Inrush Current Protection
To limit inrush current, an impedance, such as a resistor, can 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
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Current Graph Comparison
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The energy associated with the inrush currents in these two circuits can be expressed as
Ewithout protection= ½(Cint2+Cext)V2=525 nJ
and
Ewith protection= ½(Cint2)V2=25 nJ
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Note Although the peak currents are the same, the associated energy for the protected circuit is 21 times less. |