How to Identify LED Polarity: 5 Reliable Ways
Quick answer: on through-hole LEDs the longer lead is the anode (+) and the flat side of the rim marks the cathode (-). On SMD LEDs look for a green dot, line or chamfer marking the cathode — and confirm with the datasheet, because conventions vary by package.
Through-Hole LEDs
- Lead length: long lead = anode, short lead = cathode (only reliable on untrimmed parts).
- Flat edge: the flat spot on the epoxy rim = cathode side.
- Internal structure: the larger internal flag (reflector cup) is the cathode.
SMD LEDs
- Most packages mark the cathode with a green dot, a line, a chamfered corner or a "T" shape on the bottom.
- 0603/0805 chips often print a bar or dot on the back side.
- PLCC packages (2835, 5050) use a corner notch or a marked pad.
When in Doubt: Measure
Use a multimeter in diode mode. The LED conducts (and glows faintly) only when the red probe touches the anode. This is the most reliable check for unmarked or trimmed parts.
Getting polarity wrong is the most common reason an LED does not light up — reverse voltage above about 5 V can also damage the die. Full pad drawings are in every model's datasheet.
Need datasheets, samples or a bulk quote? Browse our LED datasheet center or product center. Hongcheng Optoelectronics, a member of Guangyu Group, has 20+ years of LED packaging experience with A-grade chips and 99.99% pure gold wire bonding.
