Application Notes for Pre-charge Circuits and Capacitors

Inrush currents from capacitors are the most common cause of contactor failure. Capacitors that are not fully charged can result in excessive loads being switched. Since the discharge of most capacitors is almost instantaneous, this type of event is sometimes overlooked or not recognized.

A capacitor that has a 80% state of charge can result in twice the current of a capacitor that is charged to 90%. GIGAVAC recommends charging the capacitor to a 95% state of charge. Lower states of charge can result in currents of hundreds or thousands of amps. These currents can be measured with a current clamp or shunt and an oscilloscope set to a resolution of 0.05mS.

One way to quickly diagnose a tac weld (contacts stuck in closed position) is to cycle the coil 10 to 20 times. A typical tac weld will break as a result of this coil actuation.