This AC power switch circuit senses the presence of AC current in one outlet of a multi-outlet power strip and switches power to the remaining outlet(s). It utilizes a current transformer, single transistor amplifier, voltage doubler detector, robust relay and capacitor limited AC power source.
The current transformer came out of my junk box – it has unknown specifications other than its turns ratio (1000:1) that I measured. You may obtain one from an old GFI interrupter, but beware some may be 200:1. The current transformer may be the largest circuit variable as it is used in this case as a potential transformer – its output is only 5mV @ 65mA AC. To me, this voltage seems unexpectedly low so you may expect different results – I do not believe that it will be lower. Its load (R6) was added for good measure – it makes no difference in its output voltage. If the output voltage is excessive, it may be attenuated simply by adding a series resistor.
Sensitivity of the power switch
My unit easily sensed a 65mA load current. Although I did not measure the sensing current threshold, I believe that it is the order of 30mA or so – low enough to sense almost any load – did not try sensing a wall wart.
Since the CT output voltage is so low, a transistor is required to amplify it to a useful signal level. I toyed with the idea of using an op amp, but its biasing was unreasonably difficult so I went with this circuit. Significant distortion is visible when the load current is 0.5A. This is caused by CT saturation and /or amplifier clipping – fortunately it is not an audio amplifier – in this case, distortion is acceptable.
Voltage doubler detector
A cascade voltage doubler is used to rectify the amplified AC signal. Its DC output voltage is the peak to peak of the AC voltage waveform minus the diode drops. To prevent potentially damaging gate voltage on Q2, a 10V zener limits the voltage.
Q2 is a MOSFET relay driver. Its gate to cathode threshold voltage is an additional variable – some are more sensitive than others. Keep this in mind.
Capacitor limited power supply
C1 is the limiting capacitor. Its AC output current is full-wave rectified by BR1. A 24V zener prevents the voltage from getting out of control when the relay is not picked up. A sensitive relay is required to minimize load on the power supply.
Isolation transformer required for testing
It should go without saying that this type of circuit is potentially hazardous because it is directly connected to the AC mains. Note that it must be polarized so that the neutral line is connected to the bridge rectifier – this helps to minimize the shock hazard. If an oscilloscope is connected, be sure that it is floating (not grounded to the line via the infamous ground pin). Then the real sleeper occurs when the serial or USB line grounds the scope to the grounded computer – makes sparks – been there, done that! So take my advice and use an isolation transformer when testing.
Photos of the AC power switch
The finished package should enclose everything within a plastic enclosure. It will be difficult, but not impossible to fit it inside an outlet strip – some have extra space on one end for a switch, pilot light, circuit breaker, transient protector etc.