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Fence flasher

Discussion in 'General Electronics Discussion' started by frans, Jul 5, 2018.

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  1. frans


    Jul 5, 2018
    I need to make a led flasher for my fence.
    fence is about 10kv. I use 2x 5w 2.2kohm resistors and 7 leds and it works but resistors get hot and fails. any help
  2. kellys_eye


    Jun 25, 2010
    Show a diagram of your wiring.
  3. frans


    Jul 5, 2018
    Sorry for the bad drawing

    Attached Files:

  4. duke37


    Jan 9, 2011
    Resistors have a maximum voltage rating maybe 1kV.
    You may be better to use a capacitor dropper with suitable voltage rating.
    Older fencers used neon indicators but they went black with use.
  5. davenn

    davenn Moderator

    Sep 5, 2009

    why did you put that diode in parallel with the LEDs ?
  6. AnalogKid


    Jun 10, 2015
    The LEDs should be in series, not in parallel.

    davenn likes this.
  7. FuZZ1L0G1C


    Mar 25, 2014
    You could also use inductive coupling, with 'hot' wire as the primary, coiled about 10-20 or more turns (depending on total Vf rating of LED's), around the secondary 'core', which is a single wire grounded at one end and un-grounded side feeding the series led array.
    Alternatively, an EMOSFET (Enhanced MOSFET) active sensor circuit could be used, with the gate wire (as an E-field sensor) close to but not quite touching the hot wire.
    Then use battery power of suitable voltage to bias the led array into conduction.
    Powering the load directly from the hot conductor may work, but will most likely drop the HT voltage pulses, make the 'zaps' less efficient, even to the point of not working.
    This is due to the controller being purposely designed to output high voltage pulses, but at only a few mJ (millijoules).
  8. AnalogKid


    Jun 10, 2015
    Ignoring the LEDs for a moment, 10 kV across 4.4K equals over 22 thousand watts. Most fence chargers make a pulsed output, so the average power is much less, but clearly it is much greater than the 10 W combined rating of your resistors. That kind of peak power can cause resistors to blow like a fuse. Also, you are pumping over 2 A through the LEDS. That probably is way above their pulse rating.

    Increasing the resistors to 100K each will reduce the peak current to 50 mA, a safe value for most standard red and green LEDs. Also, the peak power in the rresistors will be 500 W, so two 50 W resistors should survive.

  9. frans


    Jul 5, 2018
    Hi can you make a drawing of the inductive way running 7 leds? I need to make this as cheap as possible. To use two 50w 100k resistors will be way to expensive. Maybe also the capacitor way mentioned above? currently I use 2x 2.2k 5w resistors and the leds flash great. only resistors are getting hot and fail. if I use 4x 1k 5w resistors the led don't flash but 3x resistors it flashes but very weak.
  10. 73's de Edd

    73's de Edd

    Aug 21, 2015
    Sir frans . . . . .

    Everything that I have seen mentioned is WAAAAAAY - WAAAAAAAY - WAAAAAAAY too invasive and demanding of loading down your chargers HV output voltage from the unit.

    Instead, how about considering the use of a resistor HV divider string , a neon lamp, a photocell, a LED driver transistor, and as many BRIGHT LEDS as you want and fulfill the LED driver power aspects needs with a 9 or 12 V battery.

    Thereby, imposing an ABSOLUTE minimal loading of the fence chargers high voltage.

    73's de Edd

    hevans1944 likes this.
  11. duke37


    Jan 9, 2011
    Please tell us the type of fencer which you have and some pictures.

    Fencers come in two main types, battery driven and mains driven. The battery driven fencers are rather weak and nothing other than a neon should be connected to the output.
    Mains driven fencers are much more powerful, sometimes over 5J and they may be assessed with a 5kΩ load, simulating a long leaky wire.

    Neon bulbs will match the output much better than LEDs since they use a higher voltage and lower current.

    If all you want to do is to check if the fencer is trying to work, than connecting to the primary of the high voltage transformer will be easier than connecting to the output. This will not tell you whether the fence wire is shorted. A radio on long or medium wave can give a good indication of output and in the case of my brother,, a phone line will pick up the signal.

    Resistors have a maximum voltage rating, maybe as low as 200V. You need high voltage resistors (1kV) with at least five in series. I do not understand where the energy comes from to overheat your resistors unless the output is continuous rather than intermittent. 1J once per second is 1W.
  12. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    As per usual the OP is trying to tell us how he "solved" a problem but now needs to "fix" the solution. The OP should instead tell us WTF he is trying to DO!

    For example: I have this really powerful electric fence protecting my kraal from both four-legged and two-legged predators. I want to apply some lights along the fence to "warn off" the two-legged predators before they get fried on my fence. What is the least expensive, low maintenance, way to do this?

    Answer: Sense when the fence is charged (active) and use this information to light up a string of LEDs powered from a small, easily replaceable battery. @73's de Edd post #10 hinted on how to do this.

    Use a large-valued resistor to "light up" a small neon lamp, such as an NE2, that shines on a CdS photo-resistive cell with low current and minimal impact on the fence charging. This arrangement isolates the high-voltage fence from the low-voltage LEDs. Enclose the lamp and CdS cell in black plastic tubing to exclude ambient light.

    Use the output of the CdS cell to initiate flashing of a string of LEDs powered from the replaceable battery. Short high-intensity flashes at a rate of a few flashes per second will allow for maximum battery life. Think of the road-side LED flashers used to alert motorists entering road constriction areas. These usually run on 6 V lantern batteries and last for months on a single battery. You could get similar results from a few Li-ion rechargeable cells changed out on a monthly basis.
    FuZZ1L0G1C likes this.
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