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Oddball electric fence charger.

Discussion in 'Power Electronics' started by Biggie, Oct 2, 2021.

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  1. Biggie


    Oct 2, 2021
    This might be dangerous so I need you guys to advise me. I'm thinking of using an automotive ignition coil by itself connected to its own 12 volt battery, so it can be portable. From what I know, the ignition coil stores a big jolt of electricity which instantly flows out when it's grounded at the spark plug side. So I would feed the coil with 12 volts, and hook up the jolt wire to the spark plug output. The ground would be strategically placed sheet metal strips with a wire going to each one, then finally a wire going back to the negative post on the battery. I need this to deter bears who are damaging my property, but I can't afford to buy a real electric fence charger that's strong enough to deter bears. Anyone dare to advise me?

    [mod edit: merged from two separate threads]
    I have a serious bear problem, I think it's their hobby to break into my trailer, garage, to jump through my open windows at night, tear off the trim around my doors trying to get in, etc. Please don't tell me to not put bear attracting garbage outside, because I put all of my wet waste in my freezer until I go to the dumps. I can't afford a fence charger strong enough to deter bears. But it occurred to me I might be able to use an automotive ignition coil. The coil would be connected as usual so the positive side of a 12 volt battery. The jolt wire would come out of the spark plug output end of the coil. The jolt wire would be about 50 ft long, probably number 12 AWG bare copper wire. To ground the system I would lay strategically placed strips of galvanized sheet metal on the ground each connected to the other with number 12 AWG insulated copper wire and then back to the negative post of the battery. So when the bear touches the jolt wire he will be standing on the sheet metal strips. The one thing I can think of that's concerning is that once a bear grounds himself, the high voltage current would be constant and might actually hurt the bear. Is there some kind of automatic on-off switch that could turn the 12 volt feed to the coil on and off about four times a second? Do you guys think there's too much amperage in the system to use on a bear?
    Last edited by a moderator: Oct 2, 2021
  2. Minder


    Apr 24, 2015
    I just built one to keep deer off the garden.
    I first intended using the old style coil, one with one HV and two terminals, but I also had the modern waste-cylinder type, it has isolated primary.
    I used a 8pin Picmico, a bit of an over-kill, but modeled it on a few out there that use a 555 IC.
    I use a 100ms pulse every second.
    A WEB search will bring some up, also I believe there are previous designs posted here.
  3. Minder


    Apr 24, 2015
    You need to use a pulse, I use 1 pulse every second, timer on the primary, many designs out there.
    I laid steel galvanized 12g wire around the perimeter for GND
  4. Bluejets


    Oct 5, 2014
    Electric fences do not need a ground wire run around the perimeter as such.
    Just the isolated line.
    Usual approach is to drive an earth stake at the unit and connect ground there.
    Voltage is high enough and current low enough not to need a perimeter ground wire.
  5. Nanren888


    Nov 8, 2015
    Google for electric fence energiser circuits. You'll get lots of hits. The articles will give you a better idea fo what has to happen to make them work.
    One of the first I got was

    As above; the high voltage is a result of the coil primary-side current changing suddenly. The spark in the car is a consequence, not the cause. The cause is the circuit switching the primary-side current.
    So, needs something to do the pulsing and switching.

    Even an image search for will give you some ideas of circuits
    You will see that many choose to use a 555 ic to generate regular pulses and a MOSFET to switch the coil current.

    Animals have different reactions with different hide thicknesses; some need higher voltages. I have no familiarity with bears, so can't help with energy required guidence.

    Take care: high energy controllers can cause loss of teeth if you get them across the wrong part of your body while installing or testing.
  6. Minder


    Apr 24, 2015
    As I said in #2, I used a Picmicro, I ended up with a pulse burst of 10x .01 sec, pulse every second.
    The modern dual waste cylinder firing coil has an isolated coil primary.
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