Electric fence short finder

Discussion in 'General Electronics Chat' started by Little Ghostman, Jan 10, 2014.

  1. Little Ghostman

    Thread Starter Member

    Jan 1, 2014
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    All around our property we have a 10KV electric fence, sometimes grass or weeds touch it and cause a small short, its my weekend job to go around and clear the fence line. I have used a radio in the past to hear the clicking, it only seemed to work on one radio we had, it was one you could twidle a dial to off tune it. Since we moved in July we dont have it anymore, so does anyone know roughly what frequency I would be looking at? or more to the point how I can find the frequency?
    The plan is to build a hand held short finder with Leds to show me how close I am :D and a speaker to hear the clicks
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Do u know the model of the charge circuit used in to supply the 10KV to the fence.
    From tht we might be able to find the frequency the circuit operates
     
  3. PackratKing

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    Jul 13, 2008
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    It's been years since I dealt with an electric fence,

    ?? aren't those operating on line frequency ?? 50 or 60 Hz ??

    Edit... is there any part of the fence giving the HV pulse ? partial circle, and the shock stops at the short ? how long is the fence HV circuit...
     
    Last edited: Jan 10, 2014
  4. inwo

    Well-Known Member

    Nov 7, 2013
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    Don't think it's possible to determine how close to the short you are.
    That should bring some answers.:D

    Reason being the high resistance of the short compared to the low res. of the fence.

    I would use a neon tester. When short is cleared, holding one end of tester and probing fence, shows if it's working.
    A blade of grass works in a pinch, if you don't mind a small shock.



    I don't know the frequency of the "weed burner" types. The standard fencers use a pulse from capacitor discharge every few seconds.
     
  5. Little Ghostman

    Thread Starter Member

    Jan 1, 2014
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    I think its a trueline model, its a mains unit, I cant look as the building its in is locked untill later. With a radio I used to be able to tell distance by the volume of clicking.
    You can buy very expensive short finders for the fence, but I wanted to try a receiver.
    No idea why but for some reason 300kHz rings a bell. No idea of length, probably over 4 miles altogether, by the time you add up all the sections and strips it makes, (hence why I want a short finder :D ).
    I should have thought more before posting!!! its a bit of a how long is string question. I will go find alot more information and come back, gut feeling is its doable, Something a bit like a Lightening detector, so I need to find the frequency of the spark when it shorts on something, no spark= no short just a pulse, I can see this getting complicated! ideal world it will have a analogue meter for distance, nearer the spark the higher the stronger the signal. OR I could have all this totaly wrong!!!
     
  6. #12

    Expert

    Nov 30, 2010
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    A spark in the air radiates across a wide frequency band. That's why an AM radio can pick it up. The $10 transistor radios we had in the 1960's would do a fine job of this.
     
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  7. Little Ghostman

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    Jan 1, 2014
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    I need to experiment a bit, maybe hook up the spectrum analyzer somehow and find a frequency, that is radiated by the spark and not much else, then devise a circuit to detect this, add a few bits to measure the strength (distance) and off I go :D, the lightning detector I built a while back works ok.
    Actually hooking up some kind of small antenna to the spectrum analyzer might not be a bad idea, the electric pulse is a set frequency and duration, so spotting it on the analyzer shouldn't be too hard, We do have some 3kV overhead cables around the grounds, I am hoping they dont cause a problem.
    Sounds like a waste of time project, but we have around 40 acres of land around the house, and walking the fence line takes a couple of hours! Thats time I could be in the lab!!
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Since AM radios are quite good at picking up static discharge radiation, I would think an AM pocket radio would work if it's not overwhelmed by any corona noise from the power line. Sony, among others, still makes pocket AM radios with manual tuning.
     
    Last edited: Jan 10, 2014
  9. Little Ghostman

    Thread Starter Member

    Jan 1, 2014
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    Thats a too easy solution! I already have a old pocket radio antenna on the bench, soldering iron ready! small bit of wire for spectrum analyzer antenna.
    I am sure I can come up with something clever and over engineered!
    I refuse to do this the simple way, when I have the tools to make it complex!
     
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  10. #12

    Expert

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    OK. If you want to use a microscope to find the elephant in the refrigerator, that's fine with me.
     
