When a switch closes, a transient current flows to charge the capacitance. This current may be substantially higher than the steady-state current through the system and cause contact welding. Even though the voltage and steady-state currents are within the switch specifications, damage may occur because of this high inrush current.

To limit inrush current, an impedance, such as a resistor, should be placed in series between C_int2 and C_external. This resistor isolates the unwanted effects of the load capacitance and limits damage to the relay contacts. A protection resistance, R_p, should be selected such that

V/R_p< switching current rating of the relay

The following figures illustrate the use of the protection resistor R_p.

Resistor R_p Added to Limit Current into C_external

Current Graph Comparison

The energy associated with the inrush currents in these two circuits can be expressed as

E_{without protection}= ½(C_int2+C_ext)V²=525 nJ

and

E_{with protection}= ½(C_int2)V²=25 nJ

Note   Although the peak currents are the same, the associated energy for the protected circuit is 21 times less.