Mother's Electric Fence Charger

Image Gallery

Footloose livestock and the crop losses that resulted combined to "spark" our creative talents.

Whether you're growing vegetables or raising critters (and especially if you're doing both), you know the importance of keeping hungry livestock in their place. Of course, barbed wire will usually serve as an effective deterrent to all but the most determined animals. For those few headstrong beasties, though, an electric fence might be just the ticket . . . and it requires less sturdy, and thus often less expensive, posts than would a barbed wire enclosure.

However, if you were to buy a fence charger at your local farm supply house, that piece of equipment would probably set you back between $25 and $35. You'll be glad to know, then, that it's possible to make this component on your own-using readily available, and some easily salvageable, electronic parts and the information given here-for as little as $10 or $15.

HARDWORKING . . . AND HOT!

The charger we put together is powered by a 12-volt automotive battery, and can deliver an attention-getting 25,000 volts of electricity to the fence strands once every second. Built around a standard car ignition coil, the device hasn't enough amperage to seriously harm or to kill an animal, but its "bite" will certainly serve to reinforce the concept that territorial boundaries do exist!

Essentially, the salvaged coil does much the same job as it did under the hood of a car, but a simple relay replaces the breaker points. When that device's contacts are closed, current flows through the coil, which has the ability to store energy. Then, when the contacts come open (thus breaking the circuit), the pent-up power has no place to go, and the magnetic field collapses . . . inducing current in the coil's secondary winding. Because there are many more turns in this latter wrap than there are in the primary winding, the voltage is amplified considerably and then passed directly to the conductive fence wire.

The relay is energized for only an instant - actually about 15/ 1,000 of a second since the current flow is controlled by a single basic integrated circuit. Housing 32 individually connected transistors, this 79d electronic marvel generates a pulse (with a little help from a couple of timing circuits) every second or so, activating the relay. A variable resistor can be used to adjust the rate of pulse.

BUILD IT FOR A SONG

click here to enlarge

For the sake of your parts-shopping convenience, our materials list includes standard Radio Shack components and catalog numbers. The items can all be purchased from your local retail outlet for about $11, with the exception of the ignition coil and the printed circuit board.

The coil-which must be from a conventional automotive ignition system rather than the newer transistorized versions should be easy to scrounge, or perhaps purchase for a pittance at a wrecking yard.