Method for Detecting Light Emitting Diodes with Multimeters

A light-emitting diode (LED) is a light-emitting device that injects current directly, which is the result of photons emitted when the excited electrons inside the semiconductor crystal return from a high energy level to a low energy level. This is known as the spontaneous emission transition. When the LED The PN junction is forward biased. The injected minority carrier and the majority carrier (electron and hole) combine to emit light. It is worth noting that for a large number of high-energy particles, each column spontaneously emits a column of angular frequencies. For ν = Eg / h light waves, but there is no fixed phase relationship between the various light waves, can have different polarization directions, and the light emitted by each particle along 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 phosphorous gallium arsenide, phosphide, and other materials. It has a PN junction inside it, but also has unidirectional conductivity, but the light emitting diode emits light when it is conducting forward. The brightness of the light increases with the increase of the conduction current. The color of the light is related to the wavelength.
Ordinary light-emitting diode multimeter detection method:

Measured with Rx10K file of multimeter

Using an analog multimeter with a ×10kΩ block can roughly determine the quality of the LED. Normally, the forward resistance of the diode is several tens to 200 kΩ, and the value of 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. This kind of detection method cannot see the light emission of the luminous tube in the field, because the ×10kΩ block cannot provide a large forward current to the LED.

Measure with two multimeters

If there are two pointer multimeters (preferably the same model) can check the light emitting diodes better. Use a wire to connect the "+" terminal of one of the multimeters to the "-" terminal of the other watch. The remaining "-" pen is connected to the positive electrode (P zone) of the light emitting tube under test, and the remaining "+" pen is connected to the negative electrode (N zone) of the tested light emitting tube. Both multimeters are set to 10 Ω. Under normal conditions, it will light normally when connected. If the brightness is very low, or even does not light, you can dial the two multimeter to × 1Ω if it is still very dark, or even not light, then the LED performance is bad or damaged. It should be noted that it is not possible to place the two multimeters in ×1Ω at the beginning to avoid excessive current and damage the LEDs.

External auxiliary power measurement

The light and electrical characteristics of the light emitting diode can be measured more accurately with a 3V regulated voltage source or with two serially connected dry cells and a multimeter (either analog or digital). 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 be shown that the light emission is normal. If VF=0 or VF≈3V is measured and no light is emitted, the light emitting tube is broken.