I recently harvested 5 bright white COB LEDs from a light fixture and am trying to figure out what kind of power it would take to power them individually.

The fixture contained a transformer with an output of 36VDC 0.47A Max and was epoxy potted so I can't see if there were any resistors inside.

The LEDs were simply wired in series, so ignoring any resistors, can I assume that the voltage drop across each LED is somewhere around 7.2V? I tried measuring the Vf of an individual LED on my meter but couldn't get a reading. (36/5=7.2) If not, is there any way to calculate/estimate what it would take to power these LEDs individually?

  • 2
    \$\begingroup\$ Get a power supply, get a resistor to limit the current, bang it all together and measure the voltage dropped \$\endgroup\$
    – user103993
    Nov 8, 2016 at 16:42
  • \$\begingroup\$ A Transformer can't do VDC. Your DMM might be confused with rectified signal, between peak and RMS. The LEDs are usually around 4 - 4.2V. \$\endgroup\$ Nov 8, 2016 at 17:42
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    \$\begingroup\$ Hayman is correct. Best thing to do is to get a variable power supply (with current limiting, if possible) and get a high wattage resistor of about \$10\:\Omega\$. Place it in series with the LED and set the voltage to \$3\:\textrm{V}\$ to start and gradually increase the voltage. When the brightness is about where you think you want it, stop and measure the voltage across the resistor and measure the voltage across the LED. You will have your operating voltage and current from that. Set to measure very quickly if you don't have a heat sink for the LED!! \$\endgroup\$
    – jonk
    Nov 8, 2016 at 19:35

1 Answer 1


If there was a series resistor, see if you can probe that for a resistance value. Then you can figure a forward current and voltage of each LED. If there was no resistor, I'd say your answer of 7.2V is accurate enough. If they were all in series, you know at least they all have the same forward current of less than 470mA

Each bit of information will give you a better answer, but nothing beats a datasheet. Any model numbers or identifying marks would be the most valuable bit of info


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