r/breadboard • u/Immediate-Soil-7820 • 13d ago

Why does my LED work in reverse?

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Hey,
First of all, im an absolut beginner with this stuff, so sorry for being stupid in advance!
Every RGB LED I see online uses 3 resistors and 3 connectors to the Powersource, with one pin being for ground.
But in my setup this doesn’t work, instead I use one pin for power and the other 3 for ground, depending on which ground is connected, the color changes.

Am I doing something wrong?

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12

u/TPIRocks 13d ago

This is common anode, you must be used to common cathode LEDs 7 segment LEDs come on both varieties too.

3

u/Immediate-Soil-7820 13d ago

So this is completely normal?
Is there a reason why the colors don’t „mix“ aswell?

3

u/Useful_radio2 13d ago

at low voltages electricity likes to go the lowest resistant route and all the diodes have small little variables, so the electricity is just going the lowest resistant route here and not going through the other one

3

u/FlyByPC 13d ago

Specifically, each LED is going to have a specific forward voltage -- the amount of voltage dropped across the LED when current is flowing (forwards) through it. Longer light wavelengths correspond to 2-2.5V or so (red); shorter wavelengths (blue) correspond to somewhat higher voltage drops (3-3.5V or so). They'll keep roughly this voltage drop, no matter the current through them (within reason, anyway.)

In order to power them from a voltage source (and keep them from drawing way too much current), they're generally used in conjunction with a series resistor, sized to "take up" the remaining expected voltage difference, at the supply current.

Run different color LEDs in parallel with only one series resistor for the group, and current will start to flow through the lowest-voltage one, limiting the voltage across the others and keeping them from turning on.

3

u/Objective-Ad8862 13d ago

Define "mix"

1

u/TPIRocks 12d ago

You need series resistance to each cathode leg. This is simply three diodes in parallel, with all three anodes connected together inside. Each color has its own voltage drop, red will tend to "hog" electricity, since it has the lowest forward voltage drop. You can't just put one resistor on the anode, each color needs its own resistor.