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As current flows through the relay, power is dissipated in the contacts as represented in the following equation:
P = power absorbed by relay contacts
I = inrush current through the module
Rarc = resistance of the arc that exists between the relay contacts during bounce
R contact = resistance across the relay contacts
Rarc dissipates as heat, raising the temperature of the contacts. Inrush currents momentarily expose the contacts to very high power levels. The energy associated with the inrush current may be sufficient to melt the contact surfaces after bouncing and weld the relay contacts closed. Inrush currents usually decrease rapidly to the steady state levels. Contact bounce, however, can subject the relay to multiple inrushes per activation, causing further damage.
The following figure shows a relay within a switch module connected to a voltage source and a load. Cint1, Cint2 and Rcontact always produce an inrush current proportional to the input voltage. The duration of the inrush current increases (as does contact heating) with the addition of external capacitance at the load. Inrush current flows while load capacitance Cint2 and Cext are being charged.
where = time constant
In this figure, Cint 2 + Cexternal = C and = RcontactC > RcontactCint 2
When the voltage across Cint 2 is not the same as (Cint 2 + Cexternal), closing the relay causes momentary inrush current equal to the voltage difference across the relay, divided by the resistance of the relay and associated wiring.
To prevent damage to these modules, place a resistor in series with your load as detailed in Switching Capacitive Loads .