A Homemade Electric Fence Charger

This homemade electric fence charger was created when wandering livestock caused crop losses, includes information on the circuit board charger, a detailed diagram and materials list and instructions.

| July/August 1982


This homemade fence charger is, of course, hooked up in the same way as are the store-bought ones. We simply hung the box on a convenient fencepost, placed a 12-volt auto battery-raised off the ground -beneath it, and drove a 4-foot bare steel rod into the earth near the site.


This homemade electric fence charger was created when footloose livestock caused crop losses resulting in this livestock management idea. (See the detailed electric fence diagrams in the image gallery.)

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, this homemade electric fence charger 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.


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.

7/28/2016 11:05:45 AM

Normally fence chargers are rated by how many joules energy they deliver. Do you have any idea what output can be expected from this unit (or how to calculate it)?

7/19/2014 1:29:13 AM

It was a 12vdc relay with a current rating of 10amp 120vac/28vdc. They don't sell it in the stores anymore. Online they do sell the NTE R73-5D10-12 which I believe to be similar to the 275-216 in size and specs.

7/27/2011 6:04:47 AM

yeah, that would really help me too. I'm in uk and i haven't seen radio shack (or tandy trs as i seem to remember they are called here) for years. Also, is it me, or is the annotated picture of the circuit board a teeny bit hard to make out? I'm planning to use stripboard or breadboard to make mine to keep costs down. is there any reason this would be a bad idea? thanks!

eugene pretorius
7/21/2011 1:45:19 PM

Hi I am looking for the Specs on the Relay used for the electric fence charger I have looked on Radio Shack for the part number but it does not exist.

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