How to use multimeter to detect light emitting diode LED
A light-emitting diode (LED) is a light-emitting device that directly injects current. It is the result of photons being emitted when excited electrons inside the semiconductor crystal return from a high energy level to a low energy level. This is commonly known as a spontaneous emission transition. When the LED When the PN junction is forward biased, the injected minority carriers and majority carriers (electrons and holes) recombine and emit light. It is worth noting that for a large number of particles at high energy levels, they each spontaneously emit a column of angular frequencies. It is a light wave with ν=Eg/h, but there is no fixed phase relationship between the light waves in each column, they can have different polarization directions, and the light emitted by each particle propagates in all possible directions. This process is called spontaneous emission. Its emission wavelength can be expressed by the following formula:
λ(μm)=1.2396/Eg(eV)
Light-emitting diodes (LEDs) are generally made of materials such as gallium arsenide phosphide and gallium phosphide. There is a PN junction inside it, which also has unidirectional conductivity, but the light-emitting diode emits light when it is forward-conducted. The brightness of the light increases as the conduction current increases, and the color of the light is related to the wavelength.
Multimeter detection method for ordinary light-emitting diodes:
Measure with R×10K range of digital multimeter
The quality of the light-emitting diode can be roughly judged by using a pointer multimeter with a ×10kΩ block. Normally, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. If the forward resistance value is 0 or ∞ and the reverse resistance value is very small or 0, it is easily damaged. With this detection method, the lighting condition of the luminous tube cannot be seen in person, because the ×10kΩ block cannot provide a large forward current to the LED.
Use two multimeters to measure
If you have two pointer multimeters (preferably the same model), you can better check the lighting conditions of the light-emitting diodes. 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 electrode (P area) of the luminous tube under test, and the remaining "+" pen is connected to the negative electrode (N area) of the luminous tube under test. Both multimeters are equipped with ×10Ω block. Under normal circumstances, it will light up normally after being turned on. If the brightness is very low or even does not emit light, you can dial both multimeters to ×1Ω. If it is still very dark or even does not emit light, it means that the performance of the light-emitting diode is poor or damaged. It should be noted that the two multimeters cannot be set to ×1Ω at the beginning of the measurement to avoid excessive current and damage to the light-emitting diodes.
External auxiliary power supply measurement
The optical and electrical characteristics of the light-emitting diode can be measured more accurately with a 3V regulated source or two dry batteries connected in series and a multimeter (either analog or digital). For this purpose, just connect the circuit as shown in Figure 10. If the measured VF is between 1.4 and 3V and the brightness of the light is normal, it means that the light is normal. If it is measured that VF=0 or VF≈3V and does not emit light, it means that the light-emitting tube is broken.
