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450V electrolytic capacitor self discharges fast. Are they good to use?


2010-04-10 1:09 pm
I have collected more than a dozen 450V electrolytic capacitors of various brands. They range from 22 uF to 250 uF in capactiry.

I want to use them in a new project. So I charged them for 10 seconds with a 15V power supply, then measured the discharge rate. Only one of them held on to the charge quite well. Majority of them dropped to less than 5V within an hour.

As they all unused, can I use those fast discharge ones in my project and assuming that the high voltage will reform them in circuit?
Or should I reform them first? What is then the acceptable self discharge rate after reforming?


2007-10-10 5:21 pm
Re-form first. If it doesn’t re-form, pitch it. Self discharge rates will vary. I use a dim bulb and variac to creep them up, progressively giving them more then checking the initial discharge rate when I back off on the voltage. Initially leaky caps should get better with repeated application of voltage - if it doesn’t it’s suspect. I might fool with a power supply for a couple hours reforming the caps slowly and carefully, but thats still less time than waiting for new caps to arrive. I expect caps initially charged to 400 volts to still have several hundred on them after an hour.


2013-04-09 9:37 pm
First reform the capacitor, charge and discharge it via a resistor serveral time and measure the final leakage current, the lower the better before use. Finally connect it to at least 400V (via fuse and current limiting resistor) see it generates any heat, the cooler the better. Disconnect before you touch the capacitor to prevent shock.


Paid Member
2003-06-12 7:04 pm
Maine USA
less than 5V within an hour.

You need more than a second (100 line cycles). A minute seems utterly ample.

And yeah. Get serious. Re-form! Full 400V-450V for a few days. Stop at 10min and 30min to check for heat. If they seriously leak the self-heat will make them hiss and spit and finally blow-up, so keep your face out of the way, and put the dog out so she won't panic.


2011-04-29 8:37 pm
Suggest that you read several mfrs spec sheets about that. For example:
https://www.nichicon.co.jp/english/products/pdf/aluminum.pdf"A voltage treatment is recommended when using a capacitor stored for a long period of time.
The treatment for an individual capacitor is accomplished by charging up to its rated voltage
through a resistance of about 1 kΩ and applying the voltage for approximately 30 minutes."
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