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Electric fence charger

Discussion in 'Electronic Basics' started by Stumpy, Jan 28, 2006.

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  1. Stumpy

    Stumpy Guest

    Can you point me in the direction of a plan for an electric fence power
    supply. Ideally a diagram and a parts list. The retail models to deter
    dogs from jumping fences are ~1KV and .5mA they also claim about 1/3 of a
    joule.
     
  2. default

    default Guest

  3. Stumpy

    Stumpy Guest

    That was actually 5mA. Thanks for the very clear links.
     
  4. Stumpy

    Stumpy Guest

    I was in too much of a hurry to build my own, so I bought a model that puts
    out 1.2 Kv intermittently. It is in an urban location and I'm worried about
    liability.

    Is there a neon or LED bulb that can pulse with a rating up to ~1.5 kV for
    an indicator?
     
  5. Don Bruder

    Don Bruder Guest

    Liability for what? Hang a few of those "Warning: Electric fence" signs
    on it, and there went whatever liability there might be.

    Side note:
    The output of an electric fence charger (assuming a "store-bought" unit,
    as opposed to some homebrewed-from-a-car-coil unit) will give you a
    high-voltage (Anywhere from 5KV to 15KV or even more, depending on
    brand/maker/age) "bite", yes - After all, it's designed to do exactly
    that. But it's also (A) Too short a duration pulse to mean anything
    other than "SHIT! THAT SMARTS!", and (B) too low an amperage to mean
    anything unless somebody is stupid enough to wire a cardiac patient to
    the fence and power things up. Even then, it's not *LIKELY* - just
    *POSSIBLE* - that anything worse than "SHIT! THAT SMARTS!" is going to
    come of it. Granted, it wouldn't be any fun, but it's not likely to be
    harmful in any lasting way.
    Any standard small neon should give you at least some flash for each
    pulse of the fence if you wire it from the hot wire to ground. I
    wouldn't sweat over it, though - Simply assume that once it's plugged
    in, it's just like a gun: Loaded and could go off anytime. Only unlike a
    gun, if it goes off on you, it's only going to result in a cuss, not a
    fatality. It won't take you (or anyone else, right down to the youngest
    kiddie that might come along) long to learn not to mess with the wire.
     
  6. Stumpy

    Stumpy Guest

    It's hand labeled now, Store bought signs tomorrow.
    I assume Radio Shack can come up with a bulb, just wondering what to ask
    for.

    It is a good idea for me to do something more active than a sign because I
    don't want a wife or offspring to come from the inside and expect it to be
    turned off, or to open the gate at night and get a surprise.
     
  7. Don Bruder

    Don Bruder Guest

    Rat-shack (AKA "You've got questions, we've got blank stares and cell
    phones you-need-some-batteries-with-that?") should have them, but I
    wouldn't bet on the droids to know what you're talking about. Just look
    for a small neon lamp - An "NE-5" unit would likely be ideal.

    Failing that, go to your local hardware store or whatever that sells
    Flourescent light starters (the little tin cans with two pins on them to
    plug into the fixture - Last time I bought any, they were almost
    literally a dime a dozen) Inside the tin can is a capacitor and a small
    neon lamp (I think the type is even "NE-5" - bonus mode!) that would be
    ideal as an HV indicator light. Wire one leg to the hot wire, the other
    leg to ground, and watch it flash. For that matter, depending on the
    output of your particular fence charger, it might not even need to be
    physically wired to the system - Getting it within a few inches of the
    wire may be all the "connection" needed to get it to flash with each
    pulse of the charger - kinda like you can stand under high-tension wires
    with a flourescent tube in your hands, and have it light up without
    being connected to anything.
    Speaking from 40 years of living with electric fences, that's *ALL* it
    will be: An unpleasant surprise. I'd expect it to happen once, *MAYBE*
    twice if you've got "slow learners" in the family. For the kiddies, it
    may result in a session of bawling, but... <shrug> Oh well... Just like
    they get burned once and don't mess with the stove anymore, they'll get
    tagged by the fence once, and won't mess with it again. (But unlike the
    stove, the fence poses no significant risk of third degree burns or need
    for reconstructive surgery...)
     
  8. I don't think you will have much luck putting an idicator directly on
    the charged wire as it will likely shunt the high voltage pulses to
    ground and prevent the electric fence from having the desired "bite".
     
  9. Jasen Betts

    Jasen Betts Guest

    ["Followup-To:" header set to sci.electronics.basics.]
    a neon could do it, but ther's probably enough voltage there to run a small
    xenon tube and they're much more visible.

    Bye.
    Jasen
     
  10. default

    default Guest

    I wouldn't know for a fact, but my guess would be any ne-2 lamp with a
    fairly large series resistor might work. They use between 20K to 100K
    for neons working from 120 VAC. I'd use a few series resistors to
    prevent arcing around the resistor.

    If you kludge something together and seal it - take care that the air
    inside is dry or condensation will provide a path for voltage the
    first cold day that comes along.

    If the voltage spike has a fast rise time, it might work by just
    connecting both wires from the neon lamp to the fence and depend on
    capacitance to ground to flash the lamps.

