Help: How to build an electric fence unit
 

As a kid I built an electric fence and used a car ignition coil and the coils out of the old style voltage regulator to fire the coil. Now I need to build another one and am looking for a circuit to drive the coil. There are many on the internet, some using a 555 transistor and others using a signal generator, but every one I have looked at has some sort of problem.
Does anyone have a simple circuit that I can use? 120 VAC to a wall wart or similar would be nice instead of a battery. Want to use an auto ignition coil for the HV and not a flyback transformer or the flash unit from a camera.
If I was north of the border I would just go to TSC or similar and buy something.

Interesting project Mexstan,

What is it you are hoping to keep in (or out) with this fencer?

Would it not be more feasible to have a prebuilt unit shipped to you?

Are there no "Factory" fencers available in Mexico?

This reminds me of years ago, when I was a little kid our fence was hit by lightning and fried our fencer.

So we rigged up an old "D" Case tractor and ran a wire from the coil (or magneto, I forget) and left the tractor running for a day or two until the fencer could be replaced.

Kept the cows in anyway.

Need to keep out possums and cats. We have some fruit trees in our yard and the possums love them and then leave their 'calling cards' all over the lawn. Suspect that some cats or other critters are doing the same as it is a daily cleanup job. My wife is going nuts about this, so need to find a way to keep the peace and sanity around here. Figured a strand or two of hot wire on top of my fence will do the job.

Yes, there are fencers available, but are expensive. I will buy one next time I head north, but that may not be for another month or two. Unfortunately I was in a TSC store about three weeks ago, but at that time did no know I need this. I can have a unit shipped down, but the transportation costs and possible duty will double the cost at least.

Until I can buy a factory unit, I can going to jury rig an old car coil up and see what happens. I can do it in more than one way, but was wondering if there was an easy (modern) way to pulse the coil.

Ok, here is a simple circuit that might work using a 12 volt DC wall wart for power. The 12 volt power slowly charges the 10K uF capacitor through the 100 ohm 2 watt resistor. When the voltage on the cap gets to about 8 or 9 volts the 12 volt automotive relay will pull in and dump the capacitor charge into the ignition coil causing it to charge up magnetically. The relay will drop out at about 5 volts and the coil will dump its magnetic charge zapping the cat attached to the high voltage arrow. The big capacitor in this circuit is one of the computer caps that is about the size of a beer can. The small 0.1uF cap will cause the ignition coil to ring and provide a longer zap.

I have not tried to build this but I am thinking it will zap about once per second. Changing the resistor value or large cap value will change the zap rate. The wall wart needs to supply about 200 mA. The Automotive relay is a common one used for lighting with 30 amp contacts. Just make sure it has SPDT (single pole double through) contacts.

If you try this let me know how it works aand be sure to use a good ground rod.

Took a look at the schematic and it looks like it may work. First one I have seen using that relay. I already have most of the stuff, so will go out tomorrow and buy the rest and give this a try.
Thank you. Will let you know if it works.

Find a transformer from an old boiler system... They put out high voltage and run on 120V. Lemme' do some more research on it. Years ago, my friend and I were going to design a security system for his garage windows after it got broken into using strands of metal beads as like a curtain but then electrify them...lol

dont forget to to have a leak on the fence to test it out

My friend Jim swears by his old ballast out of a discarded florescent light fixture.
Its nailed me almost every time Ive been to his place.

Make sure you use the old-style ballast,not the magnetic type.Got a Model T spark coil?

Not seen that one. Got a schematic?

Be careful about using Fluorescent ballasts!
The Instant Start type (single round pin on ends of bulbs) puts out a whopper of an ignition pulse which will bite the hell out of you. But the dangerous aspect of using any fluor ballast is they are designed to output continuous AC, limited to around 450milliamps which is many times the amount of current needed to kill a human being!!!
You will definitely shock the hell out of a possum but will likely do a pretty fair job towards incinerating parts of him too!
You wouldn't want to do that to a person admiring your garden.


