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Old 4th April 2011, 02:01 PM   #11
oshifis is offline oshifis  Hungary
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The professional method (which I use ) is to charge the capacitor with a Megohmmeter in 10V increments and see the isolation resistance go over 10Mohm under voltage. Same can be achived with a variac.
Megohmmeter IM6 Radio Radiometer;
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Old 4th April 2011, 08:38 PM   #12
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Quote:
Originally Posted by oshifis View Post
The professional method (which I use ) is to charge the capacitor with a Megohmmeter in 10V increments and see the isolation resistance go over 10Mohm under voltage. Same can be achived with a variac.
Megohmmeter IM6 Radio Radiometer;
I don't understand this term:
"the isolation resistance go over 10Mohm under voltage"
mean you, that the isolation resistance is much lower than 10 M-Ohm without DC voltage bias between the positive and the negative connectors from your electrolytic capacitor?

BTW - I am looking for a electrolytic capacitor analyser. The most important thing for me is to get a graph about the frequency dependent electrical serial resistance (ESR) while various ripple current surge loads and various ambient temperatures.
What model is here the best choise?

Last edited by tiefbassuebertr; 4th April 2011 at 08:41 PM.
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Old 10th March 2013, 09:03 AM   #13
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I have a question on NOS caps not mounted in a circuit
In order to assess their conditions could it be useful to charge it to the working voltage and then disconnecting the PS and see how the voltage across caps terminal varies with time ?
how long should it take for a cap to discharge with open terminals ?
I think that caps are the most strange of the components ... they act in a mysterious way
Thanks and regards,
gino

Last edited by ginetto61; 10th March 2013 at 09:07 AM.
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Old 10th March 2013, 01:01 PM   #14
AndrewT is offline AndrewT  Scotland
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The current draw of the voltage measuring equipment will interfere with assessing the leakage current.
Even measuring across a 10k charging/testing resistor needs a small correction for the Meter resistance. Modern DMMs set to Volts are usually >=1M check the spec sheet.
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Old 10th March 2013, 01:05 PM   #15
AndrewT is offline AndrewT  Scotland
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Use the formula
deltaV per second = I / C

A 1uA of leakage will reduce the voltage of a 10mF capacitor by about 0.01mV per second.
i.e. about 1mV per 100seconds, or about 1V/day

If you measure (intermittantly) a loss of 8mV over 100seconds then the 10mFcap has 8uA of leakage.
Compare that to the cap datasheet.
Expect a reformed cap to perform about 100times to 1000times better than the specification leakage value, when you need to throw the capacitor away, or reform back to a very good value.
It's this reforming reducing the leakage current that I attribute many Members opinion that caps need burn in time. Balderdash!
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Last edited by AndrewT; 10th March 2013 at 01:23 PM.
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Old 10th March 2013, 02:11 PM   #16
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Thank you very much indeed for your advice.
Caps are a unavoidable nightmare
They are the only component changing with time
They are absolutely unstable. I am referring to electrolytics of course
Their characteristics vary with time, they have a life time, a shelf life time, they short, they leak, they can even explode .... what a mess of component !
But they are unavoidable
Thanks again and kind regards
gino
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Old 10th March 2013, 02:59 PM   #17
forr is offline forr  France
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I've measured the tan-delta and capacitive values of many old electrolytics.The number of them which could be said to be bad is very low. The last ones tested were four examples coming from the 10 W power amplifier fitted in the Revox A77 recorder which is 40 years old. They were FRAKO Elko 2500 µF 35-30 V. They had no voltage across them since years. For the four examples, the capacity was higher than 2700 µF, two around 3000 µF. Tan-delta was 0.1, one of them 0.12.

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Old 10th March 2013, 09:03 PM   #18
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by forr View Post
I've measured the tan-delta and capacitive values of many old electrolytics.The number of them which could be said to be bad is very low.
A refreshing breath of common sense in the recapping madness.

Yes, it is true that a number faults on older equipments can be traced down to failing Elytics.
But!!!
-this doesn't mean all older Elytics are faulty
-this doesn't mean all faults on older equipments are caused by Elytics

Measuring the leakage of E-cap is not difficult:

After some hours of reforming, use the following arrangement.

Note that a DVM in the 200mV range also behaves as a nanoamperemeter, 20nA full scale
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Old 11th March 2013, 11:58 AM   #19
timpert is offline timpert  Netherlands
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Measuring leakage is one thing, but measuring capacitance another. Not every DIYer has the resources to do the latter, while degraded (dried-up) capacitors that fall short on capacitance may still pass the leakage test and be safe in terms of not blowing up when you turn on the equipment.

I have had a couple of devices (most notably, a lab power supply that was given to me) that had no visible capacitor failures and would work on first glance, but all caps were out of spec, some so much that it didn't matter anymore that they were even there. Think of a 3300 uF reservoir cap measuring only about 100 uF. The capacitors that were mounted close to the heatsink had degraded most. These caps were significantly younger than 40 years, but they all required replacement. After replacing the caps (which was only a € 10,- job at most) the power supply worked according to spec. So old capacitors are a mixed bag at best. Some very old ones may be very good still, while others may be completely dead. Unless they are very large, they are not that expensive, so it would probably be best to replace them when in doubt about the amount of life left in them.
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Old 12th March 2013, 04:25 AM   #20
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There was an article on Nichicon site. It also specified which caps to discard if...
Don't know if it is still there.

Gajanan Phadte
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