Electronic – Strange thing with 5mm LEDs and variable PSU

groundledpower supply

I have 3 standard LEDs (not COB just the tiny regular ones used in circuits with an Anode and Cathode in epoxy lens), I was just testing them today doing random things with them:

1.9v – 2.1v = red

3.0v – 3.4v = green

3.0v – 3.2v = blue

I think they're about as standard of an LED you can get. Anyway I have a variable power supply when I connect an LED to either the negative or positive (doesn't matter) and touch it to me or touch it to the ground it lights up. I can do this for any color and any polarity. I'm confused? It doesn't work on anything unless it's a conductive thing.. ie me, metal, the ground… but the ground is for AC electricity only? DC goes from positive to negative terminals? My variable power supply is floating, I put it on a bungee cord on my ceiling just to test it out..

So basically if I connect the positive OR negative (Anode or Cathode) to the variable power supply positive or negative (does not matter, any way works) and I touch it, it lights up. My PSU can be on 0 volts 0 amps and it still lights up. The power supply itself doesn't show any amperage draw … so I am thinking it's leaking AC or something crazy and that is going through me and lighting the LEDs? can they run on like 1 volt AC? I can't imagine 1 volt would travel through my body's resistance? I can't feel it at all, I've been shocked by 15volts AC before and you can feel it pretty decently so not sure if anything lower can even travel through my skin.

I'm ok at electricity, I understand the basics so I've been able to test some things as I said above but still not really sure what to say about this. I have a multi meter that measures no resistance at all as in, infinite ohms or no connection for electricity to flow. It goes insane when I turn it to DC reading, like numbers from -300 to +200 and it sits at 20volts AC when I turn it to AC. But I can touch the power supply end to the ground or to a metal rod I put in the ground and I get no spark or anything.


Best Answer

so I am thinking it's leaking AC or something crazy and that is going through me and lighting the LEDs?

I think you are on the right path. More than likely your power supply is a switch-mode type with a floating output. Internally, to reduce DC output switching noise, capacitors are sometimes used from the DC output back to the rectified AC line. This reduces the switching noise that is passed through the internal high-frequency transfomer and, of course, reduces the output switching noise but, a side effect is that there is a capacitive impedance from your AC line to your DC output.

It's harmless to people (although you can feel a little tingle sometimes) but it can drive an AC current of up to a mA or so and this is what is activating your LEDs. The LED rectifies the current and emits a small amount of light.

You can prove this be connecting a DVM from the DC output to ground and measure the voltage. A DC voltage measurement will be close to zero but the open circuit AC voltage may be tens to many tens of volts. Again, it is impedance-limited by the internal noise reduction capacitor and should be regarded as harmless.

Here's a picture of a typical SMPS showing the "rogue" component that is likely causing this effect: -

enter image description here

It's connected between the rectified AC line and the return line of the DC output. Image from this related Q&A.