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Last Updated on: Tuesday, October 24, 2017 03:04 PM

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More  Indicator Circuits


Often there is a need to monitor when AC current flows into a load.  The circuit below works well for this application.  It uses an off-the-shelf torrid type AC-1020 current transformer from Triad, available from Mouser.  A wire carrying the AC current is routed through the hole in the center of the device.  Current flowing through that wire induces a voltage in the secondary winding of the coil.
 At a current of about 0.5 Amps, the voltage is sufficient to start turning on the green LED indicator light.  The schottky diode in the circuit prevents reverse biasing the LED and the resistor limits the peak current.  There is sufficient resistance in the current transformer winding, so even at a 40 Amps of AC current, the LED will not be driven with excessive current.  Since the current transformer offers exceptional voltage isolation, the LED can be located some distance from the transformer, using inexpensive low voltage insulated wire. 
If you wish, you can wrap more than one turn of wire around the core of the transformer to detect even lower AC current.  With one wire, the LED glows nicely with 0.5 Amps.  With two turns, the LED should glow with 0.25 Amps.  With four turns, the LED should turn on with only 0.125 Amps of AC current, which corresponds to only 15 watts of 120vac power.

Click on Drawing Below to view PDF version of Schematic

circuit self-powered AC Indicator Schematic

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