Static electricity sensor experiment

Discussion in 'General Electronics Chat' started by jut, Dec 3, 2008.

  1. jut

    Thread Starter Senior Member

    Aug 25, 2007
    224
    2
    http://www.allaboutcircuits.com/vol_6/chpt_5/9.html

    I'm having trouble with this experiment-- can't get the LED to turn on/off as experiment describes.

    First off, I've tried two different N-channel JFET and get the same results: NTE312 and MPF102 (radioshack).

    I understand that if you reverse bias the gate, by touching one hand on the 100K resistor and the other on the (-) side of battery, it should pinch off the N channel and no current should flow, turning the LED off.

    However, here's what I noticed:

    1. With the 100k resistor floating, the LED is randomly on or off. Draws about 8 mA.

    2. When I touch the floating resistor with just one hand the LED dims. Draws about 2 mA. Maybe my body has a (-) charge?

    3. When I touch the resistor with one hand and touch the (+) supply with the other the LED brightens with a current of 12 mA. This was expected.

    4. When I touch the resistor with one hand and touch the (-) supply with the other the LED brightens with a current of 7 mA. Not expected. I would have thought the LED would turn off. But the current did decrease.

    5. Connecting the resistor directly to the (-) supply brightens the LED with a current of 9 mA! Not expected!


    Any suggestions??
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Interesting experiment. Never done it though.

    You might try to make an insulated plate to increase the surface area. Just a thought.
     
  3. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    This is like Bill Beaty's detector circuit. He has a few troubleshooting suggestions on his web page you could try.

    http://amasci.com/emotor/chargdet.html

    How are you measuring the current? With an ammeter in the circuit? If so, where is it? I'm wondering could it be adding a voltage drop that's messing with the FET bias?
     
  4. jut

    Thread Starter Senior Member

    Aug 25, 2007
    224
    2
    I'm using a fluke 179 DMM as an ammeter connected between the + side of the battery and the + side of the LED. I get the same results without measuring current.
     
  5. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    Hi
    I know what you mean, I was experimenting with jfets one time and was getting weird results too, but I think I put a resistor from ground to the source of the fet, and then applied a more neg. potential to the gate and got better results.
     
  6. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    To pinch-off (turn off) a JFET you need to apply a negative voltage (less than zero) to its gate with respect to its source. By touching the gate and the negative side of the battery the voltage between the gate and the source is zero (almost but very close) thus the JFET conducts its maximum current.
     
  7. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    If you connect its source to the led side and its drain to ground it will work as the experiment describes.
     
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