LED Basics: 2 Commonly Used Detection Methods to Distinguish Good and Bad LEDs

We often encounter the detection of LED quality . Among so many detection methods that can be selected, let's learn about the two most commonly used and simplest detection methods to provide a reference for everyone to learn about LED related knowledge. The first is about the detection of ordinary light-emitting diodes . Here we can use a multimeter or an external power supply.

Using a multimeter to test is actually using a pointer multimeter with a ×10kΩ range to roughly judge the quality of the light-emitting diode. Under normal circumstances, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. If the forward resistance is 0 or ∞, and the reverse resistance is very small or 0, it is easy to be damaged. This detection method cannot actually see the light-emitting tube, because the ×10kΩ range cannot provide a large forward current to the LED. If there are two pointer multimeters (preferably the same model), you can better check the light-emitting diode. Use a wire to connect the "+" terminal of one multimeter to the "-" terminal of the other meter. The remaining "-" pen is connected to the positive pole (P area) of the light-emitting tube being tested, and the remaining "+" pen is connected to the negative pole (N area) of the light-emitting tube being tested. Both multimeters are set to the ×10Ω range. Under normal circumstances, it can emit light normally after being connected. If the brightness is very low or even no light, you can set both multimeters to ×1Ω. If it is still very dark or even no light, it means that the LED is of poor performance or damaged. It should be noted that you cannot set both multimeters to ×1Ω at the beginning of the measurement to avoid excessive current and damage to the LED.

The second method is to use an external power supply for measurement. The specific method is to use a 3V voltage regulator or two dry batteries and a multimeter (either pointer type or digital type) to more accurately measure the optical and electrical characteristics of the light-emitting diode. For this purpose, the circuit can be connected as shown in Figure 10. If the measured VF is between 1.4 and 3V, and the brightness of the light is normal, it can be said that the light is normal. If the measured VF=0 or VF≈3V, and there is no light, it means that the light-emitting tube is broken.

At the end of the article, we would like to add some explanations about the detection of infrared light-emitting diodes. Because infrared light-emitting diodes emit 1 to 3 μm infrared light, which is invisible to the human eye. Usually, the emission power of a single infrared light-emitting diode is only a few mW, and the angular distribution of the luminous intensity of different models of infrared LEDs is also different. The forward voltage drop of infrared LEDs is generally 1.3 to 2.5V. It is precisely because the infrared light emitted by it is invisible to the human eye that the above-mentioned visible light LED detection method can only determine whether the forward and reverse electrical characteristics of its PN junction are normal, but cannot determine whether its luminescence is normal. For this reason, it is best to prepare a photosensitive device (such as 2CR, 2DR silicon photocell) as a receiver. Use a multimeter to measure the change in the voltage across the photocell. To determine whether the infrared LED emits infrared light after adding an appropriate forward current. Semiconductor light-emitting devices include semiconductor light-emitting diodes (referred to as LEDs), digital tubes, symbol tubes, M-shaped tubes and dot matrix displays (referred to as matrix tubes). In fact, each light-emitting unit in a digital tube, symbol tube, M-shaped tube and matrix tube is a light-emitting diode.