Common Anode vs Common Cathode RGB LED: How to Tell and How to Wire
Quick answer: in a common-anode RGB LED the longest pin goes to V+ and each color lights when its pin is pulled low; in a common-cathode part the longest pin goes to GND and each color lights when driven high. Wire it the wrong way and nothing lights.
Identification
- The longest lead is the common pin in standard 5 mm/3 mm 4-pin packages.
- Multimeter diode mode: common-anode conducts from the common pin to each color pin; common-cathode conducts from each color pin to the common pin.
- SMD RGB (e.g. 5050, 1615): follow the pad drawing in the datasheet — layouts vary.
Wiring Common Cathode
Common pin → GND. Each color pin → own resistor → MCU pin or driver output (active-high).
Wiring Common Anode
Common pin → V+. Each color pin → own resistor → driver that sinks current (active-low, e.g. open-drain or MCU pin pulled low).
Per-Channel Resistors
Each die has a different Vf (red ~2.0 V, green/blue ~3.0-3.2 V). Calculate R = (Vs - Vf)/If per channel; sharing one resistor on the common pin makes colors fight and mix badly.
Common Mistakes
- Assuming all RGB LEDs are common-cathode — check before layout.
- One resistor on the common pin: brightness shifts with how many channels are on.
- Confusing plain RGB (4 pins, no IC) with addressable RGB (built-in driver IC, data protocol) — they are not interchangeable.
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