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Author Topic: Measuring leak current in electrolytic capacitor  (Read 2108 times)
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mikepic
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« on: September 25, 2011, 09:40:39 21:40 »

Hello all,

Does anybody have a good method to measure or at least test the leak current in electrolytic capacitor?
I find that is one of the most common failure in electronic circuit but difficult to measure with standard electronic instrumentation that everybody of us have in our homes.
I've found some circuits in the internet, but all of them uses the 220v, and I'm concerned because I'm not sure that those circuits serve to measure capacitor with different nominal voltages (16V, 40V, 63V and so on).

Thanks in advance
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flyback
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« Reply #1 on: September 26, 2011, 06:39:28 18:39 »

The first idea would be to put an microamp-meter in serie with the cap under test to measure the leakage current. BUT if your cap is shorted, you can say bye bye to your meter.
So just insert a resistor in serie with the cap, let's say 1K, supply the whole circuit with required DC voltage (16, 40V...), wait a bit for cap to recharge fully then measure voltage accross the resistor to compute leakage current. Idealy, it should be very low.
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TomJackson69
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« Reply #2 on: September 27, 2011, 12:56:39 00:56 »

@mikepic

It is almost impossible to measure capacitor leakage in circuit. You can try to measure capacitor leakage out side the circuit but not with your bench meter. You will need a very high impedance circuit to measure the leakage with error. There are some op-amp with high impedance that you can use, such as Microchip MCP6S22. The MCP6S22 is a CMOS op-amp with very high input impedance Zin = 10^13 (that is 10,000,000,000,000 Ohms).

Forget about your DMV even with 10M Ohms impedance. Let say the capacitor is charged to 5V (assume the circuit is 5V), as soon as the DMV connects to the cap, 5V/10M ohms is 0.0000005A, that is 0.5uA draws from the cap (maybe larger than the cap leakage already). For very large cap, the DMV mat be use with error.

My question to you: Are you measuring in circuit?

Tom
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mikepic
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« Reply #3 on: September 29, 2011, 11:41:46 11:41 »

No Tom,

My idea is measure leakage out of the circuit.
As I've read, all electrolytic capacitor has a leakage that is normal for the formation of the oxyde layer inside dyelectric. So, I supose that for verify properly a capacitor, I must demonstrate that the leakage is lower than that current.
Here is the table published in Silicon chip, n.259
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TomJackson69
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« Reply #4 on: September 29, 2011, 06:15:19 18:15 »


“I've found some circuits in the internet, but all of them uses the 220v”

Why don’t you convert the 220V circuit to 110V circuit and try out to see if it works? I assume you mean 220V is a supply voltage to the circuit.

Post your 220V circuit, Sonsivri members may give you an idea to convert to 110V.

Tom
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mikepic
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« Reply #5 on: October 01, 2011, 08:50:32 08:50 »

The problem is not related with the supply of the circuit.

The problem, I think is that I should adapt this voltage to the nominal voltage of each capacitor (or at least each nominal voltage of the capacitor) I want to measure
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TomJackson69
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« Reply #6 on: October 03, 2011, 06:35:54 18:35 »

@mikepic,

To measure capacitor leakage, you don't have to charge the capacitor to working voltage. For example, with a 10uF/50V, you can charge the cap to 5V then measure the leakage overtime. The voltage drops determine the leakage current.

Tom
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oldvan
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« Reply #7 on: October 03, 2011, 11:39:43 23:39 »

Tom,

Leakage current increases with applied voltage, so measuring a capacitor intended for use at 40V using a 5V supply would give a mistaken understanding of the capacitor's leakage.

A good read on the subject is HERE.
« Last Edit: October 03, 2011, 11:47:38 23:47 by oldvan » Logged

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TomJackson69
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« Reply #8 on: October 04, 2011, 01:36:02 01:36 »

Hi oldvan,

I may be wrong when said “you don't have to charge the capacitor to working voltage” if you are going to analyze the leakage current of a capacitor. Sorry for that.

But for quick solution of finding the leakage current of a capacitor, this method and the following equation can find an average leakage current fairly easy.


                    Current Drain (in Amp)
Capacitor (Farad) = ----------------------------------
                    Voltage Drop (in Volt)
                    ----------------------
                    Time (in Second)

Since, we almost never apply the voltage up to working voltage; we tend to apply a voltage lower than specify working voltage. That is why I said “you don't have to charge the capacitor to working voltage”.

I am interesting in learning your corrected way.

Thank you,

Tom

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mikepic
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« Reply #9 on: October 04, 2011, 08:13:46 08:13 »

Great article, oldvan.

I said to measure the leakage current at the nominal (or near it) voltage because is known that measuring the leakage of a capacitor with a standard tester (that ususally uses a internal battery of 1,5 to 9 volts) is not enough to verify it. The imperfection of the dielectric may cause depreciated leak at that voltage, but important, for example, if you apply 300 volts.

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« Reply #10 on: October 04, 2011, 09:38:43 21:38 »

Here's an app note from a battery maker with a measurement method on page 6 outlined

http://www.tadiranbatteries.de/eng/downloads/Both_Eng.pdf


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