How to adapt the value of a resistor that feeds 4x less LEDs - Page 1
Forum section: electronics / beginners

author: laxamar
author_rank: Contributor
timestamp: January 03, 2023, 01:45:29 am
content: I'm pretty sure this is a stupid and simple question, but I can't shake the feeling I'm forgetting something about OHM's law or LEDs.....I've used 3 variable resistors to control the color of 7 RGB LEDs. I played until I found the colors I was looking for; for the RGB LED part I was using. However, these RES values are for exactly 4 LEDs (The RGB LEDs are really 3 LEDs in one package with a common cathode).My stupid question is:For a single RGB LED, should I multiply the resistor values x 4 or is there more to it when it involves LEDs. I am trying to achieve the exact same mix of color and intensity (and power) as my test PCB.

author: Ian.M
author_rank: Super Contributor
timestamp: January 03, 2023, 02:45:51 am
content: You shouldn't have paralleled LEDs as they don't share current evenly. The one with a slightly lower Vf than the others takes a little more current, gets hotter so its Vf drops, so it takes even more current & etc. If you are lucky, and it has enough internal 'slope' resistance, it merely hogs the current, but if you are unlucky, and the total current available exceeds the max. If for a single LED, it can take so much it burns out, either slowly or near immediately, then unless it short-circuits them all, the current redistributes with one with the next lowest Vf hogging it again and the whole cycle of failure repeats more quickly as the current per LED is higher with fewer LEDs.Therefore you may well have to re-do your colour balance for a single LED. A quarter of the current (and thus four times the resistance) is only a starting point. OTOH if you aren't too fussy it may be good enough.N.B. LED characteristics can vary significantly from batch to batch and if they aren't closely binned and sold as matched (at a premium price) from LED to LED within a batch, so if you are fussy, or have to match existing lighting, you may well have to re-do the color balance anyway.

author: laxamar
author_rank: Contributor
timestamp: January 03, 2023, 03:41:24 am
content: The parallel design was just an R&D for exactly that, get approximate colors I want and have an idea of the proper resistors. I'll be using the closest E12 value anyways. Not too fussy. The production RGB will be a single LED.

author: BeBuLamar
author_rank: Super Contributor
timestamp: January 03, 2023, 03:49:36 am
content: You're better off do the test using 1 LED or series the 4 LED's. That way you know the current needed to obtain the correct color. So later no matter how many LED you use you know the current needed.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: January 03, 2023, 04:42:01 am
content: Multiplying the resistance by 4 should work, assuming all the LEDs eat up the same current (which might not be a valid assumption). It might be a good idea to use a microcontroller to change the colors using PWM so it could be adjusted, depending on your application.

author: Gyro
author_rank: Super Contributor
timestamp: January 03, 2023, 09:58:46 am
content: Quote from: Doctorandus_P on January 03, 2023, 03:29:54 amYet, for some reason, you thought it worth telling us that?

author: laxamar
author_rank: Contributor
timestamp: January 03, 2023, 06:12:57 pm
content: Quote from: EggertEnjoyer123 on January 03, 2023, 04:42:01 amOh, PWM is waaay overkill (although I did think about it with other feature creeps ..)It's simply a BCD 3 bit info I was going to use with a 7Seg LED to display on multiple boxes. But the # info was not as critical as knowing the difference between boxes, so I settled on an RGB LED that lowered the component count and complexity. These are SMD parts on a PCB an so the first attempts were abysmal with either no info or just overwhelmed with green. It was starting to get expensive to manufacture, ship and be disappointed. So I built a test bed to get a definitive resistor value that will work will all combos of R-G-B and be human (or let's say me) differentiable.I will need to of course have one more test build of the whole design with these values to be sure, but I want to put the least important stuff (color of LEDs) behind me and concentrate on actual functionality!My phone has HDR and auto corrects colors, but this is the test boards and what I get.

author: Ian.M
author_rank: Super Contributor
timestamp: January 04, 2023, 01:45:10 am
content: You've got a potential accessibility issue for colourblind users:https://accessibility.psu.edu/color/colorvisiondetails/

author: laxamar
author_rank: Contributor
timestamp: January 04, 2023, 03:24:59 am
content: Quote from: Ian.M on January 04, 2023, 01:45:10 amYou are correct. It would be an issue if the color was intended as useful information. In *this* particular case, it just will light up the correct box. The extra information about the box number is actually irrelevant and was a feature creep I added thinking it's a quick, simple and free add-on .. which it was not.Wasted way too much time, effort and money on an irrelevant esthetic. A simple red LED would have sufficed. At this point it's just pure stubbornness on my part to make it work and give an extra color cue based on the box position. Nobody but me will even notice it.