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I have two things to say before I ask my question:

  1. I am a layman with very little theoretical understanding of electronics (my question text will prove this, trust me). Sorry if my idea of the problem is a weird mess.
  2. Coming from StackOverflow, I know that asking a question about such a specific scenario/problem may not add much value for others. I really just need help understanding what's going on here and I hope there is some general understanding in the end that might also help others.

What I want to achieve

I want to build a little box, exposing three rotary dials (or potentiometers in my case), each controlling the brightness of one channel of a single RGB-LED (common ground).

What I have tried

I bought three simple PWM-Modules (link to the product page for reference, not a referrer link!), each providing a potentiometer and +/- connections for the DC in and "motor" out (they're primarily meant for motors). I don't have the schematics of those, so my sketch below shows an abstraction of the modules. I also bought a little RGB-LED module that's nothing more than the LED and three resistors (pretty superfluous).

Here is a plan of how I prototyped my build: schematic sketch

The problem

For me (as an absolute layman) this looks like it should work. But the problem is that as soon as I turn up one of the PWM pots to control a single LED (of the RGB LED package), all three LEDs light up (nice and slow, so at least that works).

When I disconnect two of the LEDs, only one works (obviously). When I measure the voltage on the disconnected "out +" (from PWM out + to the LED) - all three get the same voltage. How is this possible? I want to control the RGB colors independently.

Is it possible that the PWM modules are not made to work in such a parallel configuration? How can I solve this? Thank you in advance!

PS: I know so little I don't even know how to correctly tag this question :(

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  • \$\begingroup\$ Are your 3 LEDs connected together like you've pictured? \$\endgroup\$ Dec 9, 2020 at 20:45
  • \$\begingroup\$ They're in one RGB-LED pack on a small PCB like this, but yes, technically it's just like in my sketch. I noted the three "channels" of the one RGB-LED as separate LEDs in the sketch. \$\endgroup\$
    – bkis
    Dec 9, 2020 at 20:47
  • \$\begingroup\$ I implore you try to disconnect the "negative out" pin of the 1st module from the "positive out" pin of the 2nd module, and then try disconnecting the "negative out" pin of the 2nd module from the "positive out" of the 3rd module. \$\endgroup\$
    – TTbulaski
    Dec 9, 2020 at 21:04
  • \$\begingroup\$ Maybe try shorting the two grounds together. It's not clear if they are internally connected in the dimmer modules. \$\endgroup\$
    – Justin
    Dec 9, 2020 at 21:15
  • \$\begingroup\$ @Justin When I do that, the LEDs all turn on even with the dimmers turned down \$\endgroup\$
    – bkis
    Dec 9, 2020 at 21:19

3 Answers 3

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Your LED array is common-cathode so you cannot use it with these PWM modules. This is inherent to the LED device itself and cannot be changed.

The PWM modules have the + in and + out more-or-less connected together (there's a polyfuse in each line) and that won't work for you.

You can use individual LEDs with resistors. It's also possible to use a common-anode RGB LED + resistors however the fuses will be paralleled, which is not ideal.

The module schematic (simplified) looks like this:

schematic

simulate this circuit – Schematic created using CircuitLab

LED array:

schematic

simulate this circuit

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  • \$\begingroup\$ You are right. Thank you. That's exactly what I found out when following @TTbulaski 's answer. What problems will I be facing when I try your last suggestion (common anode RGB LED?), you say "not ideal" - but... will it work? Will my house burn down or anything? :) \$\endgroup\$
    – bkis
    Dec 9, 2020 at 21:32
  • \$\begingroup\$ @bkis I would not want to underestimate your abilities, but the most likely consequence is that a short could cause the PCB to get very hot and smoke. You can avoid that issue by adding another fuse in series with the whole lot. \$\endgroup\$ Dec 9, 2020 at 21:35
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    \$\begingroup\$ Your answer - while not the only one with the right ideas - provided the most background information about the given situation. I accept. Thank you for your help! \$\endgroup\$
    – bkis
    Dec 9, 2020 at 22:03
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schematic

simulate this circuit – Schematic created using CircuitLab

I suggest you try it this way; disconnect the LEDs from each other.

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    \$\begingroup\$ Thanks for your suggestion. But the three LEDs share a common ground (they're one RGB-LED), so I can't really disconnect the grounds. If I just cut the connection at the negative outs of two of the pwm modules, those modules don't do anything, but the third one still controls all three colors (RGB channels) :( \$\endgroup\$
    – bkis
    Dec 9, 2020 at 21:22
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    \$\begingroup\$ I think it is impossible. It is common cathode LED. \$\endgroup\$
    – user263983
    Dec 9, 2020 at 21:28
  • \$\begingroup\$ I just tried to use three separate (white) LEDs (each with positive and negative leads) instead of the RGB-LED (with R/G/B/- leads) to achieve what you suggested and it works. So the reason is the common ground of the three color channels. So your answer is correct, I'd say. But Is there a way I can build what I want with the RGB-LED? \$\endgroup\$
    – bkis
    Dec 9, 2020 at 21:29
  • \$\begingroup\$ @user263983 Yes, that's the problem. \$\endgroup\$
    – bkis
    Dec 9, 2020 at 21:29
  • \$\begingroup\$ I don't want to assume, but do you want to edit the RGB values (0-255) using the three potentiometers? \$\endgroup\$
    – TTbulaski
    Dec 9, 2020 at 21:37
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That PWM controller has a regulating element, probably MOSFET, connected between "-" output to motor and "-" power source. It is switching "-", not "+" . To control RGB LED you need common anode LED.

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