A Current Actuated Relay Driver.


The idea is to sense current drawn by an appliance and automatically turn other equipment on or off when a threshold is reached.

An immediate application, and the one that led me to create the first unit, was turning on monitors associated with a Macintosh or any personal computer. Although the Mac-II provided a switched socket for a single monitor there was insufficient power for additional monitors, a modem, and a printer, all of which needed to be powered up when the start button on the keyboard was pressed.

Other applications have since come to mind:

A lamp in a critical location can be monitored by sensing the current it draws. Sensing that current and automatically turning on a backup allows for on-demand maintenance without loss of lighting.

Remote controls which turn on television sets ought also to turn on auxiliary equipment such as hi-fi amplifiers.

Would you like to turn on your yard light when your garage door opener draws current? A latching relay would leave it on until you turn it off.


Alternating current has long been sensed using current transformers which are a loop of magnetic material, usually a toroid, through which a current carrying conductor is passed. A smaller output current is developed in a secondary winding which can be used for control with no physical connection to the wire carrying the measured current. Recent developments in very low power integrated circuits have made it possible to couple the sensed current to the coil of an inexpensive relay. The bulky circuits I built for my own use are no longer necessary.

Here is a block diagram which shows usage of the driver pictured at the top:

The current sensor is at the lower left. Its output voltage, about 20 millivolts, is amplified, detected with a rectifier, and filtered to a smooth DC using a low power operational amplifier.

The filtered DC is compared to a reference using a low power comparator with hysteresis to provide a snap action.

The output of the comparator controls a high voltage transistor capable of enough current to drive a relay.

Operating power for the amplifier and comparator is derived by allowing a small amount of current to leak through the relay coil. It has to be small enough so that it will not activate the relay by itself.

An external relay built for service at 120 volts AC is connected in series with the relay driver which includes a bridge rectifier. When the output of the bridge is nearly shorted the relay sees the AC current it expects. That's important because the inductance of the relay coil increases when its magnetic path is closed and that in turn reduces the holding current.

Sensitivity is adjustable during manufacture by changing the turns count on the sensor and the load resistor, the burden. Initial units are set for 0.5 amperes rms. That's about right for sensing an appliance that uses at least 75 watts when on. Many appliances use some power even when off but that is always less than 75 watts. More sensitivity is available by using multiple turns through the sensor. The sensitivity is purposely set rather high - 0.5 amps - because it's easy to increase sensitivity by adding turns but there is no way to reduce sensitivity.

Please note that the circuit shown is a block diagram which won't work without several more components which, to keep it simple, are not shown.


One very simple way to use the current actuated relay driver is to make up an electrical outlet box which contains two commonly available duplex receptacles. You split one of them so that you can make one socket a master. The driver is wired so that current in the master operates a relay which in turn applies power to the other sockets. The relay and driver are small enough so that the entire assembly can be fit into a 4 by 4 inch electrical box. The picture below shows the idea. More details, with better graphics, are available in the application links below.


This is what makes it reasonable. The whole thing is less than a third of a cubic inch. Dimensions are in inches. There's no 240 volt version yet but those will be in millimeters. Note that the sensing hole will pass insulated AWG12 wire. More sensitivity is available using multiple turns but they will have to be made with smaller wire.


So just what relays can be used? The standby current of the driver is a bit less than 1 milliamp so the relay must open up at something more than 1 mA. That rules out most solid state relays. The driver will exhibit about 5 volts in the ON state. 100 mA at 5 volts is a half of a watt which is about as high as I'd like to see you go. So. . . the relay chosen ought to draw something less than 50 mA in the ON state. These are some I have tested. Prices for new parts are competitive and universally less than $10.00. Surplus parts abound.


Part number


Cat number








30 A






15 A






15 A

Potter & Brumfield





15 A

Potter & Brumfield


Radio Shack



10 A







Do I offer a prewired relay? No. I refuse to pay the price for "approval" of this device by the lawyers of the world. Making you wire it up is my way of making sure that you understand what you're doing and thus assume intelligent responsibility for your actions. Please don't electrocute yourself or burn your house down.


As you can probably guess, I'm a physicist and engineer - not a businessman or salesman. If you are a manufacturer of, say, surge suppressors or outlet strips and would be interested in incorporating this technology in your products I'd like to hear from you.

Douglas P. McNutt, PhD
The MacNauchtan Laboratory
7255 Suntide Place
Colorado Springs, CO 80919-1060
voice 719 593 8192
FAX 719 594 6643 (Advise by e-mail of items sent)

More Information:

I once wrote a chapter on current measurement. The content is available as a *.PDF file at Current, it's 259kB.

A series of AC current and voltage sensors and switches is available at the SRT web site. I played a part in the design of many of them. There are also some application notes there.

An application note describing installation of a relay driver in a commercial surge arresting power strip is at PowerStrip.html.

More details about the duplex receptacles described above are in QuadReceptacles.html.

© 2001 The MacNauchtan Laboratory. All rights reserved.