Determine polarity of a capacitor electrically

Discussion in 'General Electronics Chat' started by SoftwareGuy, Sep 11, 2015.

1. SoftwareGuy Thread Starter New Member

Oct 29, 2013
29
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Is there a convenient way to determine polarity of a electrolytic capacitor that isn't visible to you (you only have access to the wires)?

In short, someone is helping me seal a large number of 10,000uF electrolytic capacitors by dipping them in plastidip. They will have black and red wires attached to the terminals before they are dipped, but I'm not the one attaching the wires so I need a way to electrically verify that the wires are on the correct pins before I put them into service. Basically I'm looking for human assembly mistakes. Is there any easy way to accomplish this electrically? I want to make a small test rig that gives me a green LED when the capacitor is connected properly, and a red LED when it's connected wrong.

2. Kermit2 AAC Fanatic!

Feb 5, 2010
3,851
967
You might try ohming out the wires to the metal case. In some varieties of electrolytic caps the negative leg is common to the can. If not electrically common it may show a low but noticeable resistance and not continue to climb to infinity.

3. Roderick Young Member

Feb 22, 2015
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This would be an interesting academic exercise, and I'm sure someone will come up with a way to do it.

In a manufacturing environment, I would suggest the potting should be the last step after all testing is complete, including polarity of wires.

If you are at liberty to say, why do you need to encapsulate the capacitors?

4. dl324 Distinguished Member

Mar 30, 2015
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This is from cap manufacturer Cornell Dubilier:

Connect the negative terminal of a cap of known polarity in series with a cap of uncertain polarity. Apply voltage and measure the voltage across each cap. If they're wired negative to negative, the cap of known polarity will get the full voltage. If they're wired negative to positive, they'll split the voltage.

Thanks for your question. I learned something today

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5. alfacliff Well-Known Member

Dec 13, 2013
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I just tested the leakage on a 13,000 mfd 50 volt electrolytic at 15 volts, it tested leaky reversed, and good foreward. and a 100 mfd, 25 volt cap at 10 volts, also leakey reverse, but good foreward. electrolytics are "formed" at their normal voltage and polarity when manufactured. other types might ot test the same, but if you keep the voltage below their ratings, they might not go bang.

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6. dl324 Distinguished Member

Mar 30, 2015
3,378
651
When I researched this, I found a reference to a NASA document (which was a dead link) that said reverse voltage ratings were conservative and that applying a sufficiently low reverse voltage would not cause permanent damage. The capacitor was said to heal when voltage of the correct polarity was applied.

7. alfacliff Well-Known Member

Dec 13, 2013
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thats why I tested them at low voltage and retested them when through.

8. MrSoftware Active Member

Oct 29, 2013
545
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@dl234 - Thanks, that is perfect!! I can make a test jig for this in minutes. I was trying to figure something out with a capacitor charging circuit that sensed whether charge went into the capacitor or not, wow did I miss the simple answer.

@Roderick - These are to be used on all-terrain vehicles. In short, if the vehicle battery is damaged or 100% dead, the motor will stop running and will not restart, even with the manual starter, and the driver is stranded. Adding a little capacitance to the system resolves the issue and the motor starts and runs fine sans-battery. It has a 3-phase alternator and I believe they're using a snubber type voltage regulator. My theory is, when the regulator shorts the coils to control the voltage, without any storage in the system the voltage drops instantly to 0, interrupting the engine ECU. I plan to put the scope on it to see if this is really what's happening, I just haven't had time to do it yet. Anyway, these caps are going to get wet, oily and dirty so we're dunking them in plastidip to protect them.

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9. alfacliff Well-Known Member

Dec 13, 2013
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thats the way that basicly people ran older motorcycles without batteries for racing. rig up a way to disconnect the lights and other loads too, when starting, that load will drain the caps too much.

10. MrSoftware Active Member

Oct 29, 2013
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That's interesting, I didn't know the older bikes had trouble running without a battery, and you have a good point with the lights. These are bikes too and we've tested some of them with the caps in place of the battery and they actually kick start with the lights on, but you have a valid point and I will definitely test this on every model affected (there are a few models). These things have EFI so the alternator is designed to put out enough juice at kick-start speeds to run the fuel pump, ECU and injector. I'm guessing the alternators on the older bikes probably weren't designed with that much low-speed output as a requirement.

Thanks again to everyone for the help and info!

11. alfacliff Well-Known Member

Dec 13, 2013
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it kind of was dependant if they had wound rotor alternators, or magnets.