0
\$\begingroup\$

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.

Thanks...

\$\endgroup\$
  • \$\begingroup\$ Make and model of PSU + link to datasheet and a photo might help. It sounds like capacitive coupling. \$\endgroup\$ – Transistor Jul 28 '18 at 9:46
  • \$\begingroup\$ Can't find any specs on it, it was just a random power supply I got 6 years ago on ebay. Similar to this, but 3 amps max: ebay.com/itm/… \$\endgroup\$ – someguy Jul 28 '18 at 19:20
3
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ Thanks for the answer & Link, I couldn't find any related info I was looking around for a few days, heh. Should I go ahead and get a new one before it zaps me, or will it always be "harmless to humans"? I've had it 6 years and it was $30 on ebay. It is a max output of 3amps 0-30 volt and I've always wanted one of the 10 amp ones for doing more stuff with it... \$\endgroup\$ – someguy Jul 28 '18 at 19:25
  • \$\begingroup\$ Ebay purchases are not always recommended so best be safe and trace the product to a reliable supplier if you can and convince yourself the part is genuine. My daughter bought a charger from ebay for her phone and left it plugged-in behind the settee only to find that my grandson had managed to dislodge the so-called protective cover and nearly electrocuted himself. Anything powered from AC mains needs to be a reliable source. In fairness to everyone, this is the only answer I can give. \$\endgroup\$ – Andy aka Jul 28 '18 at 19:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.