Why do all the LEDs in this circuit light up at once?

Question

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:

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 :(

Answer 1

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:

^{simulate this circuit – Schematic created using CircuitLab}

LED array:

^{simulate this circuit}

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

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

Answer 3

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.