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I have basic electronics knowledge and have repaired many things, but I'm a little confused about this. I have a 220 V LED PCB like this:

Seller's image of PCB

One of the LEDs is burning out and causing the whole array to flicker and die. I assumed like most things, I could replace the LED after finding out what voltage and current it was. When I desoldered and tried to power up one of the good SMD LEDs on a DC supply, I could not get it to light up, even after supplying between 3-28 V (the maximum my supply can provide). However, when I hooked it up to a variable AC transformer, I got the LED to light up fine at around 22 V.

Now I'm confused. Is this an AC-only LED? I've never heard of that.

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  • \$\begingroup\$ It would be a good idea to link to the product page - or, even better, to the datasheet. \$\endgroup\$
    – JRE
    Feb 12, 2020 at 11:53
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    \$\begingroup\$ 22 Vac peak rectified is ~31 Vdc. But sure, 28 Vdc should be close enough unless you have 29 V Vf to overcome. How did you connect it? \$\endgroup\$
    – winny
    Feb 12, 2020 at 12:07
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    \$\begingroup\$ Was your DC power backwards? \$\endgroup\$
    – user253751
    Feb 12, 2020 at 12:07
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    \$\begingroup\$ Some low power off-line power supplies use a capacitor to provide part of the voltage drop. These won't work so well off DC. \$\endgroup\$ Feb 12, 2020 at 12:09
  • \$\begingroup\$ I removed one of the good LED's completely from the PCB and soldered two wires onto it. Then i connected it to my DC power supply. I turned the voltage up slowly from 3v all to way to max and it would not illuminate. When i connected the same bare LED to an AC transformer, it lit up when the voltage reached approx 22v. \$\endgroup\$ Feb 12, 2020 at 12:26

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There are some LEDs in that size package (2835) that work at 18V, such as the Cree J-series. I assume (and I could be wrong) that they are effectively a number of junctions in series on a single die, since the die count is specified as "one" on other manufacturer's datasheets.

Perhaps there is a higher voltage version this manufacturer is using to make 230VAC lights simpler, like 36V, or (more likely, I think) they are using two 18V dies in the one package.

At the peak of 22VAC RMS (~31V) a 36V LED would certainly be emitting a fair bit of light, though not nearly at full brightness.

Good question, that's an interesting evolution in LEDs.

Edit: Here is a real-life example of a two-die 36V white LED that may be the exact one used in your lamp:

enter image description here

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  • \$\begingroup\$ If it helps, when the LED's were in situ on the PCB I measured the voltage drop across one and it was about 20v with a multimeter. Actually 20v in both AC and DC modes. For now i've replaced all the LED's with 3v white ones. The controller on this PCB is set with a resistor to provide a very low constant current and it's been working fine for a few hours, but i'm still very interested in working out what the original LED's were. \$\endgroup\$ Feb 12, 2020 at 13:06
  • \$\begingroup\$ There's no filter capacitance so the LEDs are not seeing the 36V operating voltage all the time. Average may well be 20V. Verified by the fact you're reading AC voltage across them! \$\endgroup\$ Feb 12, 2020 at 13:10
  • \$\begingroup\$ So are you saying that the chances are these LED's are 36v DC units and my DC power supply just wouldn't go high enough to illuminate them? \$\endgroup\$ Feb 12, 2020 at 18:24
  • \$\begingroup\$ Yes, exactly, if you added a 9V battery in series (additive) it should work. \$\endgroup\$ Feb 12, 2020 at 22:13

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