# Choosing the right resistor for a LED circuit

I am planning to do a high power LED lamp, and I would need some checkup for my calculations.

I plan to use some CREE LEDs from AliExpress, which claim to have an input voltage of 3.4-3.6V and a power of 3W. The idea is to run 12 of them in parallel from an ATX power supply, connected to the 3.3V rail. The leds will be connected with thermal tape to a big strip of solid aluminum for cooling.

I would try to be on the safe side and run the LEDs at only 2W each, meaning that each LED will eat up about 600 mA. So, I need to supply 0.6 * 12 = 7.2A. The good news is that my power supply can provide up to 15A on the 3.3V rail, so that should be fine.

Now, from my experience with small LEDs, usually is a good idea to insert a resistor. For the overall circuit, I would need a resistor of 3.3/7.2 = 0.5 ohm. And the resistor would need to withstand about 23.7W. Which seems big to me.

The question is: do I really need this resistor, since 0.5 ohm seems low enough for me (I'm also considering the about 1m wiring that I expect to have some resistance at this current)? Do such resistors commonly exist?

• If you have LEDs with a forward voltage of 3.4V to 3.6V how do you expect to light them with a power supply of 3.3V? You don't need a resistor at all and you won't get much light. Nov 23, 2019 at 22:40
• You have LEDs that light above 3.4V. You have a 3.3V power supply. You will get no light. A series resistor won't well if you are barely above the forward voltage of the LED.
– JRE
Nov 23, 2019 at 22:41
• No, you would need a resistor of (3.3-Vled) / 7.2A to drop excess voltage. But since Vled is 3.4V, there is no excess voltage to drop. Nov 23, 2019 at 22:44
• This answer explains how to calculate the needed resistor value.
– JRE
Nov 23, 2019 at 22:45
• @ElliotAlderson What I quoted were the values from the datasheets/documentation. In practice, without a resistor and on a small test, the leds did turn on. Nov 23, 2019 at 23:03

for a LED lamp you typically want to run the LEDS at near rated power, to do that you need to control the LED current and for that you need a higher voltage than the LEDs and some form of current limiting (like a resistor).

Use the 5V output from your ATX supply and use a 1.6V / 0.600 = 2.7 ohm resistor in series with each LED. use 1W or larger resistors for each LED.