Reforming Electrolytc Capacitors
Electrolytic capacitors occasionally need to be "re-formed", to re-generate the dielectric layer within the capacitor, especially when they have been un-used for a long time. This is a simple circuit, which will generate a suitable voltage for high voltage capacitors, and allow you to monitor the capacitor during re-forming.
Capacitor Re-former (circuit in PDF form)
The circuit was originally published in "The Radio Constructor", November 1953, and republished in "Radcom" June 1987.
The unit covers 40V to 400V with a 200-0-200 transformer. Other suitable valves are 1625, 6Y6, 6L6 etc. R4 controls the voltage, R3 ensures 0 ouput in the event of R4 failing. The meter is shunted to read 10mA, and pressing S1 increases the mater sensitivity to 1mA FSD. The capacitor to be re-formed should be connected to terminals 1 and 2, and a meter set to its 500V range connected to terminals 1 and 3. Set the unit to its lowest voltage, and allow to warm up. Then slowly increase the voltage (with R4), to the working voltage of the capacitor, checking for excessive current or heating of the capacitor - if this occurs, allow the capacitor to cool, and thn re-start at a low voltage. A couple of hours soak at working voltage should reduce the leakage to a low level. Acceptable values of leakage are about 0.5mA for an 8uF unit, and 2mA for a 32uf unit.
Copyright J.Beacon & M.Wroe-Parker 2004