Safety check request on my electric fence idea...

Discussion in 'General Electronics Chat' started by jjj, Jul 29, 2015.

Thread Status:
Not open for further replies.
  1. jjj

    Thread Starter Senior Member

    Feb 25, 2007
    111
    0
    Since my hobby electronics knowledge merely consists of trial & error, I thought of better asking you Profis, before my fence idea kills an intruder.
    Inside my house fence I thought mounting an electric fence barrier, consisting of a super simple circuit. I know there are lots of far more complex circuits around, but I reckon this simple circuit should be quite safe ...or is it?
    I wished I could somehow verify its strength. Maybe I should look at building a HV meter/tester with with Caps and neon lamp? Something that gives me an estimate of its strength so, I can adjust its power to the lower margin. Any ideas on that?
    I could turn down the Triac's switching rate or maybe I should connect a smaller transformer with 220V output to lower the voltage output, for it's only for a short fence of some 4 meters. It's only meant to discourage a would be thief. By touching one of the electrified three rows of barbed wire, he will give up and/ or consider ...to climb the neighbor's fence. :)
    Please have a quick check on it, if there's something lethal. Thank you in advance for your expert advice.
    Warm Regards from Joh, young pensioner (74) in wintry (bad sunny) St'go de Chile


    Note: Image obfuscated by moderator.
     
    Last edited by a moderator: Jul 30, 2015
  2. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    I am available to sweep away the dead bodies after your fence has sorted them out....and can put you onto a good lawyer to help you with a reduced sentence....
     
  3. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
    I think "lethal" could prove to be an understatement.

    Many years ago I worked on a product very similar to this (triac + capacitor + automotive spark coil) to serve as the ignitor for a CNC oxyacetylene cutting torch used in shipbuilding. When the triac was set to fire on each peak of the incoming AC waveform, the ignitor produced a fat, juicy spark nearly two inches long with a 120 Hz repetition rate. It was truly awesome. And scary. One of the other techs working on the thing joked that maybe if we made the capacitor just a little bit bigger, the customer could just dispense with the acetylene entirely and cut the steel armor plate with the spark alone.

    Obviously an extreme exaggeration, but still that spark was impressive.

    My advice: don't even THINK of doing this. Seriously.
     
  4. jjj

    Thread Starter Senior Member

    Feb 25, 2007
    111
    0
    Thanks for your good advice. So you reckon a 20K commercial unit of say, 0.5 joule would be much safer?
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
    3,250
    626
    Your idea could potentially kill someone. Modern electric fences probably charge a capacitor so the shock would be brief and non-lethal. I've been shocked by them; it's unpleasant, but not incapacitating...
     
  6. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,328
    890
    No, I don't recommend doing this AT ALL.

    The energy of the sparks is only part of the problem; the other part is that the sparking is continuous (100 Hz or 120 Hz, depending on your power system) and can therefore cause muscle spasm in anyone contacting it, causing them to be unable to let go. Commercial electric fences don't have that danger, because they only send out a single high voltage pulse (from a small capacitor) once a second or so.

    One more time: DO NOT DO THIS!
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    You need a shock that has controlled energy and a delay between each pulse so that it is not lethal to man or beast.
    A common way is to use an automotive coil with a capacitive discharge or flyback circuit on the primary to generate the non-lethal high voltage.
    A Google of electric fence charger circuit should give you several choices for do-it-yourself designs.
     
  8. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Consider a cactus patch in areas where you don't want intruders. Fence lines and around windows is a common place for them. BlackBerry vines also have some NASTY little needle spines on them. Booby trap is what the electric fence will be called at your trial. The stupid intruder with painful spines in their body will simply be treated to ridicule by any and all who are made aware of their trespass.
     
  9. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,693
    2,765
    A mote, with crocodiles, has traditionally worked well.

    Since when has the human body become so frail? I used to play around with the electric fence on the farm when I was a kid. It provided a great "buzz", and it was definitely powered via a transformer (the power was continuous when you touched it). Good, clean fun.
     
  10. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    Good , clean, fun? :eek: That's not exactly the words I would use to describe a nasty electric shock. :rolleyes:

    I've never seen an electric fence charger that had a continuous output, since that tends to be dangerous. All the ones I've observed put out an intermittent pulse of some sort.
    We did had a rather nasty one on the farm, called a Weed Burner, that put about about a 1 second pulse with perhaps a 2 second period. It's current was high enough to burn any weeds that might touch the fence and otherwise short the pulse to ground (which frequently happened with a normal, short-pulse controller).
    When a weed touched the fence you could see a small arc between the wire and the weed as it burned away the edge of the weed.
    You didn't want to use it in dry conditions though because it could possibly start a fire.
     
  11. jjj

    Thread Starter Senior Member

    Feb 25, 2007
    111
    0
    OK, so I need to forget about my simple circuit idea, because it might kill an intruder and look for some circuit, which only delivers unpleasant, but not incapacitating electrical shocks! How about if I build a simple flasher circuit, which charges a 0.56 capacitor to 220V every 2 seconds?
    Would that be safe? Or should I just buy a cheap commercial unit and so I can blame the manufacturer of this unit for killing the thief...?
     
  12. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    100Vdc or so would give you about 10kV out from a standard 100:1 ignition coil, which is likely sufficient to deter anything.
    You could generate that with a small line-powered 240V to 120V output transformer with a bridge rectifier.
    The capacitor can be fired into he primary of the coil with an SCR triggered by a 555 astable.

    Don't do this directly from the power line, you need transformer isolation for safety.

    If such an isolation transformer is not readily available at a reasonable price, you might try connecting two small, low voltage transformers, such as one of these dual-voltage transformers, back-to-back (low voltage sides).
    You can then put 220V into one transformer primary and take 110V from the other transformer primary (now used as a secondary).
     
  13. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
    2,804
    1,237
    Also please be advised that:

    1) Under the right (wrong?) conditions (Incl intruder health) any shock sufficient to deter an intruder could prove fatal -- Your liability in such instance would, of course, depend upon negligence and property protection rights legislation applicable to your locale...

    2) Any mains operated 'fencer' -- no matter how well isolated/protected -- may, conceivably, fail such that the line is effectively connected to the fence conductor...

    IMO there are safer, more legally sound, methods of property defense --- My $.02

    Best regards
    HP
     
    Last edited: Jul 29, 2015
  14. jjj

    Thread Starter Senior Member

    Feb 25, 2007
    111
    0
    Aha... now you admit having obtained practice at Isis, Syria... Yes, that's how we discover them all... with electric fence lures...
     
  15. jjj

    Thread Starter Senior Member

    Feb 25, 2007
    111
    0
    Thank you so much smart crutschow... that's so much more what I was after. Some truly professional advice.
    My super simple circuit has transformer isolation, too. The only thing it lacks is pulse spacing. Yet, when I turn down the light dimmer the pulses are pretty slow. Also if I lower the Cap to say 0.1MF it would barely kick the TV transformer. Lastly, I could even discharge the Cap into the primary of a small transformer's primary to get a spike of 220V. I cannot imagine this setup to be lethal.
    I can't be that hard to draw a simple circuit, which only delivers a unpleasant, but not incapacitating kicks!
    Yet, if all that is too complicated, I just buy the cheapest commercially available unit, which of course kicks much harder than that.
     
Loading...
Thread Status:
Not open for further replies.