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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 Cint2 and Cexternal. This resistor isolates the unwanted effects of the load capacitance and limits damage to the relay contacts. A protection resistance, Rp, should be selected such that
V/Rp< switching current rating of the relay
The following figures illustrate the use of the protection resistor Rp.
The energy associated with the inrush currents in these two circuits can be expressed as
Ewithout protection= ½(Cint2+Cext)V2=525 nJ
Ewith protection= ½(Cint2)V2=25 nJ
|Note Although the peak currents are the same, the associated energy for the protected circuit is 21 times less.|