technical inputs sought

Discussion in 'Shop Talk' started by agmantoo, May 10, 2005.

  1. agmantoo

    agmantoo agmantoo Supporter

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    Over on another forum I posted this when a person was wanting to install a fence and indicated that it was low budget.

    A cheap charger can be made from a turn signal flasher and a car coil. I am unsure of the amp draw if run on a battery but if you have access to AC a simple battery charger can power the homemade charger.


    I got this repy...
    Increadibly BAD idea. Don't do it.

    There is a lot more to a good charger than this.

    -Walter
    Speaking as an engineer and farmer.


    This then prompted a response by me which was not answered.....
    highlands .....Do not leave me in the dark! Elaborate on why it is an incredibly bad idea.


    Then a third person chimed in with this.....
    why dont we just go commando and cut the end off a lead cord and just hook up the hot wire..........might you want to use a ground fault outlet...lmao

    So here at the more technically qualified forum I am asking for information as to why a supposedly engineer would differentiate why the spark from a coil would be that much different from that of a fence charger?
     
  2. Travis in Louisiana

    Travis in Louisiana Clinton, Louisiana

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    I don't know about making a charger out of a coil, which may work with a battery or D.C. circuit, but DO NOT cut a line such as an extension cord (120 volts A.C.) and hook directly to the wire, with or without a ground fault circuit. One lady did such a thing several years ago putting a hot wire around her garden and killed a little girl. Just my 2 cents.
     

  3. moopups

    moopups In Remembrance

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    Assumeing your speaking about powering the coil secondary winding with a car type flasher, all well and good but how long would it last under constant useage? Flashers are usually on for maybe under 30 seconds, I doubt they would hold up to the constant heat generated in a 24 hour situtation. Possiabibly an industerial grade flasher could be used for this application. Or if you could wire in something that detects a small charge drop from contact with the fence, which would activate the flasherwhich would in turn activate the coil for a short period of time.
     
  4. Cosmic

    Cosmic Well-Known Member

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    All depends on how you come at this subject.

    The better fence chargers put out power in pulses, basically they contain circuits that limit the amount of power that can be delivered, even in a direct short situation. In general whatever is touching the fence gets a known zap, protects both that critter being zapped and the equipment suppling power.

    I suppose there are all sorts of variations to what can be built. Remember the basics. It must be intrinsic safe and not be capable of delivering a fatal shock. Not all that easy as the ground resistance will vary with each situation. Generally wants to be low voltage, probably under 32 V but somehow the equipment must be able to both limit current delivered and sense the situation enough to determine a proper amount of energy is delivered. Basically most 100% duty cycle always on type designs are going to be suspect, want something like a PWM design that can regulate the power to deliver an exact zap.

    So no home brew jury rig is probably going to do the job. You definitely don't want to be called into court to defend one if it all goes wrong and something or somebody is injured. Definitely do not want to be hooking up normal power wiring of any type as a fence charger system. Should be treated as a special class of electrical application with something special built for the purpose.
     
  5. mink

    mink Well-Known Member

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    hey agmantoo take no offense to my reply , i got to give credit for your idea. your posts always have alot of info for the people here so carry on and keep smiling.........mink
     
  6. John Hill

    John Hill Grand Master

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    It is easy to make an electric fence charger from an ignition coil, capacitor, flasher unit (or flashing LED and relay).

    But this is not a very 'good' electric fence, it will drive only a short length of fence and will completely short out and be rendered ineffective by something like a blade of grass or a spiders web, they even need good glass or porcelain insulators. Not to say such a fence is useless because for many years that was the basis of many commercial fence units. They are also very heavy on batteries running for only a few days from a fully charged car battery.

    The heavy battery consumption comes from the high duty cycle of the primary circuit, equivalent to the 'dwell' period in a car ignition. The simple flasher or relay circuit applies 12 volts to the coil primary which reaches some maximum current flow then at some later time the circuits opens and the collapsing field induces the high voltage in the secondary.

    A more effective fencer can be made by applying a short duration but much higher voltage to the primary, something in the region of 400 volts or more. This technique is approximately the same as the CDI ignition systems that were popular some years ago and are still useful for two stroke engines.


