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Base current and Darlington pair

  1. Sep 1, 2015 #1
    Hi guys and gals,
    Here's a simple circuit mostly just an LED circuit with a Darlington pair.
    My question relates to the base current to the probe.
    If the probe is connected to an unearthed metal toaster (not plugged in), the LED glows briefly then goes out.
    If the probe is connected to the toaster (plugged in) the LED glows and stays lit.

    I have my ideas as to what's happening, but I'd really like to hear your explanations.

    Is this base current Displacement current?
    Where is the base current loop?

    Sorry, I can't seem to post images. Can someone advise please?
    Last edited: Sep 1, 2015
  2. jcsd
  3. Sep 1, 2015 #2
    Where is the circuit diagram? All I see an X'ed out icon.

    If you have two transistors with beta=100, then you only need 1 uA into the probe to get 10 mA into the LED.

    When you probe the disconnected toaster, you are effectively connecting your circuit to ground through the small capacitance of the toaster.

    When you probe the plugged in toaster, you are connecting your circuit to the power grid through the small capacitance of the toaster.
    I would expect the "steady" light to have some 60 Hz modulation.

    In both cases, your circuit diagram needs two more implicit capacitors to ground, one for your circuit and one of the toaster.
    You might get interesting results connecting your probe to a cookie sheet, which will have a large capacitance.
  4. Sep 1, 2015 #3
    Yes I can help


    Is a transistor supply a battery without any connection to the mains earth ?
  5. Sep 1, 2015 #4
    Thank you Jony130,
    Yes, the circuit is powered just by the battery - no connection to Earth.
  6. Sep 1, 2015 #5
    Well if so the base current flow through all sort of a parasitic capacitor between hot/neutral wires your body and the probe.
  7. Sep 1, 2015 #6
    What exactly are Earth and Earth Ground? Where are you doing these tests? In outer space? In a desert 1000 miles from any electricity? In a spherical superconducting metal ship floating in the ocean?

    If you are in a modern building with a three-wire 120 VAC, 60 Hz electrical service and copper plumbing then you can consider the plumbing earth ground.
    The GROUND wires (GREEN in the US) are also connected to earth ground.
    The NEUTRAL wires (WHITE in the US) are close to ground, but carry current so their potential can vary.
    The HOT wires (BLACK in the US) carry 120 VAC at 60 Hz, and radiate electrical fields that fill space.

    Every point in you house is coupled to the HOT wires and to GROUND. It is hard to calculate this coupling, but it is easy to demonstrate that it exists. You can imagine the negative terminal of you battery connect to HOT and GROUND through small capacitors. The same goes for the toaster.
  8. Sep 1, 2015 #7
    Thanks Jony,
    If it's just stray capacitance, wouldn't I be getting spurious results? Like the LED flashing when I touch the probe tip etc.?
    I get a steady LED on whilst it's connected to ground, but the LED goes on for a few seconds the extinguishes when I touch the probe to any large non-grounded object.
  9. Sep 1, 2015 #8
    Yes Willy, I realise that there are EM fields all around me, but the LED only illuminates when the probe is connected to a large metal non-earthed object or of course to an earthed metal object. As I replied to Jony, if it were capacitance effects, I'd expect it to flicker almost randomly.
  10. Sep 1, 2015 #9
    Until the probe is connected to a large metal object, the small capacitance of your circuit limits the base current to the Darlington and leaves the LED dark.
    The electric field is complicated, but not random.

    The data sheet for the BC548 shows current gains from 110 to 800. This means it takes approximately 10 to 1000 nA to get 10 mA through the LED.

    Have you tried measuring the LED current with a meter, or calibrating it by eye?
    Have you tried calibrating your Darlington? It would be good to have a better idea about the gain.
    If you do these things, you can estimate what the net capacitance of your targets are.
    If you put a small speaker or ear phone in series with the LED, you will be able to tell if there is any 60 Hz component to your measurement.

    Have you tried adding various sizes of disks to you probe? Small, medium, and large pan lids for example. And then maybe a cookie sheet.
    With the right probe, you would be able to may the electric field strength in a room, you should be able to find high- and low-field locations.
  11. Sep 1, 2015 #10
    Hi Willy,
    When I was testing and experimenting with it, I found that the component values are not that critical. The 560 ohm resistor should be selected to suit the battery voltage which could be from 3 to 12 volts. For a 6 volt battery, 220 ohms will be sufficient. For a 9 volt battery, 390 ohms, and for 12 volt battery, 560 ohms. The 1M ohm resistor can be up to 5M ohm (I haven't tried greater than 5M ohms).
    I went whole hog and built it into a texta pen case. The pointy end was the probe tip, so minimising stray capacitance from leads etc.
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