Method and capacitive discharge apparatus for aluminum anodizing

Abstract

An improved electrical power control apparatus and method for use in an aluminum anodizing system includes a pulsed DC power supply and an automatic switchable shunt discharge unit connected in parallel across the anodizing cell. The cell, consisting of an anode, the aluminum part or workpiece being anodized, the electrolyte bath of sulfuric acid or the like and the cathode, forms and inherent capacitance that retains charge when pulsed with positive current flowing into the workpiece through the anode from the power supply. Between positive current pulses produced by peridoic firing of the SCRs in the power supply, the automatic discharge unit shunts the accumulated charge from the anode to cathode, thereby discharging the inherent capacitance. This markedly lowers average DC processing voltages, reduces the chance of damage to the workpiece due to overvoltage, reduces processing time, and can improve the quality of the anodized coating. The timing of the discharge is controlled automatically by control circuitry which monitors the positve current from the power supply with electrically isolated magnetically coupled sensing devices such as current transformers.