Circuit design of a new multifunctional electric energy meter-EEWORLD

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0 Introduction

Due to the constraints of its structure and principle, traditional induction electric energy meters usually have shortcomings such as small measurement range, poor stability, and low accuracy, and cannot meet the needs of social development. With the development and popularization of microelectronics technology and single-chip microcomputers, new intelligent measurement and control technologies are developing rapidly. Electronic electric energy meters with single-chip microcomputers as the core have shown obvious advantages. This paper uses a single-chip microcomputer as the main controller of the instrument, and the electric energy metering chip of Cirrus Logic is responsible for collecting data, so it has the advantages of high cost performance, strong anti-interference ability, and high measurement accuracy.

1 Circuit working principle

This design uses a switching regulated power supply to rectify and stabilize the 220 V AC mains into analog and digital +5 V voltages to power the entire meter circuit. The system detects the current signal through a current transformer, and collects the voltage signal through a voltage divider resistor, and then sends it to the single-phase power/electric energy integrated chip CS5463, and completes signal sampling, calculation and error correction on the chip. The whole process is controlled by the single-chip microcomputer PLC916. Its principle block diagram is shown in Figure 1.

2 Dedicated metering chip CS5463

CS5463 is a highly integrated △∑ analog-to-digital converter (ADC) containing two △∑ analog-to-digital converters (ADC), high-speed power calculation function and a serial interface. It can accurately measure and calculate active power, instantaneous power, reactive power, IRMS and VRMS, and can be used to develop single-phase 2-wire or 3-wire meters. CS5463 can use low-cost shunts or current transformers to measure current, and use voltage divider resistors or voltage transformers to measure voltage. CS5463 has a bidirectional serial port for communication with a microcontroller, and the pulse output frequency of its chip is proportional to the active energy. CS5463 has a convenient on-chip AC/DC system calibration function. Its "self-boot" feature allows CS5463 to work alone and automatically initialize after the system is powered on. In self-boot mode, CS5463 can read calibration data and startup instructions from an external EEP-ROM. Using this mode, the CS5463 does not require an external microcontroller when working, so when the meter is used for large-scale residential energy measurement, the cost of the meter can be reduced.

[page] The CS5463 can provide instantaneous voltage/current/power data sampling and period calculation results of active energy, IRMS, and VRMS. To adapt to low-cost measurement applications, the CS5463 can also output a pulse train on a given pin, and the number of output pulses is proportional to the value of the active energy register. The CS5463 is optimized for power measurement and is suitable for connecting to shunts or current transformers to measure current, and connecting to voltage dividers or voltage transformers to measure voltage. To adapt to different levels of input voltage, the current channel of the CS5463 integrates a gain programmable amplifier (PGA), which allows the full-scale input level to be selected as ±250 mVRMS or ±50 mVRMS. The PGA of the voltage channel can adapt to an input voltage range of ±250 mV. For the case where a single +5 V power supply is connected to both ends of VA+ and VA-, the common-mode signal voltage applied between the differential input pins of the two channels is -0.25~+5 V. In addition, during design, double-ended differential input can be implemented in one or both channels. At this time, the common-mode voltage of its input signal will be added to AGND. Each channel of the CS5463 has a high-speed digital filter that can attenuate the output of the two △∑ modulators by 10 times and integrate. The filter outputs 24-bit data at a word output rate (OWR) of (MCLK/K)/1024. In order to facilitate communication with an external microcontroller, the CS5463 integrates a simple three-wire serial interface. The serial port is compatible with the SPITM and Micro wireTM standards. The serial clock (SCLK) and RESET pins of the serial port contain a Schmitt trigger, so a slower rising signal can be used.

