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Using power from an electric fence?

Discussion in 'The Projects Forum' started by svdsinner, Mar 21, 2012.

  1. svdsinner

    Thread Starter Active Member

    May 17, 2011
    I'm toying with the idea of building a few small wireless devices that can report the status of the electric fence around my pasture, and adapt to any shorts in the line.

    I've got two options for powering them:

    • Low-tech, boring: Battery and a charge detector that will signal if the battery is getting low and needing replacement. Ongoing monitoring and replacing batteries is not ideal (Making something that makes my life easier that also makes my life harder by requiring maintenance is a bad idea.)
    • Much cooler: Collect, save and use the energy from the electric fence pulses. This would make the devices hook-up and forget.
    To make the latter idea work, I'd need an inexpensive circuit that can grab some of short (sub-second), high-voltage (15kV or so, but in various conditions the pulse could be much weaker, maybe 2kV or so) pulse and store it for gradual low-power release to my device (3.3V). Additionally, it can only use a small fraction of the pulse, since there must still be enough with several units attached to do its job of deterring animals.

    My initial guess-work would be to start with a voltage divider (25:1?) and use a transformer to drop the voltage down to something manageable (9-12V?) with a capacitor across the secondary to "spread out" the pulse. From there, send it to some kind of charge-controller that will charge a couple of super-capacitors that I'd use as my main power supply to feed the device.

    Are there any existing circuits for this type of thing? Any suggestions on how I could do it?

    FWIW, I've got a degree in Aerospace Engineering, and spend most of my professional time designing software. I'm handy with a soldering iron, but am relatively inexperienced in power conversion/storage like the above, so I'll need a little help to be able to create a prototype.
  2. crutschow


    Mar 14, 2008
    An old TV flyback transformer (those without any built-in diodes) operated in reverse direction (fence connected to secondary) would reduce the voltage to a more manageable level. From that you would probably need a series resistor, a rectifier, a large filter cap, and regulator to get your desired low voltage
  3. DickCappels


    Aug 21, 2008
    You might consider capacitivly coupling to the electrified wire with using a thick insulation that is likely to be good for 15 kv, such as a section of garden hose, with a capacitive pickup on the other side of the plastic. Between that and some capacitance to your grounded circuit's ground, you formed a capacitive voltage divider. After that, you can rectify and filter the pulses.
  4. GetDeviceInfo

    AAC Fanatic!

    Jun 7, 2009
    a solar collector would be just as cool and likely way easier.
  5. wayneh


    Sep 9, 2010
    ...and wouldn't load down your fence. Any power removal from the fence wire represents a load on the source electronics, and perhaps a reduction in efficacy of the pulse. I'd look for another way.

    Wondering what you mean by "adapt to a short". The fence is a loop, right? so a short or an open can be detected anywhere along its length, like right at the generator?
  6. SgtWookie


    Jul 17, 2007
    You might consider getting some auto ignition coils from a junkyard and experimenting with them. Their secondaries usually are designed to operate in the range of 40kv or thereabouts. The primary side is usually fired by a couple hundred volts. You may find that the primary side when used as an output gives pretty low voltage; you may need to use a step-up transformer at that point. An inexpensive audio transformer might work for that.

    You can limit the current through the coil's secondary-being-used-as-a-primary by using resistive ignition wire. The old "rule of thumb" for ignition wire was 10k Ohms per foot.