    Effective electric fences have also been made by water power turning a tractor magnetto or better still an aircraft magnetto, I guess you could do the same with wind power if you have reliable wind.
     
  7. rambler

    rambler Well-Known Member Supporter

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    1. It will have unknown output. At the least you will have many dead birds along your wires with steel fence posts. Bad it will kill your critters in damp conditions. See below for worst....

    2. It presents a fire hazard in very dry conditions.

    3. It will not be reliable made from bits & pieces that are not designed for 24 hour constant service.

    4. Can't see where it is much cheaper than an average fencer, and if you have 110v available as one of your scenerios goes it would seem to be much more expensive in materials & time to put together the parts, buy a battery & a charger??????

    5. I have enough trouble with radio & TV reception with a UL fencer. You are going to mess a lot of people up with that home-brew.

    One can make a shotgun out of a bunch of pipe bits, some black powder, a hunk of lead, and some cotton. Add a little flint & other bits, and sure you have a gun. Could even be kinda fun to achieve the goal of making it if you really never use it....

    If you want to go hunting on a regular basis & keep _yourself_ safe, you probably ought to go buy a servicable, well designed gun of the size & type that will serve you well.

    Likewise, putting any old voltage through a wire and calling it an electric fence is not a good idea.

    Do you know what happens if the flasher unit fails - does it short open or closed???? What is the output of the coil? How long is the pulse? What happens when your grandchild, or your neighbor's grandchild grabs the wire in dewy grass?

    Mixing livestock, unregulated electricity on bare wires, water, & open spaces where unknowing folks can roam _should_ open one up to lawsuits......

    Now if you are an electrical engineer with safety tendencies have fun, go at it, but as general advice to common folks owning a soldering iron, sounds like a real aweful advice to me....

    A person can be killed with a little 9v transistor radio battery if it's applied right, livestock are more sensitive than people. Mom & I were clipping weeds along the fence years ago, I was barefoot in green grass (good idea or not??? It's how I woas....) and mom was trying to hold the barbed away from me, but slipped & jabbed a barb into my skin. It more than got my attention, with a well designed, short-pulse fencer. A longer pulse or some home-made coil deal, & I don't know what shape I'd be in. The pulse is not for the fencer; it is to allow a person or critter to get loose of the wire. Low voltage shocks can freeze the muscles and do a slow-electrocution of the being if there is not a short enough pulse.

    Birds like to sit on the wires, and go after dew drops on the metal posts. When our old fencer went belly up & put out a steady jolt, we noticed it by the dead birds along the fence.

    Need more reasoning, or is that enough?

    --->Paul
     
  8. John Hill

    John Hill Grand Master

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    Hahaha Rambler, I wish I had known all that when we had miles of electric fence driven by automotive coil fence units! :)
     
  9. Cosmic

    Cosmic Well-Known Member

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    Maybe if you choke on it.

    The human body has resistance even under horrible conditions. Usually something like 40K is assumed for even bad scenarios, the path through the body counts a lot too. Critical voltage levels are probably >30 volts to deliver the current to kill in most practical conditions. Far more complicated than most present.

    Talk about raising the voltage levels in automobiles to something around 48 volts. Does seem to be pushing the envelope but maybe so. The below source goes into some of the in's and out's in better detail.

    http://www.opamp-electronics.com/tutorials/dc_theory_ch_003.htm
     
  10. caballoviejo

    caballoviejo Well-Known Member

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    Wow you guys are good! REally.

    So tell me this (and observe my level of knowledge): when I was 10 and at a S.A.C. base my brother and I got my train transformer and hooked up to it coiled wire and antenna parts and aluminum foil, etc. in a weird array which we secreted in our garage. Then we cranked it up.


    None of the B-52's crashed or seemed to dramatically alter their flight paths due to the patterns of our grand electronics disruptor. That was a good thing cause boy would we have been in trouble. Why? Did we just need more voltage or a better array? Little kids minds want to know.

    We ended up discovering how to melt plastic checkers and fuse them together while they were still molten. But we had to use matches for that.
     
  11. agmantoo

    agmantoo agmantoo Supporter

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    rambler ....What about all those birds sitting on the high voltage power transmission lines? Do they not know they are dead?
    With a DC input to an auto coil you can only get a pulsed output!