL "keep in place for a minute or two": 10kOhm * 20µF = 0.2s \$\endgroup\$
    – Curd
    Aug 11, 2015 at 14:19
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    \$\begingroup\$ Perhaps not the most constructive comment, Curd, but point taken - a minute or two was a figure of speech as opposed to an exact time. Will edit - although would have been good if you had proposed an edit. \$\endgroup\$
    – stefandz
    Aug 11, 2015 at 14:22
  • \$\begingroup\$ Discharged it with a 1K resistor (first one I found). Could actually see final few volts dropping by the resistance of the multimeter. Then stripped, applied heatshrink, soldered, and heated/shrunk heatshrink. Mower works fine again :) \$\endgroup\$
    – Wannes Sels
    Sep 28, 2015 at 7:58
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Stefan's suggestion on how to discharge w/ resistor, etc., is a good one. The RC discharge/decay will have a time constant of R*C, so if you have a 10kohm R, that would be about 0.2 seconds. Give it 5 time constants to discharge 99% of the energy. If you wind up with a different resistor value than 10k, recompute the time constant and multiply by five. It's still good to check the voltage after you've made each discharge attempt since it's sometimes hard to be confident you've made a solid connection.

A few HV safety tips can be found here. If you're nervous (hand shaky, it's ok, mine do), wear rubber gloves, like the kind for washing dishes. And tape the wires down to some insulating material so they don't squirm.

The energy on the capacitor is 0.5*C*V^2. Even at full charge this is only 2.5 Joules. It would hurt but would probably only get you in trouble if you had a heart condition and applied the leads across your chest, or if you held one lead in your right hand and the other in your left. (Don't do that!)

Regarding the repair job, it should be fine to strip/solder/heatshrink.

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Just take a piece of wire and touch one end of the wire to each lead of the cap so that you have good metal to metal contact. That should do it.

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