Posted on May 4, 2012

The circuit protects a solid-state relay from overloads. The circuit limits current, automatically disconnects the load after

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detecting a short circuit, and develops a fault-condition output signal. In normal operation, the controlling sets the flip-flop, IC1, which turns on transistor Ql. When Ql turns on, current flows through the solid-state relay"s input, thus activating the relay. If an overcurrent or fault condition occurs, the excessive load current flowing through the relay develops enough potential across sense resistor R5 to turn on one of the optoisolators, IC4A or IC4B. The optoisolator"s output transistor diverts current around the solid-state relay"s input, which limits the current that the relay"s output can pass. If the overload is severe enough, the optoisolator pulls the input of the Schmitt trigger above its threshold, thus clearing the flip-flop and turning off the solid-state relay. R2 has two functions: It keeps the input of the Schmitt trigger below 5 V max. to prevent latchup, and it forms an RC filter in conjunction with CI. The RC filter prevents spurious triggering of the Schmitt trigger. You can use the output of the flip-flop to signal overload conditions to the controlling uP.

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