Resistors - Series, Parallel, And Both Together

applearcher

Joined Apr 21, 2019
6
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?

ScottWang

Joined Aug 23, 2012
6,981
The power dissipation should be use at least 3 times of the calculation values in the real world.

Dodgydave

Joined Jun 22, 2012
9,869
Can you draw what you're trying to do...?

Tonyr1084

Joined Sep 24, 2015
5,559
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.