Problem: Identify Positive and Negatve Capacitor Leads

EchoWars · 26th May 2005, 09:03 PM

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 · 26th May 2005, 09:30 PM

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

EchoWars · 26th May 2005, 09:44 PM

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 · 26th May 2005, 10:18 PM

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 · 26th May 2005, 10:26 PM

Quote:

Originally posted by TwoSpoons
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!

moamps · 26th May 2005, 10:38 PM

Skinned electrolytic caps (in loving memory of Fred D.)

EchoWars · 26th May 2005, 11:02 PM

I have access to the leads, but not the top of the cap, otherwise your method would work.

kilowattski · 30th May 2005, 06:05 AM

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 · 30th May 2005, 10:50 PM

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.

26th May 2005, 09:03 PM	#1
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	Problem: Identify Positive and Negatve Capacitor Leads 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)

26th May 2005, 09:30 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Hook up a scope across it, apply a small AC signal across it through a resistor, and see which way it distorts. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

30th May 2005, 06:05 AM	#8
kilowattski diyAudio Member Join Date: Aug 2003 Location: Long Island, New York	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. __________________ ----------------------------------------------- Kilowattski

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26th May 2005, 09:44 PM	#3
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	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.

26th May 2005, 10:18 PM	#4
TwoSpoons diyAudio Member Join Date: Oct 2004	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.

26th May 2005, 10:38 PM	#6
moamps diyAudio Member Join Date: Oct 2002 Location: Croatia	Skinned electrolytic caps (in loving memory of Fred D.)

26th May 2005, 11:02 PM	#7
EchoWars diyAudio Member Join Date: Dec 2002 Location: Left of the Dial	I have access to the leads, but not the top of the cap, otherwise your method would work.

30th May 2005, 10:50 PM	#9
TwoSpoons diyAudio Member Join Date: Oct 2004	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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