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PWM as driver for electric fence

Discussion in 'General Electronics Discussion' started by Skin, Feb 6, 2016.

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  1. Skin


    Aug 16, 2011
    Hi Guys
    I was wondering if any of you know if a PWM can be used to drive an ignition coil to make an electric fence?
    The PWM was a kit one I built to run hydrogen cells I was experimenting with, rated at 20A-30V variable frequency.
    Looking for high Voltage out put but low Ampage, I don't want to fry the goats lol!

    I like doing stuff with electronics but I'm not that knowledgeable so thanks for any help or ideas.

  2. Colin Mitchell

    Colin Mitchell

    Aug 31, 2014
    Fence circuit needs flyback not PWM.
    duke37 likes this.
  3. duke37


    Jan 9, 2011
    Most fencers I have seen use a flyback high voltage generator. This charges a capacitor to 200 to 300V which is then discharged through a 1:20 transformer.
    Boost convertors can be obtained which generate up to 80V limited by the transistor switch using an inductance. If this is changed to a transformer, then the higher voltage can be obtained.
    If you wish to play, then the flash circuit out of a disposible camers will give the high voltage but it will bite and you may want to use more than a 3V power supply.

    I have made push/pull invertors but these are often much more powerful than you want, also they do not like being shorted when the capacitor is discharged. The flyback convertor doesn't mind this since it puts out a sequence of pulses of constant charge.

    If you have a battery or mains power supply which will provide a high current pulse, then you can use the ignition coil as used in a car. You will need a high voltage (300V) transistor switch. Early fencers used mechanical switches with problems of reliability.

    An ignition coil has a higher resistance than is really wanted but will work over short distances with a well insulated fence.

    You should look at a pulse output of 1J or so. At 1 pulse/second, this means 1W average, however, the efficiency will be lower than 100%!
  4. Skin


    Aug 16, 2011
    Thanks guys,

    Duke I used a flash circuit from a camera years ago and added several capacitors to it and made a crude stun gun, about 800v off of a 9 v battery. It would weld to metal.
    The reason I posted was I have seen circuits using 555 timers and thought that a PWM would do the same job?
    Thanks for your replies
  5. davenn

    davenn Moderator

    Sep 5, 2009
    As Colin and Duke have said, there no need for a PWM circuit
    a 555 circuit with a pulsed output driving the HV generator is all you need :)
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