Detecting the Polarity of a Large Electrolytic Cap

On a sunny day (5 Dec 2006 10:27:52 -0800) it happened "EdV" wrote in :

Yes, charge via say 1 MOhm, to 25V. If it does not reach 25V in a few minutes, or if a constant current keeps flowing (uA meter) it is the wrong way around likely. No guarantees, just try some tha tway to look for how it behaves.

--
Connect a 510 ohm 2 watt resistor to the + side of a 25 volt supply
that can source at least 50mA.

Connect a voltmeter from  the free end of the resistor to the - side
of the supply.  It should read 25V.

Now connect the cap under test to the voltmeter leads.

The voltmeter reading should momentarily dip and then climb close to
25V.  If it doesn\'t, the capacitor\'s in there bassackwards.

Check Agilent

The device has problems with some manuafacturers radial-lead caps, but maybe this will help.

Richard

1e6 ohms * 3.9e-2 farads = 3.9e4 seconds: over ten hours. The rated leakage current for such a large capacitor may well cause significant drop even after days. Better use a lower value resistance. Even John Fields' suggested 510 ohms yields a time constant in excess of 20 seconds, so his "momentary dip" in voltage is actually going to go on for rather a long time; it would be over half a minute to reach 20 volts. I'd expect the technique to work, and you don't need to wait till the voltage reaches that high a value to know if it's backwards, but do set expectations correctly about the duration of the test. You probably do need to test to near the rated forward voltage; I just ran a similar test on a 25V 10,000uF cap, using 25V and 1100 ohms, and noted that it allows 10V reverse. Your mileage could vary--a lot.

Perhaps an even better way to test would be to use a current-limited supply that will deliver, say, a constant 100mA until the voltage reaches the preset voltage limit, say 25V. With 3.9e-2F and 100mA, the charging rate will be about 2.5V/second. Ten seconds should get you to close to 25V. If you reach 25V and the current drops to a low level, you're OK, but beware the effects of dielectric absorption that may result in noticable current (significant fraction of a mA) for quite a long time (minutes).

Cheers, Tom

Don't test them. Instead, find an employee with good vision and give him/her a felt-tipped marker for circling the damned plus signs. We've done the same thing for LED's. Works fine.

-- Joe Legris

"J.A. Legris" wrote in news: snipped-for-privacy@n67g2000cwd.googlegroups.com:

Give them a good lighted magnifier to examine the caps,too. Maybe use a monochrome LED light source to better illuminate the markings?

--
Jim Yanik
jyanik
at
kua.net

Or maybe better, a white paint pen if it is on black plastic. And while that independent person is at it, have them mark the outside sleeve of the capacitor. I am assuming that these are screw-mount caps with no marking on the sleeve covering the can. If you mark that sleeve (boldly), then a simple visual inspection after assembly will let you know if it is in right.

Cheers, Tom

Good idea. And maybe run a test through the soldering process too. You don't want your hard-earned marks washing off.

--Joe Legris

On a sunny day (5 Dec 2006 13:27:54 -0800) it happened "Tom Bruhns" wrote in :

You are 100% right, I was wrong, yes lower resistor, better have some diode acros the uA meter as protection.

C R1 R2 uA meter plus -----|| |-----------===------===---O------- minus + - | | - k | ^ a | |_____________|

Correction: This works better C R1 R2 uA meter minus -----|| |-----------===------===---O------- plus + - | | - k | ^ a | |_____________|

In theory a small alternating current should create an average dc offset voltage, due to the leakage current being greater one way than the other, as to what frequency or magnitude that maybe needs a bit of experimentation, but obviously should be low enough to not cuase degradation.

However I think the idea of better visual identification and checking mentioned already is a better first step.

Colin =^.^=

These must be huge screw-terminal types. Suggest the assembly people go through the parts first and mark the + screw with a red Sharpie.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I would make a "+" metal stamp and either use paint, or heat the stamp enough to distort the plastic when it is pressed lightly to the can.

Some of Microdyne's oldest equipment had a linear power supply. We tested the capacitors bu watching the line current as the AC voltage was ruin slowly from 0 towards full line voltages. If it rose a little and stayed there, they were all installed correctly. One or more would show as a sharp rise in input current, and the unit was sent back to assembly to correct the problems.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Big caps like that usually have some residual voltage on them unless they have been sitting around for a long time. Use a Hi-Z voltmeter and see if you can measure something.

charge it with a constant current if the voltage doesn't follow linearly something's wrong.

for this comparison purpose some sort of bridge could possibly be used.

buy someone a red paint pen and an illuminated magnifier and get them to mark, and confirm, polarity on the caps before assembly..

replacing 1 in 15 seems like a it'd take longer than marking 30

Bye. Jasen

1. Find a replacement that is marked properly.

2. Invite the Nippon salesman over and make sure he understands why you cannot use their product until they change the markings.

Regards,

Mike Monett

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