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I made a Darlington pair from two 2N3904 transistors, and connected the collector (of the entire Darlington pair) to the 3.3V output of a breadboard power supply (powered by an adapter that plugs into the wall socket), connected the emitter to a voltmeter, and grounded the other end of the voltmeter. The voltmeter began registering a voltage that was greater than the original voltage (~4.6V), when I touched the base connector, via a wire. The effect was improved when I touched a radiator with my free hand, and the voltage increased to 36.5V. Even more ridiculously, the effect persisted when I shut off the supply. I am 99% sure that the effect was not due to any sort of capacitors in the power supply, or another energy store. Is this due, perhaps to inductive energy harvesting, or RF energy harvesting? I have included a picture of the schematic and a photo of the circuit (the squiggly wire bit is the bit I touch with my finger).

https://i.stack.imgur.com/jInwS.jpg

schematic

simulate this circuit – Schematic created using CircuitLab

===EDIT=== It still works when I unplug the power supply, and I can even light an LED very very dimly, and charge a capacitor. Now I am nearly fully sure that it is due to inductive coupling from the transformer in the power supply.

16-bit-apps
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3 Answers3

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powered by an adapter that plugs into the wall socket

and

grounded the other end of the voltmeter

The DC output voltage of an average wall-wart may only be a few volts but, the AC common mode voltage of both DC output wires to ground/earth can be dozens to the low hundreds of volts but, don't worry, that common mode voltage is sourced via a very high impedance and you'll only feel a tingle but, on a high impedance voltmeter, it will display tens of volts.

Add to that is your hand acting as a capacitor plate for the AC voltages within your room. Or, your body could be lowering the impedance to earth. Both will change the voltage seen on the meter.

Again, these won't hurt you but, they will register on a meter connected to a transistor amplifier.

Is this due, perhaps to inductive energy harvesting, or RF energy harvesting?

Do the math; the power levels will be microwatts and just not worth exploiting. I mean; the power is coming from your AC supply no matter how hard you might argue this so, you are paying for it anyway. No free energy thing going on.

Andy aka
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    The transistors probably act as rectifying diodes in this configuration, making the AC voltage visible even in DC range of the multimeter. – jpa Mar 17 '24 at 06:59
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A typical digital voltmeter has an input impedance of 10 Megohms. That's is a very high impedance, and it takes very little radiated energy to develop a measurable voltage across it. Your body is acting as an antenna, picking up electric fields in the area. I have a 5-1/2 digit meter. If I lay the probes on the bench, not connected to anything, it's displayed value changes when someone walks by. Literally, a disturbance in the (electromotive) force.

AnalogKid
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  • My handheld meters can't do that. That's for sure. I wonder what is up with OP's meter since it's a cheap battery powered meter. – DKNguyen Mar 16 '24 at 19:59
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    The transistors are probably half-wave rectifying too. Even on DC the meter could show a significant voltage. – Simon Fitch Mar 16 '24 at 20:31
  • This is an old experiment, picking up electrons from what seems like room air. There are lots of factors but this answer gives a good, strong reason why this happens. Also see this question and answer for more technical details. – Stephan Samuel Mar 18 '24 at 14:53
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Transistors have parasitic capacitance themselves that can store small amounts of energy. Now the voltage of these stray capacitance may not be definite but it can deviate a lot depending on the other parts of your circuit (i.e. you wire, your hand, etc). Parasitic capacitance (and inductance) are a messy thing but in the presence of AC small signals in the air these voltages manifest in these invisible capacitors and thus can be measured by a voltmeter.

The voltage doesn't go away because your circuit has no load but if you put a resistor or LED it will discharge the voltage into somewhere but it's really a small amount of energy (because these stray capacitors are small) so the energy they store cannot kill/hurt you. You may not even feel it.

micropyre
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