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I have 29 RGB LEDs connected in parallel and will control them with PWM.

They can handle 20 mA per color. I was planning to supply them with 5V and measured the voltage drop per block at 20x29 = 580mA input 5V.

  • Blue drops 3.06V so resistor needed: 3.9 ohm
  • Red drops 2.15V so resistor needed: 5.6 ohm
  • Green drops 2.95 so resistor needed: 3.9 ohm

Combined resistance: 1.45 ohm

The cable is a common UTP CAT6 cable I merged the ends so 8m of wire and measured with 2 multimeters and it's 3 Ω. This is the problem. The total resistance is 4.45 ohm and that is max 1123 mA instead of the desired 1740 mA

Can I do a 9V supply to get the current with adjusted resistor values or will it hurt the LEDs?

They won't be running at ful power all the time, PWM will adjust the colors so power dissipation should not be an issue.

Unfortunately I can't find the datasheet. Part number EVERLIGHT EVL-504-90-CA

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    \$\begingroup\$ How will you limit current on 1st LED with 9V and fixed R? That’s too much. You need heavy power rails hestore.hu/prod_10037597.html \$\endgroup\$ Feb 21, 2021 at 17:38
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    \$\begingroup\$ For 10% load regulation with 3.9 Ohms your cable loop resistance must be lower \$\endgroup\$ Feb 21, 2021 at 17:42
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    \$\begingroup\$ translate.google.com/translate?sl=auto&tl=en&u=https://… These specs are very wide tolerance so your performance will suffer with your design. 1.8..2.4 / 2.8..4 / 2.8..4V assume nominal for RGB is 2V , 3.4V, 3.4V \$\endgroup\$ Feb 21, 2021 at 17:47
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    \$\begingroup\$ Are you planning to use AWG 30 magnet wire? \$\endgroup\$ Feb 21, 2021 at 17:59
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    \$\begingroup\$ This is the design I'm stuck with not a lot of room. Parallel is not the best, but LEDS are from the same batch and first tests seem to be even lit. So I would do 9v with a resistor of 12ohm to get the 580mA combined current for one color. Would a led survive it or that 4v limit is the max and not the current that count? \$\endgroup\$
    – Reider
    Feb 21, 2021 at 18:03

3 Answers 3

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Can I do a 9V supply to get the current with adjusted resistor values or will it hurt the leds?

So long as the current through each LED is correct, the LEDs don't care what the supply voltage is.

But if you are controlling the LEDs individually, beware that the voltage drop along the cable will depend on the current, which will depend on how many LEDs you have on at one time. You could find that turning one LED on and off causes the other LEDs to flicker.

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This isn't any different for RGB LEDs than for any other set of 3 LEDs:

You can't put them in parallel with a single series resistor. Every color LED needs its own resistor.

but my 4m power cable is 3 ohm an this is the problem

Yes. This means you need a better cable. I don't know which cable you're using, but even a single 0.180 mm diameter copper wire strand (AWG 33, that is literally as thin as the average human hair) has lower resistance than that... My guess is you have a measurement artifact in there, and there's no problem at all.

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  • \$\begingroup\$ The cable is a common UTP CAT6 cable I merged the ends so 8m of wire and measured with 2 multimeters ant it's 3ohms. But those have a really good price for a 8 strand wire also twisted pairs will help with high speed I2C and PMW. Leds will be parallel the design is too complex and not a lot of room for individual resistors, but they are from the same batch and tested them, seem to be even so that is what I'm stuck with for now... \$\endgroup\$
    – Reider
    Feb 21, 2021 at 17:55
  • \$\begingroup\$ you're still not measuring properly, or you got bad cable. Cat-6 is AWG 24 at least, so 4m would have about 0.33 Ω of resistance. Twisted pair helps not that much with non-differential signalling like in I²C or PWM signals. Shielding might help, but you're much more the interference aggressor than the receiver of interference, so meh. \$\endgroup\$ Feb 21, 2021 at 17:58
  • \$\begingroup\$ Price is the main selling point, I could get 20m dirt cheep. I twisted the end of a pair, and measured at the end. The combined length is 8m and it is 3ohms unless both my meters are off but unlikely. \$\endgroup\$
    – Reider
    Feb 21, 2021 at 18:06
  • \$\begingroup\$ 24AWG = 84.22mOhm /m * 8m * 2 wire = 1.35 Ohm \$\endgroup\$ Feb 21, 2021 at 18:23
  • \$\begingroup\$ 5m Stripleds are dirt cheap with a remote control \$\endgroup\$ Feb 21, 2021 at 18:25
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It seems like there is no voltage limit for LEDS if you adjust the current and power dissipation accordingly. So it should work with 9V and the proper resistor. The parallel connection is not a great idea but that would have to do for now. Thanks everyone.

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