This circuit will detect power line currents of 250 mA or more
without making electrical connections to the line. Current is detected
by passing a line of alternating current through an inductive pickup
(L1) as a screen diameter of 1 inch U-bolt wound with 800 turns # 30 – #
35 magnet wire.
The pickup could be made from rings of iron or other type of
processor core that allows enough space to pass one of the AC lines
through the center. Only one of the power line is the line or neutral
must come from the center of the pickup to avoid the fields cancel.
I tested the circuit with a 2 wire extension cord that had separated
the twin wires a small distance with an exacto knife to allow the U-bolt
to encircle a single cable. The magnetic pick-up (U-bolt) produces
about 4 millivolts peak for a series of alternating current of 250 mA,
or AC load of around 30 watt
The signal from the pickup has grown about 200 times the output of
pin op-amp 7, followed by the peak detected by diode and capacitor
connected to pin 7. The second op-amp used as a reference point, which
detects voltage increases above the drop pass.
The minimum mark required to cause the comparator output stage is a
positive change of 800 mV peak around a power line 30-watt. The 1458
swing op-amp output in just a couple of times the ground so a voltage
divider (1K/470) is used to reduce the voltage signal at about 0,7 volt.
An additional diode added in series with transistor base to ensure
that off when the voltage op-amp is 2 volts. You can get a piece “of
relay chatter if the AC load is close to the switching point so a larger
load 50 W or higher recommended. The sensitivity can be increased by
adding more becomes a pick-up
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