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Today I was making a little circuit with an LED strip (3 LEDs in series) and I noticed a curious fact that I am not sure how to explain. I wasn't sure of the operating voltage of the LED strip, so I tried to power it with 3 volts to see whether it worked (with a DC power supply.)

I noticed the following:

  1. if I connect the positive terminal of the LEDs to 3 volts and I touch the negative terminal with my finger, the LEDs turn on very dimly.
  2. If I connect the negative terminal to the ground of my power supply they turn off (even if I still touch them with my fingers).
  3. If i connect them to a >9 V power supply they light up brightly

I am not sure how to explain this phenomena, my theory is that 3 volts is below the threshold voltage of the 3 LEDs (series) so they don't work with the power supply, but my body is capturing electromagnetic noise so the voltage at my finger sometimes drops and activates the LEDs for a brief moment. Do you have any explanation? Thanks for your help!

enter image description here

leopicchio
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    Please show your setup with a schematic or block diagram. – winny Jun 28 '22 at 14:48
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    I’m betting on y capacitor leakage - we get one of those here every week. Take a multimeter and measure from the positive terminal to your body or a grounded desktop computer and tell us what you get – Bryan Jun 28 '22 at 15:25
  • Probably high voltage leakage current from the power supply passing through the diodes and then through you to ground. The human eye is very sensitive to light, so even nanoamps of current can generate visible glow from an LED. – user1850479 Jun 29 '22 at 13:47
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    Bit of a silly question, but are your LEDs connected backwards? That would explain why case 1 doesn't light despite case 2 lighting. – Willa Jun 29 '22 at 14:27
  • I am quite sure they are not connected backwards, because if I power them with a higher voltage they light up – leopicchio Jun 30 '22 at 13:15
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    A good explanation about Y capacitor effects can be read here: https://electronics.stackexchange.com/questions/559118/why-do-i-have-44v-ac-between-ground-and-the-v-and-v-of-a-switch-mode-power/559128#559128 – Jens Aug 07 '22 at 20:50

2 Answers2

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First of all, 3 leds connected in series adds up to about 9.9 volts. Three volt power supply would work if leds are connected in parallel, if the supply carries enouph current. The leds lighting up when touched happens because our bodies are like a capacitor, storing electrical energy. The three volts can turn into many volts for just a few milliseconds. In fact, an led can survive much more voltage for a split second than it is rated for without damage, only for a few milliseconds. But to light an led for any significant amount of time, they need to be powered within their designed current handling specifications. I say current, because an led is a current driven device.

Dr. Dorfus Dingle
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  • It is more likely that the power supply has significant leakage from the AC mains, and your body provides a path to ground, which could be due to capacitance, or resistive leakage if other parts of your body have conductive paths to earth. – PStechPaul Aug 08 '22 at 03:52
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Case 1: The LEDs don't light up since the applied DC voltage, for three of them in series, is as low as or even lower than the forward voltage of one of them.

Case 2: The LEDs light up dimly on account of the small capacitive leakage current from the mains,through your body, to ground. It's apparent that the power supply is not earthed through the third pin.

Case 3: The LEDs do not even light up dimly since the capacitive leakage current to ground bypasses the LEDs, through the negative terminal of the power supply.

enter image description here

vu2nan
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