Five Practical Methods to Determine the Positive and Negative Terminals of LEDs on a PCB Board
1. Lead Length Differentiation: For unclipped LEDs, the longer lead is the positive (anode), and the shorter lead is the negative (cathode). This is the most intuitive way to identify them.
2. Internal Structure Observation: Observe the internal electrodes through the LED casing. The smaller metal piece corresponds to the positive terminal, and the larger bowl-shaped structure (with a reflective surface) corresponds to the negative terminal.
3. PCB Board Marking Identification
• "+" sign or anode symbol: A "+" sign is usually marked next to the positive pad.
• Angled or missing corner markings: The angled end of the PCB silkscreen indicates the positive terminal.
• Pad shape differences: Square pads are mostly positive, while round/strip pads are mostly negative.
4. Multimeter Testing: Using the diode setting, when the red probe touches the positive terminal, the LED will emit a faint light, and the digital multimeter will display a forward voltage value (usually 1.8-3.3V). Reversing the probe will result in no response.
5. Power Supply Test (Low-Voltage DC Only)
Using a 3V battery and a resistor (100-300Ω): When the LED is emitting light normally, the positive terminal of the battery is connected to the positive terminal of the LED. Reverse connection will not damage the LED (under low-voltage conditions).
Precautions: Reverse connection exceeding the reverse breakdown voltage (usually exceeding 5V) may damage the LED. It is recommended to use a multimeter for testing first.
