Resistors - Series, Parallel, And Both Together

Thread Starter


Joined Apr 21, 2019
I have an LED array, 4 x 4 LEDs, 3.2v, 150mA, 20v power supply. In the current configuration using 56Ohm 1w resistors the power dissipated by each resistor is about 950mw. This is really close to the resistors rating.
If I add 3 - 100Ohm resistors in parallel branching out to each group of 4 LEDs and change out the 56Ohm for say 27Ohm. This puts me a little over 56 for each group. Will this affect my light output much? Will adding the 3 in parallel reduce the power dissipated by each resistor?


Joined Sep 24, 2015
The power dissipation should be use at least 3 times of the calculation values in the real world.
While I respect the opinions of my contemporaries, sometimes I take a different approach. Scott recommends three times over the required rating. In other words, if you dissipate 1 watt then the resistor should be three times, or 3 watts. That will certainly work and will never give you any issues with heat. However, in my opinion that's a little overkill. I've often seen others here suggest twice the rating (2 watts), but I've gone with what I've learned to be standard engineering practice to design at 150% normal operation. So if you're operating at 1 watt using my principal, 1.5 watts would be sufficient. However, since you can't get a 1 1/2 watt resistor the next best thing would be 2 watts.

@ScottWang I respect your opinion. I just disagree with over building a circuit. You certainly wouldn't put a 50 watt resistor on the circuit, but at 50 watts you most definitely would never suffer a failure. But expense and real estate are thrown out the window. I think 3 watts is too big. That's all. Disagree with me if you like, I welcome differences of opinions and the discussions that follow.