Electric fence tester

  • hi all
    after advice on an electric fence sensor node for mysensors

    so I have a small 5KV Electric fence energizer which sends pulses of 5KV down the line in 1-2 second pulses
    I'm trying to work out how to monitor it..
    Ideally id love it to be self powered, but I'm tipping that's out of the questions, as the capacitor requirement I think would be huge to stop overloading....

    so my two options I thought of were either a voltage divider (Research shows this probably wont work)
    or a hall effect current sensor, but I've never worked with one of these and am unsure of how this one should go.

    I know my main issue is going to be protecting to Arduino from the raw voltage as it would fry it in seconds.

    has anyone worked on a similar project or able to advise on a good place to start?

  • Hero Member

    You could make a non-contact detector very simply using a simple MOSFET. FET stands for Field Effect transistor, and that's exactly what you want: something to detect an electric field. Just leave the gate pin floating (unattached) and put it near your electric fence to detect the electric field. Simple as that. If it proves to be too sensitive, you could weakly ground the gate pin with a suitably high ohm resistor (probably something in the megaohm to gigaohm range).

    As for powering the sensor from the electric fence, I presume you could use a suitable step-down transformer to get a low-enough AC (pulsed) voltage, which you could then rectify and then filter to get a useable DC voltage. If you go this route, then you could simply detect the 1Hz low voltage pulses on the input pin of an MCU as each one is sent, but before it gets filtered.

    Take care, because high voltage behaves differently from even mains voltage, and you can get very dangerous plasma arcs in places you might not expect and/or even violently explode a multimeter that's not rated to handle it. If you're at all uncertain as to your capability/abilities, enlist the aid of a qualified electrician, or else simply go the non-contact route. A properly built non-contact sensor could run for 10 or more years on just two AA lithium primary batteries (and 20+ years with bigger batteries), and so it will likely be easier and cheaper to build anyway, and without the safety risks of a direct connection to high voltage.

  • that quite likely a good idea... as its outside i could even drop a solar panel on it... as in this situation with 1 read every second (give take) the node wont get a a chance to sleep at all as i would be sending back to the gateway constantly...

  • @markjgabb
    If you want to run on batteries, which is a good option, then just make a reading every 10sec, and make the MCU sleep and save precious battery. It's not needed to measure each pulse every second

    Pseudo code:
    wakeup after 10sec
    wait until you get a reading, unless you have waited 5 sec (timeout to save battery)
    send pulse detection

  • @NeverDie This may well be a different scenario, but most agricultural electric fences generate the HV from a 12v battery, and have an inbuilt indicator if the fence is live.... Viz you could tap into the indicator...
    Some I've seen running 24/7 for years in remote locations (deer fences) kept charged up only by DC wind generator..

  • You might get some tips from this interview on the Amp Hour podcast: https://theamphour.com/481-an-interview-with-paul-thompson/. If I remember correctly, Paul Thompson goes into details of the various issues he's encountered creating circuitry to work together with high voltage. If nothing else it is a fascinating interview to listen to.

  • Hero Member

    There are a bunch of non-contact voltage detector videos on youtube:
    Homemade Non-contact Voltage Tester – 04:19
    — Thomas Kim

    Most are of a 3 transistor design, which from this video looks like it would be good enough for your purpose. If you wanted something even more sensitive (or greater range), you could use op-amps instead.

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