AC current controlled relay
AC Current Controlled Relay schematic

Self powered solid state relay switches on when the AC current through the sensing loop exceeds the preset limit.  The trip point is adjustable in the circuit as shown from about 40 to 150 watts of load current and consumes only about 50mW.  Sensitivity can be adjusted by changing the number of turns on the sense winding (Here set to 5).  Safety isolation is intrinsic to the design.  Any SSR with an LED control input will work with this circuit, the one chosen here has a 60V-AC/DC contact function rated for 400mA maximum.

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This page last updated on June 28, 2009