Around '81/'82 timeframe, I had a new Toyota diesel PU truck that I parked under an attached carport.
I had trouble with feral cats from the surrounding woods sleeping on the warm hood of my white truck and chewing blood-filled fleas which stained the paint. Whenever I would open the apt. door, the cats would scratch the heck out of my hood while running away. I thought about rigging up a high voltage transformer from a radio transmitter but it would output way too much current...
I went down to TSC and bought a current limited, AC powered fence charger, wire, a box of insulators and a couple screen door springs. I made a perimeter fence around my carport about "nose high" on a cat. The screen door springs allowed me to drive over the wire at the entrance.
I initially tried it out with a bit of tuna set on the floor in my carport while I watched as the first feral cat approach.. He approached the wire which was new to him which made him pause, but Charlie the Tuna did the trick.
They almost always leaped backwards before running off at high speed. And if it was their first time, the would turn back around to look as if to say "what the heck was THAT?!?" . It usually took two hits before the most persistant cats left my PU alone.
Possums are another matter.. They would touch the wire and they would "flinch" and you can almost count 1 one-thousand, 2 one-thousand before they turn around and begin accelerating away from the wire. Almost like it took the message that long to travel from their nose to their primitive brain. [duhhh]
BTW, thusly "educated" cats can sense when the wire was energized via their whiskers and wouldn't dare touch it.


Mexstan, I installed the fence charger indoors(nearest ac plug) and used some leftover solid-core sparkplug wire to bring the HV juice outside to the perimeter wire.
I took an ordinary 4' fluorescent bulb and wired it in series between the fence charger HV terminal and the sparkplug wire. The bulb would barely glow indicating the HV was on but the wire not shorted-out. But whenever an animal would touch the wire outside, it would make the fluorescent bulb give a satisfying flash as the circuit was completed to ground:D. If it glowed bright continuously, something had shorted out the fence, like when the BBQ pit was touching it, or when the grass grew into it diminishing it's bite. Weed eater time...
The fluorescent bulb only drops about 150volts from the several thousand available from the transformer. It was still P-LENTY potent.

Frequently, while I would be watching TV in the evening, the bulb would flash and I could see through the open window curtains, a very high-speed feline streaking away from my yard with a new found respect for my property.

The problem with commercial fencers is that they are supposed to be humane.

I had a local Great Dane that would always pee on my truck, he missed the tires and hit the fender and I tried my 6-volt fencer and it didn't stop him the only thing that got his attention was when I made a circuit using a 2n3055 transistor and a GM HEI ignition coil, supposed to be around 60,000 volts.
I saw a blue flash and the she dog never came around again.

If that didn't work I was going to use my arc welder.

I used to get hit from 60,000-volt flyback circuits at work they really get your attention.

Have you thought about using ultrasonic sound to keep them away?

Just put the word out that possum is some of the best tamales that you ever eat.[duhhh]
Problem solved [laugh][laugh][laugh]

Yes i'm a redneck [duhhh]

Finally got around to building this and guess what? It does not work, but have not had any time to play with it and figure out the problem. Even tho it is such a simple circuit, maybe I did something wrong. Discovered that I only have about 1 volt after the 100 ohm resistor, so something is sucking the voltage down. I am using a 12V 2A power supply.
Is 85 and 30 supposed to be connected together (with the large cap) as per the circuit? Yes, the diode is the right way around and tests OK.
Will play with it some more maybe tomorrow.

Mexstan, not knowing the characteristics of the relay you bought, your relay may be so heavy duty that the coil draws more current than a 100ohm series resistor can pass to pull it in.
As a quick test, try disconnecting the relay from your ckt and JUST hook up the coil to the 12v PS WITH the 100ohm resistor in series and no diode either... If the coil won't pull in smartly, it'll never work with a 100ohm resistor. If this is the case, you'll have to lower the series resistance to deliver more current for the heavier relay coil.
You can try substituting a small 12v automotive light bulb to provide a convenient "ballast resistor" for your experimenting. Ideally, select a lamp just large enough to pull in the relay. Then try again in-circuit.

I'm used to relays being lettered with "N.O.", "N.C." and "C" to tell me which is the normally open, normally closed, and common terminals.
Automotive relays use numbers but they've never made sense to me. I wind up having to check first which one is NC, NO, Common and the coil terminals.[duhhh]

OK, Looking at the skizmatic it appears that you would want to connect the Ign coil hot to the N.O. or "87" terminal.
The resistor(lamp) lead goes to the N.C. or "87a" terminal, with the big electrolytic cap connected to the COM or "30" terminal.

If it just lightly buzzes but won't pull in far enough to fully touch the N.O. contact to the ign coil, try connecting the resistor directly to the big electrolytic cap(COM,term 30)instead, and rely on the load from the ign coil to discharge the electrolytic cap to drop the relay out, thus interrupting the ign coil primary and generating a HV pulse. (some relays drop out at just a few volts)
Thus connected, too large a lightbulb, (low resistance) and the relay will just latch up and never release even though it's powering-up the ign coil primary. You'll likely have to adjust your resistance (lamp size) to obtain the optimum pulse rate.
Too small a lightbulb, (high resistance) the relay will never pull in..
This 2nd scenario sounds like where you're at right now.