LED powered with my finger?!

Discussion in 'General Electronics Chat' started by Voltboy, Jul 26, 2010.

  1. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    Hello, I was experimenting with some LEDs to do a night lamp, when something really weird happened. One of the LEDs, the only white one that I have, if it was grounded, or connected to Vcc, and I thought the other electrode with my finger, the LED would like up, not as if the battery was connected, but a very faint light that could be easily seen. The voltage came from a cell charger that outputs 5V, I stripped the wires and used it as my voltage source. I thought this might happen because it is grounded to the mains (via the charger?), but when I connected it to Vcc and touch it with my finger on the cathode it still lighted up.

    Any ideas? This is very weird. I also tried with other LEDs and didn't happen.
     
  2. timrobbins

    Active Member

    Aug 29, 2009
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    Are you saying your LED is not connected in any way "across" the 5VDC power supply terminals (ie. one terminal is completely isolated), but the LED is connected to one 5VDC terminal of the power supply, and the other side of the LED is only connected to you and that you may be connected to 'earth'?

    If so then do you know if the power supply DC output is isolated from earth - it will be if the power supply is modern wallwart with no earth pin.

    Ciao, Tim
     
  3. retched

    AAC Fanatic!

    Dec 5, 2009
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    I would be interested in seeing a photo of this phenomenon in action.
     
  4. tom66

    Senior Member

    May 9, 2009
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    I've seen this phenomenon. I had my scope probe connected to the Vss of a PIC and was touching the cathode of an LED connected to Vdd. It dimly lit, because the power supply was connected through the scope to earth ground and I was earthed.
     
  5. retched

    AAC Fanatic!

    Dec 5, 2009
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    I was thinking the LED was grounded and he touched the anode and lit the LED.

    I guess both.

    Maybe you have an electric personality!? ;)
     
  6. sage.radachowsky

    Member

    May 11, 2010
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    My personal resistance is about 1 MegOhm from one hand to the other.
     
  7. timrobbins

    Active Member

    Aug 29, 2009
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    I was thinking capacitive coupling from hot primary side of power supply through one DC terminal of power supply, via LED to earth and back to neutral of power supply - power supply would have no earth (double insulated) but have a switchmode with high capacitance coupling to secondary.

    I recall seeing light at less than 1mA in a LED earlier this year - I'll recheck tomorrow what it can get down to and still be visible.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    It will be. LEDs are fairly nonlinear in how much light they produce to current. A little current goes a long way, same with neon bulbs.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    You are in a high electromagnetic area.

    Ur body acting as an antenna of some sort
     
  10. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    I rechecked myself and I wasn't grounded, was wearing socks, my floor is wood. And yes, the wallwart has no earth pin.


    Actually my WiFi Router was about 3m from where I was.




    I will try to see if I can get a picture of it, and even better if I could measure amperage or voltage across.
     
  11. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    Here are the pics, sorry for the bad quality, they're from my phone.

    The multimeter shows a reading of 0.00 amps even when using the 200μA range.
    The voltage is a surprising 1.4V


    [​IMG]
     
  12. tom66

    Senior Member

    May 9, 2009
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    What power supply are you using?

    I bet if you use a battery it won't work.

    As I have said before it is most likely leakage current through you to earth ground. With sweaty palms you can have a resistance as low as 40kohms. That is enough to make an LED glow dim.
     
  13. Voltboy

    Thread Starter Active Member

    Jan 10, 2007
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    On the photo the LED is grounded to a 5V phone charger that I use as a mini power supply, also its connected to a 150Ω resistor.
    Supposing that I was connected to ground somehow would only explain this if I connect the LED to Vcc and my finger touch cathode, but it works both ways. And anyways I dont believe I'm grounded, this time I was wearing shoes.

    EDIT: Using a battery this doesnt happen. I removed the resistor and still works. What is really weird is that my body acts as Vcc and also as ground, depending which way I'm touching it.
     
  14. timrobbins

    Active Member

    Aug 29, 2009
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    Voltboy - the concept that you think you are not grounded is what a lot of people think, up until the time that they accidentally touch a mains active wire - then they rethink that concept!

    The phone charger is most likely a smps, and the current used to turn the led on is most likely the high switching frequency used in the smps, and it gets 'around' the loop through capacitive coupling and electrically conducting parts (the led, the mains cable, the body), of which your body makes quite a good capacitor plate to couple back to the general earth around you.

    Reminds me of those screwdrivers with neon bulbs in them that glow when you touch active.

    You could do some good experiments with trying to increase/decrease your capacitive coupling to earth!

    Ciao, Tim
     
  15. sage.radachowsky

    Member

    May 11, 2010
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    So Voltboy, let me get this straight... You can ground one leg of the LED, and then touch the other leg, and see a light in the LED, and you are not touching anything else except the floor with your feet?

    I have in the past rectified power from my finger, using a Schottky diode and a 10 uF capacitor. I could charge the capacitor up to some voltage -- I forget, but it was around 1 Volt.

    But I never thought I would see an LED lit up if I did that. It looks bright, too -- not just a faintest glow.

    I am guessing that your body is picking up the electromagnetic waves produced by the current in your walls, and the diode is rectifying it and glowing.

    I doubt it is from a higher frequency RF source, like 2.4 GHz of your access point.

    I'm really interested in being able to gain power this way.

    What if you send the current through a very tiny transformer? I am sure it would not work with any normal sized transformer, but Coilcraft nowadays makes very tiny transformers for energy harvesting. I should try it out with one of those and see if I can get a higher voltage. Since it's an AC source, that would be an easy way to harvest it. Less would get lost in the rectification.
     
  16. tom66

    Senior Member

    May 9, 2009
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    AFAIK, neutral and earth are connected at the substation or power pole. So if you're earthed, then you're also connected to neutral, which explains why you get a small current from the SMPS even though you haven't got a direct return path.
     
  17. ELECTRONERD

    Senior Member

    May 26, 2009
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    I've wired LEDs with a battery and resistor and by replacing the switch with my hand, it will dimly light the LED. To confirm this, try pinching the wires harder and see if they get any brighter.
     
  18. VoodooMojo

    Active Member

    Nov 28, 2009
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    Voltboy lives up to his name!
    Over unity is not a myth!!!
     
  19. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Yaaa.. He is a cell of some sort, I think :D
     
  20. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    That means you definitely were grounded. Or do you think that the neon mains-checker will not light up when you are standing in socks on a wooden floor? Usually it lights up even when standing in rubber shoes.
     
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