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Mystery base current in darlington pair

  1. Jul 14, 2010 #1
    I was trying a simple Darlington circuit with two Darlington-connected NPN transistors. There is an LED in the shorted collectors, connected to a 5V supply, and the base is open. The trivial objective was to see that when you touch two fingers to the base and the supply positive, the LED turns on. That's all very straightforward and worked as expected, but to my surprise, there is a (very faint) glow in the LED (with about 150 mV across it) when I touch just the base as well! I initially thought it might be discharge from built-up static but the glow is not momentary. It sustains for a long enough time (although I haven't stared at it for longer than about 15 seconds at a stretch) to make it seem like it isn't capacitive discharge- yet isn't that what it must be? Where else can the current come from?

    I also notice that when I momentarily touch either the positive or the ground of the 5V supply and then touch just the base, there is a split-second flash in the LED, after which it returns to the low glow (making it seem like a capacitive discharge). Is the body cap providing the base current which then gets amplified? What circuit gets completed though?

    When I complete the circuit betn the base and the supply, I measure Ic = 100 mA and Ib = 20 uA. So hFE=5000.

    Base current is too small to register in the one-finger case, but Ic is 5-7 uA. Even if hFE stays the same (it might actually be lower at low Ic), that implies a sustained base current of at least 1 nA.

    The last observation is that if I touch the body of the (linear) power supply with one hand while holding the base with the other, the low glow of the LED goes off completely (but promptly reappears when I let go of the power supply). This makes it seem like it is indeed current being sourced by the body, and the circuit is completed through earth ground. This raises two questions:

    1. Aren't linear power supplies isolated from the AC input side? How is earth ground related to the DC ground of a linear power supply? Is there effectively a massive resistance from DC gnd to earth ground through which the body cap is discharging?

    2. When I let go of the body of the supply, the LED promptly comes back on (of course with the low glow). Shouldnt the body cap get discharged once I touch ground? How is the charge getting instantaneously replenished?
     
  2. jcsd
  3. Jul 14, 2010 #2
    Do you have a circuit that looks something like this? And resistor replaced with a diode..

    darling.gif


    That could be a collector-emitter leakage current of the first transistor Tr1 driving the base of the second transistor Tr2.
     

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    Last edited: Jul 14, 2010
  4. Jul 14, 2010 #3
    Your body could be acting as an antenna and picking up microvolts of ac signal that gets rectified in the base of Tr1. Install a 1 meg pulldown resistor from the Tr1 base to ground.

    Bob S
     
  5. Jul 15, 2010 #4
    waht: yup, that's the circuit, with resistor replaced by an LED, and base kept open.

    Bob S: Just tried what you suggested, and that worked! Now there is no glow when I touch the base of Tr1. Although- with the pulldown installed, shouldn't the current generated by the body antenna still divide such that some flows through the base (and get amplified as before)? It seems like the path through the transistors would be a lower-resistance path compared to the 1M to ground?
     
  6. Jul 15, 2010 #5
    With the 1 meg resistor installed, it would take over ~1.2 volts (over ~1 microamp) across it to get a base current.

    Bob S
     
  7. Jul 15, 2010 #6
    Got it. But what exactly happens when the base is floating? Does it float at about this voltage (so that the noise voltage generated by the body pushes the B-E diodes into conduction on positive excursions)?
     
  8. Jul 15, 2010 #7

    Averagesupernova

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    Yes.
     
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