Switching Capacitive Loads

NI Switches Help

Edition Date: April 2015

Part Number: 375472H-01

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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.

Resistor Rp Added to Limit Current into Cexternal

Current Graph Comparison

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.

Related Topics

Inrush Current


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