Design of intelligent power limiting, lightning protection and fire protection controller based on Renesas microcontroller

　　0 Introduction

　　The management of electricity use in school dormitories has always been a difficult problem, especially in brick-wood structure dormitories, where the use of various illegal high-power electrical appliances is rampant. Fire accidents caused by the use of illegal electrical appliances occur from time to time, causing serious losses to the country's property and posing a certain threat to students' lives. The intelligent power limiting and fire prevention controller studied in this paper can well ensure the safety of electricity use in student dormitories and maintain the safety and stability of the school.

　　1 Design of intelligent power limiting and fire protection controller

　　1.1 System Hardware Design

　　The intelligent power limiting and fire prevention controller uses Renesas MCU (M3062P) as the control core, and the peripheral circuits include power supply circuit, key circuit, relay drive circuit, clock circuit, 128×64 LCD display circuit, current sampling, voltage sampling, leakage sampling, E2PROM circuit, optocoupler input, and communication RS485 interface. The system hardware block diagram is shown in Figure 1.

Figure 1 Hardware structure of intelligent power limiting and fire protection controller

　　The current sampling circuit is used to detect the size of the circuit current and is used for overload, short circuit and other protections; the voltage sampling circuit is used to detect the high and low voltage of the circuit and measure the frequency; the optocoupler circuit isolates the sensor interface and the control circuit interface; the leakage sampling may detect whether there is leakage in the line; the communication circuit may realize remote 24-hour online monitoring; the E2PROM circuit can realize power-off storage of parameters and fault information; the power supply circuit provides power for the entire system.

　　1.1.2 Power supply circuit

　　Since the operating voltage of the system's control core and peripheral circuits is 5V, and the relay coil voltage is 5V, and considering the cost and anti-interference performance, a three-terminal regulator 78L05 is used to power the system. T1 is a 5W power frequency transformer.

Figure 2 Power supply circuit diagram [page]

　　The current sampling uses a threaded current transformer, and the voltage sampling uses a resistor after passing through the transformer, and then the amplification circuit is used to sample the single-chip microcomputer to calculate the effective value, and then calculate the power and power factor. According to the active power, it can be detected whether there is a pure resistive load connected to realize the intelligent power limiting function. Leakage sampling can detect whether there is leakage in the line. If the single-chip microcomputer (M3062P) detects that there is leakage in the line, when the leakage current reaches the set value, the single-chip microcomputer sends an action signal to drive the relay to disconnect the full load, thereby avoiding the occurrence of fire caused by leakage. Ia1, Ib1 and Ic1 are threaded current transformers. The voltage sampling and current sampling circuits are shown in Figure 3.

Figure 3 Voltage sampling and current sampling circuit diagram

　　1.1.4 LCD display circuit

　　The LCD display uses 128×64 dot matrix liquid crystal, which has advantages that other displays cannot match, such as small size, light weight, ultra-thin and programmable drive. It can not only display numbers and characters, but also Chinese characters, and can realize screen scrolling, animation, reverse display, and display flashing. The design uses a Chinese graphic display interface to make the human-computer interface more friendly.

　　1.1.5 Optocoupler Isolation Circuit

　　The optocoupler circuit isolates the temperature sensor, smoke sensor, combustible gas detector and 1-way DC24V remote fire control signal output interface from the control port of the CPU to improve the anti-interference ability of the system.

　　1.1.6 Communication interface circuit

　　The communication interface circuit adopts RS485 mode to send the system's measurement data to the main monitoring terminal of each dormitory through the serial bus, where the power consumption and safety conditions of each dormitory can be understood in real time.

　　1.1.7 Power surge protection circuit

　　In order to prevent the surge voltage on the line from damaging the electrical equipment, a lightning protection (power surge protector) circuit is added to the incoming end of the power supply, that is, a varistor is connected in parallel to each phase line and neutral line. The surge protection circuit is shown in Figure 4.

Figure 4 Three-phase and single-phase surge protection circuits

[page]

　　The main program first initializes the internal registers and working mode of the main control chip, checks whether the parameters stored in the E2PROM are within the set range, and uses the default parameter settings if they are out of range or the parameters are empty. Then the display module and keyboard module are initialized and the default interface is displayed. In the main loop of the program, the sampling of voltage and current, leakage detection, and input detection of various sensors are mainly implemented, and whether protection action or alarm action needs to be executed is determined based on the measurement results. The main program flow chart of the system is shown in Figure 5.

Figure 5 System main program flow chart

　　2 System installation, debugging and summary

　　After the entire system design was completed, we conducted verification and debugging based on investigation and research, according to the school's schedule and allowed power consumption. The intelligent power limiting, lightning protection and fire protection controller has all the functions of intelligent power limiting, lightning protection device and residual current fire protection device. The results show that this system can operate stably and the effect is ideal.

　　At present, many student dormitories in Guangxi still have the same electrical safety hazards as our school. The research, development and application of this project has significant practical significance for maintaining the safety and stability of schools and has high promotion and application space and application value.

References:

[1]. RS485 datasheet http://www.dzsc.com/datasheet/RS485_585289.html.
[2]. 78L05 datasheet http://www.dzsc.com/datasheet/78L05_99997.html.