If you have a bunch of capacitors on the shelf, surely you will even face this. Once upon a time, I plan to use my old capacitor (we are talking several ‘few’ years, not 15 or 20 years or so). Off course as wise ‘technician’ says: better safe than sorry. So test your old stuff first before putting them into the real circuit.
When I test with a basic Ohm-meter testing with my Sanwa analog meter, I find some of my capacitor not totally back to infinity during the testing. It means there is some leakage on the capacitor. Mostly, such leakage is recoverable with reforming them (I will discuss this later as I need to use a power supply, some high wattage resistor, etc). So don’t throw them to the recycle bin directly.
See below the measurement result. The needle doesn’t move back to infinity.
The better way to reform such capacitor is by giving a controllable voltage at its rating point with a resistor in series. Then we can see the current movement inside the circuit with a Volt-meter across the resistor.
Off course the best way is to use specific reformer device, like Sencore LC-102 (which I’m too lazy to power her up).
So, what is the laziest way to solve this (and hope it works)?
The laziest way is by giving the capacitor a voltage (good if you can source an exact voltage with the capacitor’s rating). If not, using a lower one should be fine. I use 9V battery pack to charge the capacitor. My capacitor is rated at 25V though.
Then we have to discharge the capacitor. You can use 100-220K resistor across the capacitor. I prefer a simpler method by connecting the capacitor to a load (the load is my LC meter). Capacitor with 2200uF size could power the LC meter for several seconds. Nice! You can see my LC meter is running with capacitor powered power source 😉
Repeat this process for several times. After repeating them few times, do another measurement with your analog Ohm-meter. Make sure you discharge the capacitor first before testing it. Otherwise, it may bring some trouble to your analog Ohm-meter. As you can see below, after discharged, the capacitor only holds around <1V DC voltage. Anything under 2VDC should be ok for testing with your analog Ohm-meter (or any LC meter). Never test capacitor when it holds a charge (trust me, you don’t have to try it).
Now the capacitor could turn to infinity during testing with analog Ohm-meter. If you haven’t got luck yet, please repeat for several times (charge-discharge process). If you still don’t get the luck, please wait for next article (or search around on the Internet about reforming capacitor).