AC power relay switch circuit

Smart Outlet Strip: AC Power Switch with Relay

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.

Schematic of the AC Relay Switch Circuit

AC power relay switch circuit

Current transformer

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.

Transistor Amplifier

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.


Oscillograph Vce

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.

Relay driver

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.


Join the conversation!

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  • Joe Marowski

    This circuit has the following use: If you have say a light operated from a photoswitch and wish to control other lights or indicate when this light is on then this circuit would work great.

  • C.Mitra

    I would like to present my observations regarding this circuit. To me this circuit seems to be more of academic interest. As a practical project, it is overkill. What does it actually do? It simply senses one power socket, and if there is no current drawn in this, it switches off the other socket or sockets, as the case may be. What can be the practical use of such a circuit? A simpler, cheaper and safer alternative is a power socket strip with multiple sockets and on e common power switch. Just switching off the main switch will automatically switch off all the other sockets. Isn’t that magic? Well, jokes apart, can anyone enlighten me the advantage of this complicated circuit?

    • FrictorOTS

      Problem: People are lazy, and often believe their devices are “Off” when they are actually acting as power vampires, resulting in a gaming console or other TV peripheral that is electrically on, in “sleep” or “power saving” mode.
      Solution: Implement a twin-vampire solution. Vampire 1 is the TV or other primary device on the power strip. Vampire 2 is the monitoring circuit used to detect whether or not Vampire 1 is in a fully-on state. When Vampire 1 is on, Vampire 2 turns on the peripheral devices and vampires.
      Why do this? If Vampire 2 draws less power than the peripheral devices and vampires, it saves you money on your electric bill.
      Potential issues that result: Improper shutdown of devices because, people are too lazy to turn them off before turning off the TV or other primary peripheral. Potential power supply failures on said devices.

      Of course, these are the normal arguments.

      In a workshop though they can have a special place for another reason. For small wood shops you can use them to activate a local shop-vac hooked up to that specific device. For those who cannot afford full-shop systems, it is more economical to have a single shop-vac that gets moved from tool to tool and connected as needed, and adding a delayed shut-down to the secondary outlets lets you clean up before the system shuts down.
      This is great for when you’re turning the tools on and off to check your work or change settings, but will keep working with the same tool for a while.

  • Robert

    This is great article, and very usefull.
    Any update? I mean can you make the circuit board and a list of all pieces? Thanks!