    When I was a kid I got the idea that it would be neat to light a neon
    with static. My aunt had this killer carpet and walking over to the
    radiator was a sure fire way to get a surprise. NE2 and NE45 lamps
    would only flash once, not very brightly, and burn out . . . The
    correct size resistor is necessary even with very low current sources.
    They do make neon gate hooks for cattle farmers. What I've seen is a
    neon lamp mounted in a cylindrical yellow plastic housing. It has a
    hook on one end (the gate side) and loop on the other with some larger
    flanges to keep one's hands from slipping off and onto the wire

    The idea is that in order to safely cross the fence you unhook the
    wire and step over the fence, then replace the wire. It doesn't have
    a ground connection that I could see so it either depends on
    capacitance to light the lamp or is in series with the fence and drops
    the 65 volts or so the neon needs to light.

    Your signs won't be visible in the dark . . .

    An LED probably wouldn't work. They require a few milliamps to run,
    your charger probably puts out 1-2 milliamps. There's also the
    (probability) possibility that inductive ringing in the output of the
    charger or length of fence wire would be more than enough to break
    down the ~4 volt reverse voltage Leds can withstand.

    A second cable with low voltage "walk lights?" Or a small cable with
    a number of LEDs running from a LV source?

    Is it the idea of neon lamps flashing and lighting the perimeter that
    appeals, or just the gate?
     
  11. Stumpy

    Stumpy Guest

    I bought a couple of 120volt neon panel lamps. Rated 1/2 watt @ 120v. This
    should mean P=IE, 1/2=I*120 .:. I=4mA That seems awful high for a dim
    little lamp.

    Using that value the resistance of the bulb is I=E/R, 4mA=120/R .:. R=
    30kohms

    So a voltage divider requires 9*30K=270Kohms

    If I buy a 270Kohm resistor it needs to pass 4mA. That would require a 6
    watt resistor (P=IE, .004*1.5K=6)

    That's ridiculous. What am I doing wrong?
    Good point. I'm planning on putting the lights panel mounted on the sign
    itself.
    Too expensive!

    Just the gate, where any law abiding citizen might expect to gain access.
     
  12. Don Bruder

    Don Bruder Guest

    Just the gate, where any law abiding citizen might expect to gain access.[/QUOTE]

    The solution becomes even simpler: A hot wire doesn't need to be a
    continuous loop - Simply run the wire UP TO the the posts on either side
    of the gate, and terminate it right there. Hook the charger to the wire
    anywhere happens to be convenient, and you're in business. As long as
    all the wires that are supposed to be hot are in contact someplace, your
    entire fence will be hot, and no need for worrying about the gate.
     
  13. Stumpy

    Stumpy Guest

    No, the gate has a charged up piece of hardware cloth on top to prevent the
    dog from going over. Someone could reach for the latch with one hand and
    touch the charged up portion at the same time.

    I realized my concern about impedance was a waste of time. I might as well
    just put 10 of the lamps in series since they are rated at 1/10 of the
    supplied voltage. The local Rad Shack only has 4 amber bulbs so I might as
    well get a few green and some red ones to give my sign a classy, christmassy
    type decor.
     
  14. Don Bruder

    Don Bruder Guest

    No, the gate has a charged up piece of hardware cloth on top to prevent the
    dog from going over. Someone could reach for the latch with one hand and
    touch the charged up portion at the same time.[/QUOTE]

    Ahhh... Now we have one more snippet of needed information that wasn't
    provided to begin with.

    And I go back to the "they'll only do it once or twice" school of
    thought.
    As mentioned, simple neon bulbs, like an NE-5 or similar, will likely
    flash *WITHOUT* needing to be electrically connected to the wire - Just
    placed "close" to it. Use the legs to attach them to the wire like
    twist-ties, and I'd expect them to flash brightly each time the charger
    pulses.

    Keep in mind that the charger isn't putting out continuous juice (or at
    least, it SHOULDN'T be - "constant" chargers went out decades ago) It's
    pulsing, and the pulses are *SHORT* - Very possibly too short to
    usefully light a non-discharge type bulb.

    I still say the simplest solution is posting a wanring sign at the gate,
    and letting the fence "train" anyone coming in.
     
  15. Rich Grise

    Rich Grise Guest

    If you can get a naked neon bulb with leads sticking out, you could just
    dangle it by its leads from just one of the fence wires - a spike like
    1.2KV should fire neon just by capacitance to air. =:-O

    Cheers!
    Rich
     
  16. Stumpy

    Stumpy Guest

    Good point. I used 10 neon bulbs and then added 2 10Kohm resistors in
    series to reduce current. I have difficulty measuring the actual voltage
    because it does not stay constant long enough to get a real reading. My
    digital multimeter is useless and I don't really trust the ancient analog
    one.

    One thing I do know is that the dog has touched the fence 3 times and has
    not done so again. I was too wimpy to grab the wire. I touched the wire
    above the 2 resistors and after 10 bulbs - it was very unpleasant. I don't
    want to do it again.

