Monochromatic LED detection method
  Light-emitting diodes (LEDs) are junction-type electroluminescent semiconductor devices that can convert electrical signals into optical signals.
1, the main features of LED


(1) A sufficiently high brightness can be obtained by operating at a low voltage (1.5 to 2.5 V) and a small current (5 to 30 mA).
(2) The luminescence response speed is high (10-7 ~ 10-9 s), and the high frequency characteristics are good, and the pulse information can be displayed.
(3) Good monochromaticity, common colors include red, green, yellow and orange.
(4) Small size. The shape of the light emitting surface is divided into a circle, a rectangle, and a special shape (triangle, etc.). The outer diameter of the circular tube is φ1, φ2, φ3, φ4, φ5, φ8, φ10, φ12, φ15, φ20 (mm) and other specifications, and the diameter of 1mm belongs to ultra-miniature LED.
(5) Anti-vibration and anti-impact performance, low power consumption and long life. Because the PN junction of the LED works in the forward conduction state, the power consumption of the present shot is low, and as long as the necessary current limiting measures are added, it can be used for a long time, and the lifetime is more than 100,000 hours, and even up to 1 million hours.
(6) Flexible use, can be made into digital tubes, character tubes, level displays, dot matrix displays, solid light emitting diodes, LED flat type TV screens, etc. as required.
(7) Easy to match with digital integrated circuits.
  
2. The principle of light-emitting diodes


Inside the light emitting diode is a PN junction with light emitting characteristics. When the PN junction is turned on, luminescence is radiated depending on injection of minority carriers and subsequent recombination. The shape, symbol, and volt-ampere characteristics of a common LED are shown in Figure 1. The positive volt-ampere characteristic curve of the LED is relatively steep and there is almost current before the forward conduction. When the voltage exceeds the turn-on voltage, the current rises sharply. Therefore, LED is a current-controlled semiconductor device, and its light emission luminance L (unit cd/m2, read candela per square meter) is approximately a positive double with the forward current IF, and has a formula


L =K IFm


In the formula, K is a proportional coefficient, in the range of small current (IF = 1 ~ 10mA), m = 1.3 ~ 1.5. When IF>10mA, m=1, Equation (5.10.1) simplifies to L=K IF


That is, the brightness is proportional to the forward current. Taking phosphorous gallium arsenide yellow LED as an example, the relationship between relative luminous intensity and forward current is shown in Figure 2. The forward voltage of the LED is related to the forward current and the semiconductor material of the die. When using, according to the required display brightness to select the appropriate IF value (usually selected about 10mA, for high-brightness LED optional 1 ~ 2mA), both to ensure that the brightness is moderate, it will not damage the LED. If the current is too large, it will burn the PN junction of the LED. In addition, the service life of LEDs will be shortened.
 
Due to its low power consumption, small size, bright colors, fast response, and long life, light-emitting diodes are commonly used as level indicators, tuning indicators, and power indicators for radios, radios, and electronic instruments. Light-emitting diodes have a certain voltage stabilizing effect during forward conduction. They can also be used as zener diodes in DC voltage regulators to provide a reference voltage and double as a power indicator. At present, there is a light emitting diode tube (such as BT104-B2, BT102-F) with a reflective cavity and a fixing device that is easily fixed on the instrument panel.
 
The output spectrum of the LED determines its luminescent color and purity of light radiation, and also reflects the characteristics of the semiconductor material. The common die materials are gallium phosphide (GaP), gallium arsenide (GaAsP), gallium arsenide (GaAlAs), gallium arsenide, gallium nitride (GaN), and blue light.


3. Use caution


(1) The polarity of the tube must not be reversed. In general, the longer the lead is the positive electrode, and the shorter the lead is the negative electrode.


(2) Each parameter in use must not exceed the specified limit value. The forward current IF must not exceed the limit operating current IFM, and must be derated for use as the ambient temperature increases. Long-term use temperature should not exceed 75 °C.


(3) The welding time should be as short as possible, and the solder joint cannot be at the root of the pin. When welding, tweezers should be used to clamp the heat of the base of the pin. Neutral flux (rosin) or rosin solder wire should be used.


(4) Do not soak or wash with organic solution.
 
(5) The LED driver circuit must add a current limiting resistor, which is generally from 100 ohms to several hundred ohms, depending on the supply voltage.


(6) The use of pulse voltage drive can save power consumption when the light emission brightness is basically unchanged. For LED dot-matrix displays, the use of a scanning display can greatly reduce overall power consumption.


