Testing Ordinary Light-Emitting Diodes Using a Multimeter
An analog multimeter with an ×10kΩ range can roughly judge whether a light‑emitting diode is good or defective. In normal condition, the forward resistance of the diode ranges from tens of kΩ to 200 kΩ, and the reverse resistance is infinite. If the forward resistance is 0 or infinite, or the reverse resistance is very small or zero, the diode is damaged. This test method cannot observe the actual light emission of the LED, because the ×10kΩ range cannot supply a large forward current to the LED.
Dual-Multimeter Combined Measurement If two analog multimeters (preferably of the same model) are available, the light-emitting performance of the LED can be inspected better. Use a wire to connect the "+" terminal of one multimeter to the "−" terminal of the other. The remaining "−" probe connects to the anode (P-region) of the tested LED, and the remaining "+" probe connects to the cathode (N-region) of the LED. Set both multimeters to the ×10Ω range. Under normal conditions, the LED will light up after connection. If the brightness is very low or there is no light, switch both multimeters to the ×1Ω range. If it is still dim or does not light up, the LED has degraded or failed. Note: Do not set both multimeters to ×1Ω at the start of measurement, as excessive current may damage the LED.
Measurement with External Auxiliary Power Supply A 3V regulated power supply or two series dry cells together with a multimeter (analog or digital) can be used to accurately measure the optical and electrical characteristics of the LED. Wire up the circuit as shown in Figure 10. If the measured forward voltage VF is between 1.4 V and 3 V with normal brightness, the LED works properly. If VF = 0 or VF ≈ 3 V and no light is emitted, the LED is broken.
How to test LEDs with a multimeter. The new Fluke 18B digital multimeter is recommended for this application.