  11. inwo

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    Nov 7, 2013
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    Are you looking for an arc or a short?:confused:
     
  12. #12

    Expert

    Nov 30, 2010
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    Almost all shorts are arcs because the pulses carbonize any vegetable that dares to touch the wire.

    ps, I tried the "blade of grass" trick. It knocked me on my can!
     
    Last edited: Jan 10, 2014
  13. Little Ghostman

    Thread Starter Member

    Jan 1, 2014
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    yes sorry, a short on most electric fences tend to arc a little as te grass or whatever moves, a radio picks up the crackle really well. Its been rare to get a complete short. maybe short is the wrong word????
    We use what is called a bull fence, it has a very painful kick to it! more than once I have been tickled by it!!! Very unpleasant, some fences are pretty weak, this one puts out the max allowable voltage, it has a switch to drop it to 4kV, but even that wakes you up if you touch it! Wet days are the worse :D
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    LGM,

    You could build two battery-powered 555 timer circuits and inject them into the wire. One at 540 kHz and one at 400 hz with attenuation control and output cloth linked to one leg of a capacitor. Put the other capacitor on the electric fence near the power supply. You can then walk the fence listening for shorts with your am radios eat to 540 KHz. The 400 hz tone Should get quieter where the fence is shorted to ground.
     
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  15. GopherT

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    Nov 23, 2012
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    Another option, since you have about 20000 feet of fence to work with, can you think of a way to take advantage of the wire resistance? Lots of options!
     
  16. MrChips

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    Oct 2, 2009
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    How about a cheap portable AM/FM radio from the dollar store?
     
  17. GopherT

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    That was ruled out earlier - he is looking for an in-naturally complicated solution that he can build himself. See #12's comment. " using a microscope to find an elephant in the fridge.
     
  18. 4pyros

    New Member

    Jan 3, 2014
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    How about a crystal radio?
     
  19. inwo

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    Nov 7, 2013
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    Think, inductive pick-up timing light.

    Picks up differentiated high rise time pulse. May ignore the slower rise time into the clear part of the fence because it may be more capacitive.

    Probably not, but it's complicated.:D
     
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  20. Little Ghostman

    Thread Starter Member

    Jan 1, 2014
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    The resistance isnt constant, we use three types of wire. First off tho I was totaly wrong with distance, the unit we have is a hotline (a model they no longer make) it is dual output, 4kV @3-5 stored joules and 10kV @6-15 stored joules (hurts!), we have approx 4 miles of thick multi strand stainless steel wire meant for zoo's!! (very low resistance) and 5 miles ish of the nylon type rope with 4 single strands of copper (high resistance), also we use poly tape which is alot like the rope but even higher resistance.
    I have measured resistance on it before, but its all over the place depending of weather etc.
    In the past I used the radio, I tuned to the frequency where I got the click click, of the pulses, with a short you get a click with interference type noise (hard to describe), with no short you get a clear click click every 850 mS or so. What I used to do was walk in the direction the interference was loudest.
    I would like to have a go at the injecting but would I need to switch the fence off? As I cant really do that, we have pigs in a space by the side of our wood, infact lets make this easy, if you google map DG8 0LG our postcode, it centres on our house then you can see where I am doing this. we have around 70 acres apparently (I got that wrong as well sorry). Anyway the pigs are by the side of the wood, there only enclosure is electric fence, if I turn the fence off I have to spend 3 weeks getting pigs out the wood!
    Yes its totally a microscope and fridge job! most of my projects are purely for learning and fun! dad wants to buy a short finder, I would be horrified to use one :D
    Can I have some feedback on the whole test the frequency with a spectrum analyzer thing, I might be on the wrong path, But logically I think I can do a variation to my lightening detector (this works great, but is a bit iffy outside in a storm with a 2 meter collapsible antenna in hand :D) .
    Sorry for the madness of it all, but most the stuff I make is more for learning and tinkering, I like a purpose at the end tho, also if I go the injection route then its pic 18f and DACs :D with Led bar graph lol and a bleeper, with changing tone :D:D

    edit
    just found out the model isnt on there website, apparently its a unit used by zoo's that dad got when doing consultancy for a zoo, its known there as a bear unit :eek: no wonder it hurts!!
     
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