Method for Testing Ordinary Light‑Emitting Diodes with a Multimeter
① An analog multimeter with the ×10kΩ range can be used to roughly judge whether a light-emitting diode is good or defective. Under normal conditions, the forward resistance of the diode ranges from tens 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 defective. This test method cannot visually observe the light emission of the LED, because the ×10kΩ range cannot supply a relatively large forward current to the LED.
② Measurement with Two Multimeters in Cooperation If two analog multimeters (preferably of the same model) are available, the luminous condition of the LED can be checked effectively. Connect the positive terminal of one multimeter to the negative terminal of the other with a wire. Use the remaining negative test lead to connect to the anode (P region) of the LED under test, and use the remaining positive test lead to connect to the cathode (N region) of the LED. Set both multimeters to the ×10Ω range. The LED should light up normally once connected. 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 light-emitting diode has poor performance or is damaged. Note: Do not set both multimeters to the ×1Ω range at the start of measurement, as excessive current may damage the LED.
③ Measurement with External Auxiliary Power Supply A 3V regulated power supply or two series-connected dry cells together with a multimeter (either analog or digital) can be used to measure the optical and electrical characteristics of the LED relatively accurately. Wire up the circuit as shown in Figure 10. If the measured forward voltage VF is between 1.4 V and 3 V and the LED lights up with normal brightness, the LED works properly. If VF equals 0 or is close to 3 V and there is no light output, the LED is defective.
To test LEDs with a multimeter, the Fluke 18B new digital multimeter is recommended.
