|[AMRadio] RE Electronic Capacitor Reforming|
k2glo at jkasystems.com
Sat Jun 7 01:07:26 EDT 2008
There has been alot of discussion regarding electrolytic capacitor leakage. Here are the "official" specifications. The JAN-C-62 specification addresses the maximum acceptable allowable leakage for electrolytic capacitors. (There is also an EIA specification but it is the same as Jan-C-62). The formula is I = (K)(C)+0.3 where "I" is the maximum allowable leakage current in milliamperes, "C" is the capacitance in ufd, and "K" is a constant relative to the working voltage of the electrolytic capacitor. "K" = 0.01 for voltages up to 100 volts; "K" = 0.02 for working voltages between 101 and 250 volts; "K" = 0.025 for working voltages between 251 and 350 volts; and "K" = 0.04 for working voltages between 351 and 450 volts. Here are some an examples, a 20 ufd @ 450 volt capacitor has a maximum acceptable allowable leakage current of (0.04)(20)+0.3 which works out to 1.1 ma.; while a 20 ufd @ 250 volts would have a maximum acceptable allowable leakage current of (0.02)(20)+0.0.3 which works out to 0.7 ma. K4KYV hit the nail on the head with his comment that the leakage must be proportional the the capacitance. As you see by the formula, the allowable leakage is proportional to the capacitance.
It seams that everyone has their own opinion and method on reforming so I will throw method in the pot since it works well for me. I sell NOS can electrolytic cans so I built a couple custom reformers to condition them. One is for 150 volt cans and the other for 450 volt cans. Each reformer can reform up to 30 electrolytic sections at a time. I will only discuss the 450 volt reformer. It is basically a 450 volt supply that is regulated so it does not exceed 450 volts as the reforming currents decrease. Each of the 30 reforming legs has has a 90K current limiting resistor in series with the capacitor which limits the current to 5 ma in a short circuit condition. Typically an unreformed capacitor will draw about 2 to 3 ma initially and leakage current usually drops of considerably in the first few hours. I can moniter the leakage current of each independent reforming leg with a 30 position switching system. The meter is switchable to read full scale currents of 8 ma, 1.6 ma, 0.8 ma and 0.16 ma. As an example, an unreformed capacitor that starts off drawing 2 ma will only have 270 volts applied to it because if the current limiting resistor. As the capacitor reforms, the leakage current decreases and the voltage increases upward to 450 volts. Many my 20 ufd capacitors will reform to leakage values well below 0.05 ma @ 450 volts. I typically reform my capacitors for 3 days and then in addition, operate them at a full 450 volts for an additional 4 to 7 days to endurance test them. One advantage of the 7 to 10 day reforming and endurance testing period is that on rare occasions some electrolytic fluid will weep through where the terminal pentrates the bottom seal. The extended testing period allows this condition to manifest itself. The 5 ma current limit is pretty safe and I have never had a capacitor explode on me even when they have had excessive leakage. I have reformed many capacitors using the above method I find the capacitors respond very well to this method.
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