[ Huaqiang Security Network News ] Although many people know LED display, do you know how it works? This chapter will explain for you. The principle of the display scanning circuit is the dynamic scanning mode, and the operating current cannot be measured statically. Therefore, to calculate the operating current, it is necessary to analyze the dynamic parameters. Figure 2 is a working circuit diagram of an LED. In the circuit, Q8 is the driving circuit, the positive terminal is connected to the power supply, the control terminal is connected to the output of 74HC138, the output terminal is connected to the LED light-emitting diode D, and is connected with the current limiting resistor, and the resistor is connected to the data output end of the 74HC595. LED lighting mode is: control 74HC138 chip select signal is invalid, is not strobed, after 74HC595 output level, low level is lighting signal, and then strobe 74HC138, control output strobe signal, at this time, there is current I0 is output from Q8. After flowing through D and R1, it enters the data output of 74HC595. 
In the figure, Vab is the voltage applied to the LED. The voltage drop of the red and green high-brightness LEDs is about 2~3V. Vbc is the voltage applied across the current limiting resistor. By adjusting the value of the current limiting resistor, It is possible to change the operating current I0 of the circuit. When the resistor R1=0, the circuit relies on the output active resistor of the 74HC595 as the current limiting resistor.
It can be seen in the scanning circuit that the circuit structure is relatively simple, and the reasonable adjustment of the working parameters of each part can make the circuit work in an optimal state. When selecting a circuit, it is also necessary to accurately grasp the performance of each company's circuit, and the difference in technical parameters between them. The technical parameters of different types of devices are also different. Table 1 is the technical parameters of 74HC595. The technical parameters of 74HC595 of Texas Instruments, ST and PhiliPs are given in the table. In the table, it can be seen that the circuits produced by different companies are slightly different. Therefore, a display screen should use the same company's circuit components as much as possible to avoid the display effect of the display due to the difference of parameters.
In Table 1, Iik is the input spike current, Iok is the output spike current, I0 is the continuous output current, Vcc is the highest supply voltage, and fmax is the maximum operating frequency at 25 °C (working with load capacitance) The frequency is also different), ta is the operating temperature. The corresponding components in the table are SN74HC595, M74HC595, and 74HC595. The company is Texas Instruments, ST, Philips.
It can be seen in the scanning circuit that the circuit structure is relatively simple, and the reasonable adjustment of the working parameters of each part can make the circuit work in an optimal state. When selecting a circuit, it is also necessary to accurately grasp the performance of each company's circuit, and the difference in technical parameters between them. The technical parameters of different types of devices are also different. Table 1 is the technical parameters of 74HC595. The technical parameters of 74HC595 of Texas Instruments, ST and PhiliPs are given in the table. In the table, it can be seen that the circuits produced by different companies are slightly different. Therefore, a display screen should use the same company's circuit components as much as possible to avoid the display effect of the display due to the difference of parameters.
In Table 1, Iik is the input spike current, Iok is the output spike current, I0 is the continuous output current, Vcc is the highest supply voltage, and fmax is the maximum operating frequency at 25 °C (working with load capacitance) The frequency is also different), ta is the operating temperature. The corresponding components in the table are SN74HC595, M74HC595, and 74HC595. The company is Texas Instruments, ST, Philips.
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