Detecting the Polarity of a Large Electrolytic Cap

[EdV]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

 

[Jan Panteltje]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

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.

 

[John Fields]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

---

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.

 

[Richard Seriani, Sr.]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

Check Agilent
http://www.ate.agilent.com/pcb_test/ent_all/3070/3070_manuals/User_WN/OPTION05.PDF

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

Richard

 

[Tom Bruhns]

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.

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

 

[J.A. Legris]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

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.

 

[Jim Yanik]

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.

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

 

[Tom Bruhns]

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.

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

 

[J.A. Legris]

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.

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

--Joe Legris

 

[Jan Panteltje]

1e6 ohms * 3.9e-2 farads = 3.9e4 seconds: over ten hours.

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 |
|_____________|

 

[Jan Panteltje]

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

 

[colin]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

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 =^.^=

 

[Spehro Pefhany]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

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

 

[Michael A. Terrell]

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

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

 

[qrk]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

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.

 

[jasen]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

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

 

[Mike Monett]

a 39000 uF 25V Nippon Chemicon. It is hard to see the tiny stamped "+"
and the the terminals are symetrical to the case so incorrect
installation is pretty easy to do. Currently about once every 15
assemblies.

The vent hole has worked succesfully to prevent anyone from being
injured but boy it would be nasty if it didn't work just one time.

This assembly (terminals, cap and inductor) is an open circuit until it
is installed in the system.

Any thoughts on a non destructive test?

Thanks,
Ed V.

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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