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schematic

simulate this circuit – Schematic created using CircuitLab

I have +9v connected via 180 ohm resistor to the collector of an NPN transistor. The emitter is connected via 680 ohm resistor to the anode of an LED, with the cathode connected to ground.

If I allow a small current to flow into the base, the LED lights up and the voltmeter reads about +2V as I expected.

Here's the thing that doesn't make sense: with the base connected to nothing, I can touch the base with my finger to make the LED glow faintly. And when I measure the voltage across the LED in this circumstance, I find it reverse biased at -4v with one multimeter and -5v with a second multimeter. I'd love some one to explain this. Thanks.

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  • \$\begingroup\$ You could use the built in circuit diagram editor to make this more visual. \$\endgroup\$ – PlasmaHH Mar 27 '15 at 9:08
  • \$\begingroup\$ Thanks for that idea. I will try and find the circuit editor and post a circuit diagram. This was my first post, so I wasn't aware of this feature. \$\endgroup\$ – Steve Malikoff Mar 27 '15 at 9:16
  • \$\begingroup\$ Switch your voltmeter to AC volts and measure the voltage between your fingers and the (-) of your adapter for the answer. Swap the adapter around (the mains plug) and repeat the measurement. \$\endgroup\$ – Spehro Pefhany Mar 27 '15 at 11:57
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One possibility is 60hz being coupled in by you acting as an antenna. try using the AC range of your meter and see whats across the LED.

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  • \$\begingroup\$ Now I'm totally confused. The 9V supply in the original circuit was from a power adaptor connected to 240V AC. When I replace it with a 9V battery the strange behaviour goes away. \$\endgroup\$ – Steve Malikoff Mar 27 '15 at 11:18
  • \$\begingroup\$ if your body was transferring small amounts of 60hz energy into the base of the transistor (which is a very high input impedance), you might be turning the transistor on and off at 60hz (but just barely as you are not transferring that much energy). so you might see a small 60hz signal across your LED. When you measure AC on your meter DC range you get all sorts of odd results. so use the AC range and see whats there! \$\endgroup\$ – hwengmgr Mar 27 '15 at 11:22
  • \$\begingroup\$ Thanks for the idea, but now I'm totally confused. The 9V supply in the original circuit was from a power adaptor connected to the mains AC. When I replace it with a 9V battery the strange behaviour goes away. But now I notice that when I connect the 0V line of the power adapter to COM on the multimeter (leaving the +9V of the power adaptor AND the V lead of the multimeter totally disconnected) the Multimeter reads 2.5V AC and If I touch the V lead on the multimeter, it gives a reading of 12V AC. Is something dangerous going on here? \$\endgroup\$ – Steve Malikoff Mar 27 '15 at 11:26
  • \$\begingroup\$ its not dangerous at all. Your body is acting like an antenna and picking up 60hz which is all around us essentially being broadcast by our power wiring. if you had an oscilloscope handy, and you touch the scope lead with your finger you will see 60hz waveforms on the scope. \$\endgroup\$ – hwengmgr Mar 27 '15 at 11:52
  • \$\begingroup\$ By the way the voltmeter across the LED in AC was reading about 2V. I agree that explains the strange result when on DC range. I still not sure why an AC voltage reading was present with just the single wire between the meter and the power adaptor. Could the positive lead also have acted as an antenna? I'll try and get a CRO to look for the mains frequency signature. \$\endgroup\$ – Steve Malikoff Mar 27 '15 at 11:57

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