Positive and Negative Terminal Judgment of Light-emitting Diode

• Observe the appearance of the LED

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Lead length:
In most cases, the longer lead of the LED is the positive terminal (anode), and the shorter lead is the negative terminal (cathode).

Housing structure:
The bottom of the LED housing usually has a flat surface, and the lead corresponding to this flat surface is typically the negative terminal (cathode).

Internal structure:
By looking through the transparent housing, the larger part (similar to a cup shape) inside the LED is the negative terminal, while the smaller part is the positive terminal.

• Use a multimeter for testing

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Diode mode (or resistance mode) on a digital multimeter:

1. Set the multimeter to diode mode or resistance mode.
2. Connect the red probe to one side of the LED and the black probe to the other side.
3. If the LED emits a faint light (or shows a voltage drop), the red probe is connected to the positive terminal, and the black probe is connected to the negative terminal.
4. If the LED does not light up, swap the probes and test again.

• Use a simple circuit for testing

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Prepare a low-voltage battery (such as a 3V button cell or two 1.5V batteries in series) and a resistor (e.g., 1kΩ resistor).
Connect the positive terminal of the battery to one lead of the LED through the resistor, and the negative terminal to the other lead of the LED.
If the LED lights up, the side connected to the resistor is the positive terminal; otherwise, swap the leads of the LED and try again.

• Use a power supply module

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Use an adjustable power supply or a low-voltage power module, set the voltage to around 2V–3V, and connect it across the LED terminals.
If the LED lights up, the lead connected to the positive terminal of the power supply is the positive terminal of the LED; otherwise, swap the polarity and test again.

• Check the datasheet

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If the LED has a model number, you can look up the product's datasheet, which will clearly indicate the corresponding positive and negative terminals of the LED.

• Summary

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The simplest and quickest method is to observe the lead length and the flat surface of the housing. Alternatively, you can verify using a multimeter or a simple circuit.

[When operating, make sure the voltage is within the LED's working voltage range to avoid damaging the LED.]

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