View of LEDs and IR LEDs

“Using an analog multimeter with a x ten kΩ block can roughly determine the quality of the light-emitting diode. In normal conditions, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. Assuming that the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easy to damage. This inspection method cannot see the light-emitting condition of the LED on the spot, because the ×10kΩ block cannot supply a large forward current to the LED.

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1. Usually the inspection of light-emitting diodes

(1) Check with a multimeter. Using an analog multimeter with a x ten kΩ block can roughly determine the quality of the light-emitting diode. In normal conditions, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. Assuming that the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easy to damage. This inspection method cannot see the light-emitting condition of the LED on the spot, because the ×10kΩ block cannot supply a large forward current to the LED.

It is assumed that there are two pointer multimeters (of the same type) that can better check the light-emitting status of the LEDs. Use a wire to connect the “+” terminal of one of the multimeters to the “-” terminal of the other meter. The remaining “-” pens are connected to the positive electrode (P area) of the LED under test, and the remaining “+” pens are connected to the negative electrode (N area) of the LED under test. Both multimeters are set to the × ten Ω block. Under normal conditions, it can light up normally after it is turned on. If the brightness is very low, or even no light, you can turn both multimeters to ×1Ω. If it is still very dark, or even no light, it means that the LED is not functioning properly or damaged. Care should be taken not to place the two multimeters at × 1Ω at the first measurement to avoid excessive current and damage to the light-emitting diodes.

(2) Measurement of external power supply. The optical and electrical characteristics of light-emitting diodes can be measured with a 3V stabilized voltage source or two series-connected dry batteries and a multimeter (either analog or digital). For this purpose, the circuit can be connected as shown in Figure 10. Assuming that the measured VF is between 1.4 and 3V, and the brightness of the light is normal, it can be explained that the light is normal. Assuming that VF=0 or VF≈3V is measured, and no light is emitted, it means that the light-emitting tube is broken.

View of LEDs and IR LEDs

2. View of infrared light-emitting diodes

Because of the infrared light-emitting diode, it emits infrared light of 1 to 3 μm, which is invisible to the human eye. Usually, the emission power of a single infrared LED is only a few mW, and the angular distribution of the luminous intensity of different types of infrared LEDs is also different. The forward voltage drop of infrared LEDs is usually 1.3 to 2.5V. It is precisely because the infrared light emitted by it is invisible to the human eye, so using the above-mentioned visible light LED inspection concept can only determine whether the forward and reverse electrical characteristics of its PN junction are normal, but cannot determine whether its light-emitting condition is normal or not. To this end, prepare a photosensitive device (such as 2CR, 2DR silicon photovoltaic cells) as a receiver. Use a multimeter to measure the change of voltage at both ends of the battery. To distinguish whether the infrared LED emits infrared light after adding the appropriate forward current. Its measurement circuit is shown in Figure 11.