Transistor turning on without base voltage?

Discussion in 'General Electronics Chat' started by iinself, Feb 8, 2013.

  1. iinself

    Thread Starter Member

    Jan 18, 2013
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    Hi,
    could someone please explain why the LED glows in the following circuit without any base voltage when I touch the base lead. I observe that when I touch the base the Vbe voltage reads as 3.7V, when not touched it is fluctuating at 300mV. The transistor is MJE3055T.

    I thought the transistor can conduct only when the base-emitter is forward biased but without any direct voltage just by touching the base the transistor seems to conduct??

    Thanks
    Arvind
     
    Last edited: Feb 8, 2013
  2. MrChips

    Moderator

    Oct 2, 2009
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    3,370
    The base-emitter junction of a bipolar junction transistor (BJT) responds to current, not voltage. The LED will light up with a collector current of less than 1mA.
    If the typical current gain of a BJT is about 300, it only takes about 3μA from your finger to turn on the LED. Your body acts as an antenna and is picking up a lot of electrical signals, mainly 60Hz AC power from around your room.
     
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  3. Audioguru

    New Member

    Dec 20, 2007
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    I doubt the transistor will turn on when the base is touched if the circuit is in the middle of a desert hundreds of miles away from civilization (no AC mains signals).
     
  4. killivolt

    Active Member

    Jan 10, 2010
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    Do "EM's" affect the Transistors P/N junction?

    Is it the same reason why on occasion the transformer lighting would set off my, capacitive Touch circuit.
     
  5. iinself

    Thread Starter Member

    Jan 18, 2013
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    Thanks, got that. Current amplification happens whether base-emitter is forward biased or not. So a BJT is a current amplification device and not a voltage amplifaction device. When the LED turns on can I say the BJT is in the ON state? One more question still remains as to why is my voltmeter showing 3.7V when my supply was only 3.1 and all of this 3.7V is on the base-emitter junction ??
     
  6. #12

    Expert

    Nov 30, 2010
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    An idea that is not normally told to noobies is that transistors have a conduction ratio that goes all the way down to microvolts Vbe. They don't, "break over" like zeners or LEDs. If you want to say your transistor is, "on", that is OK with me. Fact is, a bipolar transistor can be manipulated to "on" way down in the nanoamp range.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    A voltmeter can be set to show AC or it can be set to show DC. You did not say which.
    3.7V is problably the level of the AC signal picked up by nearby mains electrical wiring.
     
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  8. iinself

    Thread Starter Member

    Jan 18, 2013
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    I checked again, it is DC volts ! When I don't touch the base then Vbe is fluctuates a lot around 300mV and when I touch it it is 3.7V. Yes it seems the only explanation is that by touch I am supplying the additional .6V, interesting !
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    DC can be positive or negative. The DC voltmeter shows which but you did not say which.
    I suspect the AC signal picked up by you caused the base of the transistor to swing 0.7V positive and 4.4V negative. So it shows the difference which is -3.7V.
     
    killivolt likes this.
  10. killivolt

    Active Member

    Jan 10, 2010
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    Ok, then.....
     
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