I'm wanting to make a light box of sorts from LEDs and wanted to run it by you to confirm I have the right idea...

I have ordered 500 super bright LEDs off eBay and want to wire loads of them together. The forward voltage on the specs says 3V and my supply voltage is 12V (max 2A). I want to provide 20mA of current to the LEDs for a nice brightness.

I have calculated that if I wire 3 LEDs in series with a 150 Ohm resistor, I will get the desired 20mA of current. Is this correct?

Resistor = (12V - (3*3V)) = 3/0.02A = 150 Ohm

Now I have 3 LEDs using 20mA of current, can this be up-scaled to the full 2A power by simply duplicating the circuit as shown in the picture? So in theory I could run 300 LEDs from the power?

One last thing, If the LED's forward voltage is 3V, does that mean I can used 4 in series without a resistor with a 12V supply?


IMAGE IS SUPPOSED TO SAY 12V NOT 5V - I've been working with Arduino for the last while

Proposed Layout


You might find, that at 20mA, the LED forward voltage is slightly less than 3V and will increase the current a bit so go for a slightly higher value resistor with 3 LEDs.

Upscaling to several strings of LEDs should not be a problem.

You can't put 4 in series because there is nothing to limit the current thru them and, if all four were dropping 2.9V at 20mA, you'd be applying 12V across something that expects 11.6V and this may take a lot more current than 20mA.

  • \$\begingroup\$ Thanks. So it's probably best to go with a 180 or 220 Ohm resistor? \$\endgroup\$ – Adam May 12 '13 at 14:57
  • \$\begingroup\$ If the maximum current is greater than (say) 25mA you'll be probably OK with 180 ohms Adam. If you can link to the LED you are planning on using a more precise answer can be given. \$\endgroup\$ – Andy aka May 12 '13 at 16:15
  • \$\begingroup\$ Thanks Andy. There's no mentioned of maximum current on the page, but I read elsewhere that they can usually take a bit more current than the 20mA I was after. These are the ones I bought (No affiliation) link \$\endgroup\$ – Adam May 12 '13 at 22:53
  • \$\begingroup\$ The only thing I could see was the forward voltage rating of between 3.0V and 3.4V - devices at 3.4V may need a lower value of resistance if 3 are in series. \$\endgroup\$ – Andy aka May 13 '13 at 7:22
  • \$\begingroup\$ Well I have a load of resistors, so I'll try a few out on load for a while and see if they can handle it. Cheers. \$\endgroup\$ – Adam May 13 '13 at 18:04

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