Switching Capacitive Loads

Using reed relays to switch capacitive loads, especially with high voltages, requires special care. When a switch closes, a transient current flows to charge the capacitance. This inrush current may be substantially higher than the steady-state current through the system. Reed contact welding may occur because of this high inrush current, even though the voltage and steady-state currents are within the switch specifications. Inrush currents can be controlled with series impedance, such as a resistor or ferrite, between the switch and the capacitance. Any capacitance in the system can contribute to inrush currents, whether it is in a reactive device under test or from a shielded cable.