# Problem: Identify Positive and Negatve Capacitor Leads

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#### EchoWars

Assume you have a number of polarized electrolytic capacitors. Also assume that you have access to the leads of these caps, but you have no markings on the caps to denote which lead is '+' or '-'.

Develop a way to determine which lead is which.

(I was hoping a capacitor tester would work, but it apparently doesn't care about polarity and shows the same reading with test leads connected either way...obviously it sends an AC signal to determine the capacitance)

#### SY

Hook up a scope across it, apply a small AC signal across it through a resistor, and see which way it distorts.

#### EchoWars

As a test, I grabbed a 47µf 63V cap of known polarity, and a 600 ohm resistor to ground. I monitored the voltage on the 'far' side of the cap at the '+' side of the resistor, and applied 20V P-P to it. Seemed not to matter which way the cap was oriented...waveform on the 'gnd' side of the cap looked the same either way.

#### TwoSpoons

Purely theoretical, but ...

Apply a DC bias and monitor the leakage current. I'd expect the leakage current to slowly rise if the bias is reverse polarity.

#### EchoWars

TwoSpoons said:
Purely theoretical,

Theoretical hell...it works great!! I just checked a known cap and indeed it leaked like a sieve when installed backwards. Simple... why wasn't it obvious to me?

The only thing I did different was to add a 12V 70mA lamp in series with the supply as a saftey valve.

I haven't checked any of the mystery caps yet, but that's next.

Thank you!

#### kilowattski

Be carefull not to reverse the capacitor for too long. I have seen a computer grade capacitor pop it's pressure relief and spray boiling electrolyte all over the place after only about 90 seconds of power.

#### TwoSpoons

And the smell! UGH!

I'd suggest only a volt or two of bias, most electro's can withstand this for a while. Note that this is not true of tantalums, which will die immediately.

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