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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
