Reforming Electrolytic Capacitors

Warning: The voltages involved with most electrolytic capacitors used in valve based equipment are lethal. If in doubt seek expert help. The old adage - keep one hand in your pocket while it is switched on or the capacitors are charged - should be followed.

1. Current and Voltage Limited Method

The electrolytic capacitor is a critical part of both old and modern electronic equipment and it must be  used correctly in order to get the longest and safest operational life and is particularly important with high voltage versions of these components. Electrolytic capacitors rely on a chemical process to provide the insulator between the two metal plates and this process can degrade over a period of years if the capacitor has not had power applied. The result is that the working voltage of any electrolytic capacitors in equipment gradually falls. If full power is applied to long unused equipment then the electrolytic capacitors can pass excessive amounts of current that could cause a catastrophic failure to the entire equipment and a potential fire hazard to surrounding property.

The correct course of action is to ensure that each electrolytic capacitor’s insulation layer is ‘reformed’ by the application of a current and voltage limited DC supply to each individual capacitor. Current limiting ensures that the heat generated within the capacitor is kept at a sufficiently low level that damage does not occur. My preferred method is to carefully disconnect each electrolytic capacitor and apply a voltage, equal to the working voltage of the respective capacitor, via a suitable current limiting resistor to that capacitor.

For example, for a 450V working capacitor, I apply 450V DC, observing the correct polarity, via a 470K 2W resistor to the capacitor and measure the voltage drop across the resistor with a volt meter - see the following circuit.

The circuit assumes that the negative line of the power supply is connected to ground but this is not mandatory. Should the positive line be grounded instead then move the 470K resistor to the positive side of the capacitor. Care is needed if the capacitor case is internally connected to one of the capacitor wiring tags.

Over a period of time, which can be up to 24 or more hours for older components, the voltage across the resistor will fall and eventually stabilise at some much lower value. My rule of thumb is that if the voltage drop across the resistor after 24 hours is significantly more than 22V (indicating a leakage current in excess of 50 microamps) than I repeat the reforming process. If no improvement is obtained then I replace the capacitor with a new one. You may also find that very old capacitors have dried out and cannot be reformed in which case they must be replaced. A similar process may be required for new electrolytic capacitors that were manufactured only one or two years ago - I always check to make sure.


Capacitors with higher values will have higher natural leakage currents and may require a correspondingly lower value current limiting resistor. For lower voltage capacitors like 10,000uF 25v I use a 10K series resistor on a 25V DC source.


Once the reforming process is complete then the capacitor may be fully discharged with a resistor (not a short circuit), disconnected from the reforming supply and reconnected to its original circuit. As soon as any further inspections or tests on the equipment are completed then it may be powered up.


NB Another well known but separate problem with older equipment is that solid carbon resistors will gradually show an increase in their resistance and it is not unusual to see increases from 50% to 500% after periods in excess of twenty years. All resistors outside of their original tolerance should be replaced - check that the wattage and voltage ratings of the replacement resistors are suitable for the proposed application. Carbon film resistors do not appear top suffer from this problem.


2. Alternative Reforming Method

It has been suggested that a variac and a low power incandescent llight bulb wth the same working voltage as the incoming supply connected in series with the AC supply to the unit in question may be used to reform electrolytic capacitors without the need to remove them from circuit. The unit is connected to the variac and bulb and powered up with the variac set to zero. The variac is increased to say10V or sufficient to make the bulb glow weakly and then left for a few minutes after which the bulb should be glowing less brightly. The process is repeated until the variac is set to output the correct mains voltage for the unit under test.

This may well work but my concern is that the lamp may not provide sufficient current limiting to prevent damage.  One downside of this method is its use where there is a constant current drain by the unit under test - for example where there are valves present as this will cause the lamp to glow continuously and damage may take place to the valve heater emissive area when operated on low voltages.

There is an additional problem in selecting the bulb with the correct  power rating for each application.

My preference is for method 1, even though it may be more complicated and require more patience.