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Darlington Pair question.

Discussion in 'General Electronics Discussion' started by bonkerz, Mar 13, 2012.

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  1. bonkerz

    bonkerz

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    Jan 7, 2012
    I set up a wee circuit with two transistors with an LED at the collector of the second transistor. I left a wire open so that I could use my fingers to switch on the circuit.

    The thing I'm wondering about is when I put my finger very close or onto the base resistor the LED lights briefly even though I've not also touched the wire connected to the positive of the 9V battery.

    Is current flowing through my finger to the base of the transistor?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    YOur finger and the base lead make a very small capacitor. Since everything is capacitively coupled to earth, a very small current may flow, enough to light up the LED if the gain is very high, as with a darlington pair.
    You can observe the same effect with open SMOS inputs, that's one of the reasons why those should always be tied to a fixed potential.

    Harald
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I suspect Harald meant CMOS.

    Yes, anything with a MOSFET or JFET front end is even more sensitive to this sort of thing than your darlington.
     
    Last edited: Mar 13, 2012
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Sorry,
    of course I meant CMOS - typographical error.

    Harald
     
  5. john monks

    john monks

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    Mar 9, 2012
    No. You are observing some AC coupling between your finger and the rest of the circuit. The AC may be leaking in from your utility or from a radio station. This phenomena is not unusual. Your finger is acting as one plate of a capacitor and the base lead is acting as the other plate of the capacitor. No electricity is passing between your finger and the transistor. Only a charge of electrons on the base lead is being effected by the AC charge of your finger. If you are standing on carpet your finger is probably developing a static charge as a result of moving around on the carpet.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    John, you are partly right, but why "No" to the previous posts?

    The AC coupling happens because there is a capacitance between finger and circuit. And while you are right that no electrons are flowing across the gap, a so called displacement current (http://en.wikipedia.org/wiki/Displacement_current) will exist. This leads to the electrons in the attached wire/transistor/finger to move to and fro with the frequency of the AC signal.
    From the point of view of the transistor, the movement of electrons is a current, the transistor can't "see" that there is a capacitor in the path.
    This is how capacitors work.

    With respect to the static: yes, ststic usually develops. But discharging the static to the device will
    1) happen only once. Unless you move around further, once dischrged, you will no re-charge.
    2) be destroyed if the charge is injected directly into the electronics without limiting the Energy. That's why you usualy have to take care of ESD protection (http://en.wikipedia.org/wiki/Electrostatic_discharge).

    Harald
     
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