Finger on base of transistor!?

Discussion in 'General Electronics Chat' started by ISB123, Sep 27, 2015.

  1. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    Hello, I have been messing with a basic circuit,just a LED,transistor and resistor.
    I noticed the "problem" when I was removing the base resistor and while I was doing so I accidentally touched the base of transistor with my finger and LED turned On,there was no closed loop since I never touched the ground pin. At first I thought that there was a problem with AC mains and that somehow I was closing the loop with my feet,so I put on my thick rubber boots and tried again but LED turned on again.
    Then I though that there was a problem with my PSU so I unplugged it from the socket and run the LED just using capacitor charge and LED turned on again although I never made contact with the ground line.

    Holding a diode in hand and touching the base pin with Anode end doesn't turn the transistor on so I must be closing the loop somehow.

    So does anyone know what's going on?


    Capture.PNG
     
  2. dannyf

    Well-Known Member

    Sep 13, 2015
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    static. This can work especially for high beta (darlington for example) small current transistors or mosfets.
     
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  3. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think you induced a voltage to the base due to the mains in the naberhood.
    It is like showing a sinewave of the mains on an oscilloscope by touching the tip of the probe with your finger.

    Bertus
     
  4. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    This is due to the electromagnetic fields that surround us and which induce currents in our electrically conductive bodies; you can hear it if you touch the input of an audio amplifier.
     
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  5. #12

    Expert

    Nov 30, 2010
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    Ancient diagnostic tool: Finger Hum. First invented for testing audio amplifiers.:D
     
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  6. dl324

    Distinguished Member

    Mar 30, 2015
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    That won't stop your body from acting like an antenna and picking up radiated power from nearby transmission lines or wiring.

    I used this "feature" to implement a snooze function on a digital clock I made in the 70's. Instead of using a switch, I used an exposed contact (screw head) attached to the gate of a JFET. The person I gave it to never complained that snooze didn't work...
     
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  7. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    I wouldn't leave home without it.
     
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  8. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    Thanks guys never really thought that electromagnetism could switch a transistor.
     
  9. joeyd999

    AAC Fanatic!

    Jun 6, 2011
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    If it couldn't, then a whole class of technologies would be non-existent. We'd still be early in the industrial revolution.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    If you going to some wild or wide place and make sure that there is no any city power around that place, and to do your experiment again, remember to use battery to be the power supply.
     
  11. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    It's the same principle as touching an oscilloscope probe with your finger--you'll see the 50/60 Hz waveform. It's that signal that is switching the transistor.
     
  12. ISB123

    Thread Starter Well-Known Member

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    I live 100ft from 25kV power line so no wonder strong magnetic field is present.
     
  13. DerStrom8

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    That could be part of it, though you will see this effect anywhere with mains wiring.
     
  14. ISB123

    Thread Starter Well-Known Member

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    LED was very bright I thought it was going to burn out o_O.
     
  15. ramancini8

    Member

    Jul 18, 2012
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    It is easy to in-circuit test a transistor for dc function with a clip lead and a resistor. AC in-circuit testing would be complicated except our body is a large antenna and just touching the base lead injects an ac signal. Dead ac transistor response equals no noise on output, and live is lots of noise.
     
  16. dannyf

    Well-Known Member

    Sep 13, 2015
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    If that's the case, there may be something else going on. Even with an efficient high brightness LED, you would see it lights up faintly. For yours to be THAT bright, something other than rectified static is going on.
     
  17. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    Here is how bright the LED is,it gets equally bright if i connect it to antenna or my finger.
     
  18. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    That image shows a red LED, but your original schematic says green. What other differences are there between the original circuit and the one you have built up?

    Not necessarily. We don't have enough information to know how brightly it should be lit.
     
  19. ISB123

    Thread Starter Well-Known Member

    May 21, 2014
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    It says green but that's only because I was too lazy to find the red one in library. But here is the test with green one, shines a bit less brightly because of slightly higher voltage drop.
     
  20. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    And you're feeding 2V into it?

    A red LED generally has a voltage drop of 1.8V. Using a 2V supply and a 300 ohm resistor, that means that the current flowing through the LED is (2V-1.8V)/300 = 0.6mA. Something seems wrong there. If the LED is only seeing 0.6mA it would not be shining very brightly. So what dannyf said (besides "rectified static", which is total rubbish) is correct--it should only be lighting very faintly, even when the transistor is in saturation.
     
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