    I'm a little embarrassed about how tacky the sign looks. I probably should
    have looked for a 300Kohm 10watt resistor and just used 2 bulbs.
     
  17. Don Bruder

    Don Bruder Guest

    Good point. I used 10 neon bulbs and then added 2 10Kohm resistors in
    series to reduce current. I have difficulty measuring the actual voltage
    because it does not stay constant long enough to get a real reading. My
    digital multimeter is useless and I don't really trust the ancient analog
    one.

    One thing I do know is that the dog has touched the fence 3 times and has
    not done so again. I was too wimpy to grab the wire. I touched the wire
    above the 2 resistors and after 10 bulbs - it was very unpleasant. I don't
    want to do it again.

    I'm a little embarrassed about how tacky the sign looks. I probably should
    have looked for a 300Kohm 10watt resistor and just used 2 bulbs.
    [/QUOTE]

    Word of advice from an old hand with electric fences: When in doubt as
    to whether it is or isn't "hot", touch it with the *BACK* of your hand -
    The resulting "jerk" if it is turned on will (usually) fling your hand
    away from the wire, rather than onto it, reducing the duration of the
    "zap" (although not having much effect on how hard the "bite" feels)

    Another method that gramps swore by was pick a fresh green blade of
    grass, at least 3-4 inches long, and lay that on the wire - If it's hot,
    it'll "zing" you, but the bite won't be anywhere near as bad as direct
    contact.

    Of course, there's always the "grab a stick" method - Using a stick or
    similarly non-conductive object, push the "hot?" wire to within about
    1/8-1/4 inch of something grounded and hold it there for a couple fo
    seconds (to give the charger a chance to pulse at least once). If it
    says "SNAP!", it's hot. :)
     
  18. default

    default Guest

    Try point four milliamps. I see in Jameco they list their ne2 lamps
    at point six milliamps.

    Wattage? You only need to dissipate the average power not the peak
    the charger can put out. There's probably a 1:100 duty cycle you're
    dealing with.

    The dog will have learned and the fence will be moot . . .

    I had this cat . . . the former owner let the cat in the second floor
    window, so it didn't understand doors but insisted I open the window.
    That got old fast and was hard on the screen. I rigged a auto spark
    coil in series with a car signal lamp and flasher and laid two tracks
    of wire down on the window sill. The cat learned in just one day.
     
  19. Stumpy

    Stumpy Guest

    I will keep the fence active. Maybe the dog is smart enough to know what
    the flashing lights are for.

    I did notice all the suggestions about freestanding lamps. It's a very
    interesting proposition, but I can't understand no clear reference to
    ground. Would you have to respect the combined load and limit the number
    used? Heck - you could put an infinite amount on a charged up wire and
    get no current lighting.
     
  20. default

    default Guest

    The dog can probably make the connection between the lights and shocks
    relatively quickly. Animals are a lot smarter than we credit them -
    they just don't have the same priorities or abilities that we do.

    Then you just need the lights?

    There's no magic with operating gas discharge lamps with only one
    connection. High frequency (20 KHZ and up) high voltage electricity
    has a relatively low X sub C (capacitive reactance - expressed in ohms
    but applied to AC).

    AC will pass through a capacitor even though there is no DC path
    between the leads. The higher the frequency, the lower the resistance
    to AC, or/and the higher the capacity the lower the resistance to AC.

    Remember seeing "plasma globes?" Low pressure gas breaks down at a
    lower potential than high pressure gas - that and the capacitive
    reactance makes the arc follow a hand brought near the globe.

    Neon tubes and lamps have a partial pressure of gas in them and ionize
    fairly easily. (Too low and it won't work - too high and it won't
    work).

    Getting back to fences and neon lamps . . . your fence charger almost
    certainly develops a high voltage low current pulse by using the
    collapsing magnetic field of a transformer to generate the voltage.
    The faster the collapse the higher the voltage. The more turns of
    wire the higher the voltage also.

    With that type of generator, you have a fast rise time that is "rich
    in harmonics." The faster the rise time the more/higher high
    frequency present. That and high voltage may be enough to break down
    the gas in the neon bulb and ionize it. The capacitive reactance to
    ground and nearby grounded objects, completes the return path for the
    electricity.

    There is no free lunch - but if it works at all there should be no
    limit to the number of neon bulbs that will flash when the spike comes
    along. The bulbs are just turning some of the energy that would be
    lost to capacitive reactance anyway (actually it may take a smidgen
    more energy because the surface area is greater - make a capacitor's
    plates larger and you increase the capacity and lower the capacitive
    reactance).

    It won't be as bright as with a hard wire connection - but you really
    have to try it and see what happens.

    Tesla coils can light fluorescent lamps from twenty feet away without
    wires using similar principles.

    Touching an ordinary incandescent lamp to a high voltage terminal on a
    working spark coil will light the lamp with a faint blue glow (partial
    pressure of nitrogen in the bulbs - so they work like plasma globes).

    Some lamps light green when subjected to high voltage high frequency -
    vacuum filled bulbs with 100,000 volts on them produce X-rays, and
    flint glass fluoresces green with X-rays.
     
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