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Given an LED constant current supply (a.k.a integreated led driver, with an ac/dc conv.) with 3A output and b/w 14-24V depending on the load. The load is a series of 6 RGBW LEDs. The 3 loads I mention are the R, G, and W lines each with different forward voltage requirements at 3A. I've looked up that load switches or discrete power mosfets can be used, but that's the extent of where I've gotten. I'm thinking maybe load switches on the R, G and W lines might be the easiest solution but then again these switches have so many additional elements that i'm concerned if the output to the load is going to fluctuate the current.

Below is a visual of what I mean (not a schematic, sorry)

enter image description here

The drawing on the right was taken from the datasheet of a SMT RGBW LED. Just imagine 5 more of those side by side with each color connected in series. I want to switch the source between the three colors based on an input.

Tl;dr What is a good solution to switch the (a) supply between 3 power rails based on an input from a mcu?

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    \$\begingroup\$ Please draw a schematic. \$\endgroup\$
    – winny
    Commented Jun 7, 2023 at 8:01
  • \$\begingroup\$ What are you trying to do? make three different colours? \$\endgroup\$
    – Jasen Слава Україні
    Commented Jun 7, 2023 at 9:58
  • \$\begingroup\$ Yup that's right, three different colors, the real estate of the application calls for using RGBW led's. Sure, i'll put up a schematic. \$\endgroup\$
    – roaibrain
    Commented Jun 7, 2023 at 13:53
  • \$\begingroup\$ @winny, I put up a schematic \$\endgroup\$
    – roaibrain
    Commented Jun 10, 2023 at 16:43

1 Answer 1

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A lot of RGB diodes are controlled by putting all colors in series and then a MOSFET in parallel with each:

enter image description here

This is taken from the g8208 datasheet, but it's a general circuit used for RGB LEDs. The idea is that the MOSFET on shorts out the diode but still passes current to the others in series. The constant current source then regulates the current, adjusting the voltage as mosfets turn on or off.

Note that you will need a high side gate driver for the MOSFETs that are not referenced to ground.

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  • \$\begingroup\$ I'm a little confused without a visual when you mention a FET in parallel and how it shorts out the diode but passes current to the others in series. Can you please help elucidate. \$\endgroup\$
    – roaibrain
    Commented Jun 7, 2023 at 14:06
  • \$\begingroup\$ @Roshan Added a picture from a commercial RGB LED. \$\endgroup\$
    – user1850479
    Commented Jun 7, 2023 at 21:57
  • \$\begingroup\$ Thanks that helps, any recommendations on a power mosfet that fits the supply criteria, there's just so many out there with a lot more to the part name than I understand. \$\endgroup\$
    – roaibrain
    Commented Jun 9, 2023 at 14:45
  • \$\begingroup\$ Pick one with Rdon less than 50mOhm to avoid heating. One thing I just noticed, your supply has a lower voltage limit of 12v, which you might violate if you had only one or two LEDs on, the rest off. Make sure whatever current source you are using is ok with having however many LEDs shorted. \$\endgroup\$
    – user1850479
    Commented Jun 9, 2023 at 16:20
  • \$\begingroup\$ based off your drawing i'm instead going to have like 6 led's per color in series, so three series strings instead of the 3 individual. And will only one color to light up at a time. Is using either a NMOS or a PMOS fet alright? I'm also assuming that with this higher current rating the Vgs is gonna be high, will I potentially be able to drive this with an arduino? \$\endgroup\$
    – roaibrain
    Commented Jun 9, 2023 at 16:23

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