How to use multimeter to detect light emitting diode LED

A light-emitting diode (LED) is a light-emitting device that directly injects a current.It is the result of the photons emitted when the excited electrons inside the semiconductor crystal return from a high energy level to a low energy level, which is commonly known as spontaneous emission transition. PN junction plus forward bias, the injected minority carriers and majority carriers (electrons and holes) recombine to emit light. It is worth noting that for a large number of particles at high energy levels, they spontaneously emit a column of column frequency It is a light wave with ν = Eg / h, but there is no fixed phase relationship between the columns of light waves, which can have different polarization directions, and the light emitted by each particle propagates in all possible directions.This process is called spontaneous emission. The emission wavelength can be expressed by the following formula:

λ (μm) = 1.2396 / Eg (eV)

Light-emitting diodes (LEDs) are generally made of gallium arsenide, gallium phosphide and other materials. There is a PN junction inside it, which also has unidirectional conductivity, but the light-emitting diode will emit light when it is forward-conducted. The on-current increases and increases, and the color of light depends on the wavelength.

Multi-meter detection method of ordinary light-emitting diode:

Measure with R × 10K file of multimeter

A pointer multimeter with a × 10kΩ block can be used to roughly judge the quality of the light-emitting diode. 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 small or 0, it is easy to damage. This kind of detection method can't actually see the light emission of the light-emitting tube, because the × 10kΩ block cannot provide a large forward current to the LED.

Coordinate measurement with two multimeters

If there are two pointer multimeters (preferably of the same model), you can better check the light emission of the LED. Use one wire to connect the “+” terminal of one multimeter to the “-” terminal of the other table. The remaining "-" pen is connected to the positive electrode (P zone) of the luminescent tube under test, and the remaining "+" pen is connected to the negative electrode (N zone) of the luminescent tube to be tested. Both multimeters are set to 10 Ω block. Under normal circumstances, it will glow normally after being connected. If the brightness is very low, or even does not emit light, you can dial two multimeters to × 1Ω, if it is still dark, or even does not emit light, it means that the LED has poor performance or damage. It should be noted that the two multimeters cannot be placed at × 1Ω at the beginning of the measurement to avoid excessive current and damage to the LED.

External auxiliary power measurement

Using 3V voltage stabilizing source or two series of dry batteries and multimeter (either pointer or digital) can more accurately measure the optical and electrical characteristics of the LED. To do this, 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 can indicate that the light is normal. If VF = 0 or VF≈3V is measured and does not emit light, it means that the arc tube is broken. (Organization of China Educational Equipment Purchasing Network)