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.
Reforming Electrolytic Capacitors The process of reforming an old aluminum electrolytic capacitor consists of the application of rated voltage, through a resistor, for a period equal to five minutes plus one minute per month of storage. The electrolytics appearing on the surplus market have often been in storage for a very long period indeed.
Principle of electrolytic capacitors Electrolytic capacitors consist of two electrodes (anode and cathode), a film oxide layer acting as a dielectric and an electrolyte. The electrolyte brings the negative potential of the cathode closer to the dielectric via ionic transport in the electrolyte (see Fig. 2).
With non-solid electrolyte aluminum electrolytic capacitors the aluminum cases connect to the negative terminals by contact with electrolyte. The resulting isolation resistance may vary from a few ohms to a few thousand ohms. For axial leaded capacitors and flatpacks the case is connected to the negative lead.
1. Current and Voltage Limited Method The electrolytic capacitor is a critical part of both old and modern electronic equipment which must be used correctly in order to get the longest and safest operational life and this is particularly important with high voltage versions of these components.
These are available for momentary-duty AC applications like motor starting and voltage-reversing applications, but the high DF of aluminum electrolytic capacitors – from 2% to 150% – causes excess heating and short life in most AC applications.