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Experimentally determine capacitor polarity
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jcosper:
All,

When working on a recent project, I was stupid and clipped the leads off my polarized electrolytic capacitors before discovering that the packages were not marked, and relied on the lead length to determine polarity.  :palm:

After a few moments of disbelief, I figured there must be a way to experimentally determine capacitor polarity.

I did some googling, but the methods I found seemed hokey at best (measuring can voltage when reverse biased), and manufacturer dependent at worst. Any suggestions?

Thanks!
Zero999:
I don't think I've ever seen a polarised capacitor, without a marking to determine the polarity. Please post a picture.

Electrolytic capacitors behave like poor diodes and will conduct, if reverse biased. Ensure it's fully discharged Connect it to 5V, via a 1k resistor. If it charges up normally, as expected, then the polarity is correct. If it sits at under a volt or so and never charges, then it will be reverse biased.
SiliconWizard:
Yep, and if you're potentially reverse biasing a polarized cap, do limit the current to a small value, or the cap may just explode in your hands in a matter of a couple seconds. Yes it happened to me once and I still remember it. Not fun. :-DD
duak:
Try measuring the voltage with a sensitive voltmeter - I'm assuming a DVM with a 200 mV range with a 10 M input impedance.  I just tried this on new and unused caps from 100 to 1000 uF and saw at least 50 mVDC on most of them.

I would then try charging them from 10 V (or less if the cap's rated voltage is lower) through a 1M0 resistor and observing the voltage across the cap.  You should see the voltage reach 9 V or so within five time constants (t = RC).  If the voltage goes part way and sticks there, try the reverse connection and see if it improves.
wraper:

--- Quote from: jcosper on February 29, 2020, 06:06:06 pm ---that the packages were not marked, and relied on the lead length to determine polarity.  :palm:

--- End quote ---
Sounds like nonsense. The only electrolytic caps without polarity marking I've ever seen were non-polar type. Even if there is no polarity marking, sleeve likely has certain orientation which you can compare with capacitors which have full length leads.
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