4. Check the quality of light-emitting diodes


Light-emitting diodes have unidirectional conductivity, using R × 10k file can measure its positive and negative resistance. The general forward resistance should be less than 30k ohms and the reverse resistance should be greater than 1M ohms. If both the positive and negative resistances are zero, it indicates an internal breakdown short circuit. If the positive and negative resistances are infinite, prove that the internal open circuit.


The types and main parameters of common light emitting diodes are shown in Table 2. Need to explain two points: First, for the same kind of material, due to the different dopants, the light color is also different; Second, LED is a current-controlled device, VF changes with the IF, the standard VF value only for reference.


In addition, the positive and negative electrodes of the light emitting diode can also be distinguished based on the external shape. The tube produced earlier was provided with a metal tube seat and an optical lens was placed over it. The tube had a protrusion on its side and the positive electrode near the protrusion. The current production of LEDs, all encapsulated with transparent or translucent epoxy resin, and the use of epoxy resin to form a lens, play a role in amplification and focusing, this type of tube leads longer positive.
 
 
Precautions: 


It is not recommended to use the R×1k file to measure the forward and reverse resistance of the LED. Because of the battery voltage E


Measure the forward and reverse resistances only, and do not check whether they emit light normally. Since the forward voltage VF of the light emitting diode is generally 1.5 to 2.5 V, and the battery voltage of the multimeter R×1 or R×10 gear is 1.5 V, the tube cannot be turned on and emits light. The battery voltage of R×10k is high, but because of the large internal resistance, the forward current supplied is very small and the tube will not emit light normally.


Double table method can be used to check the light-emitting diode light. It is best to select two multimeters of the same type, all of which are assigned a R×1 or R×10 gear and used in series as shown in Fig. 1(a) to provide a higher forward voltage.


Assume that the two multimeters all use the MF30 type and are all set to R×1. Since the battery voltage of a watch is E=1.5V and the ohm center value is R0=25 ohms, the total voltage and total resistance are


E′= 2E= 2×1.5=3V


R0' = 2R0 = 2 x 25 = 50 ohms


If they are seen as a new table, the equivalent circuit is simple. The full meter full-scale current is:


IM'= E'/ R0'=2E/ 2R0= E/ R0=IM


It can be seen that the full-scale current value has not changed.


When the LED is used, an upper limit current resistance R should be added. It is advisable to limit the forward current IF to 10-30 mA to avoid power consumption damaging the tube. Typical typical forward current is 10mA, IF is calculated as


IF= E-VF/ R


(c) R0' in the figure can act as a current limiter, so no additional current limiting resistor is required. Phosphorus gallium arsenide light-emitting diodes have a low forward voltage drop of about 1.7V. E′=3V R0′=50 ohms, the forward current measured by the double-table method can be calculated as


IF= E'-VF/ R0'= 3-1.7/50=26 mA <30 mA


So there is no danger to the pipe. After the circuit is turned on, the tube emits brilliant red light.


If the two selected multimeters have different Rx1 octahedral center values, set them to R01 and R02, respectively, and the R+1 battery voltages of both tables are E(E=1.5V).


IM'=2 E / R01+ R02


IF=2 E -VF / R01+ R02


Example: Measurement of an unknown light emitting diode.
 
The first step is to determine the positive and negative electrodes. Using the MF30 multimeter's R×10k file, the forward resistance was 26k ohms, and the reverse resistance was close to infinity. When measuring the forward resistance, the black pen is connected to the positive pole.
 
In the second step, the two MF30 multimeters are all set to R×1 and the double-table measurement is used. The tube under test emits bright red light. If the polarity of the light emitting diode is reversed, the tube cannot emit light when reverse voltage is applied.


Then set two multimeters to R×10, and the tube glows dimly. This is because the total resistance R0' = 2x250 = 500 ohms, resulting in a smaller forward current. at this time 


IF≈3-1.7/500=2.6 mA


Precautions: 


(1) The ohm zero of two multimeters must be adjusted before using the double meter method.
 
(2) In order not to damage the light-emitting diode under test, the IM' value should be calculated before measurement. If IM' ≥ 50mA, R × 10 must be selected. For example, the total resistance R0 = 20 ohms for two 500-meter multimeter R×1 gears in series, IM′=IM=75mA>50mA. When using R×1 gear, IM′=7.5 mA, which is close to the typical forward current IF=10mA.


In fact, the light-emitting diode itself still has a voltage drop of 1.5 to 2.5V, so the above results all leave a certain margin.




If you do not know the forward voltage of the LED under test, the IM' value is not known. It is recommended to set both tables to R×10, if the light is very dark, then change R×1.