The main features of CS5463 are as follows:

◇Within the dynamic range of 1000:1, the linearity of its electric energy data is ±0.1%;

◇It can measure instantaneous voltage, current and power, including IRMS and VRMS, apparent power, active and reactive power, as well as active fundamental and harmonic power, reactive fundamental power, power factor, frequency, etc.;

◇It has electric energy-to-pulse conversion function;

◇It has system calibration and phase compensation function;

◇It has a temperature sensor;

◇It has two reactive power calculation methods;

◇It can be intelligently "self-booted" from the serial EEPROM without the need for a microcontroller;

◇It can be calibrated in AC or DC systems;

◇It has a mechanical meter/stepper motor driver;

◇It complies with IEC687/1036, JIS industrial standards;

◇Power consumption <12mW;

◇With an optimized shunt interface;

◇With a single power supply ground reference signal;

◇With an on-chip 2.5 V reference voltage (maximum temperature drift is 25ppm/℃);

◇With a simple three-wire digital serial interface;

◇Built-in power monitor.

[page]3 Power supply circuit design The

switching voltage-stabilized power supply has the advantages of low power consumption, wide voltage-stabilized range, small size, light weight, safety and reliability. Therefore, the transformer voltage-stabilized power supply of electronic equipment is gradually being replaced by the switching voltage-stabilized power supply. The power supply circuit in this system is shown in Figure 2. Since CS5463 is used in the detection circuit part, in order to avoid interference between analog and digital, this system needs to have two +5 V outputs in the switching voltage-stabilized power supply part.

The AC first passes through a protection circuit composed of a fuse and a varistor. The fuse mainly plays an overcurrent protection role; the varistor plays an overvoltage protection role. Under normal working conditions, the varistor of the circuit is equivalent to a circuit breaker. When the grid voltage fluctuates or is affected by external factors (such as lightning, sudden power supply connection, etc.) and causes a sudden surge in voltage, the varistor will automatically turn on, and the varistor and the fuse will form a loop. Since the resistance of the varistor under high voltage is small, the loop current is large, and the fuse automatically disconnects, which plays a role in protecting the internal circuit. The fuse is self-recoverable, that is, it can be used repeatedly without replacement.

L1 and C1 can eliminate the spike wave in the signal and suppress interference. The signal filtered by L1 and C1 is connected to the rectifier bridge, and the high-voltage DC signal output from the rectifier bridge is filtered by C2 to obtain the voltage Vi. In Figure 2, T1 is a high-frequency transformer, which can quickly store and release energy. After high-frequency rectification and filtering, a DC continuous output can be obtained. Pins 1 and 3 of the high-frequency transformer T1 are the input initial windings, pins 4 and 5 are the feedback windings, and the secondary windings are pins 6, 9 and 7, 10 (where 1, 4, 6, and 7 are the same-name ends of each winding). And the 6, 9 windings are the main windings, and the +5 V DC regulated power supply generated by it after further transformation can be used as the main voltage source, that is, the digital voltage source (Vo1); the 7, 10 windings are auxiliary windings, and the +5 V DC regulated power supply generated by it after filtering by capacitors C8~C11 and stabilization by 78L05 is the auxiliary voltage source, that is, the analog voltage source (Vo2). In view of the fact that at the moment when the power MOSFET in TOP221Y is turned off, the leakage inductance of the high-frequency transformer will generate a spike voltage. In addition, an induced voltage (i.e., reverse electromotive force) will be generated on the initial windings 1 and 3, and the two will be superimposed on Vi, causing the voltage to increase seriously, so the power MOSFET is required to be able to withstand high voltage. At the same time, a clamping circuit must be added to the drain to absorb the spike voltage and protect the power MOSFET in TOP221Y from damage. This design uses R1, C3, ZD1, and D1 to form a clamping circuit. When the MOSFET is turned on, the voltage polarity of the initial winding of T1 is (terminal 3) positive and terminal 1 negative. At this time, D1 is turned off and the clamping circuit does not work; at the moment when the MOSFET is turned off, the initial winding becomes positive at terminal 1 and negative at terminal 3. At this time, D2 is turned on, and the spike voltage is absorbed by R1 and C3.

[page]4 Design of the metering circuit

The power detection circuit is the core of the meter design. The P89LPC916 of PHILIPS is a low-cost 16-pin single-chip package microcontroller. This chip is suitable for many occasions that require high integration and low cost, and can meet various performance requirements. P89LPC916 adopts a high-performance processor structure, and the instruction execution time only takes 2 to 4 clock cycles, which is 6 times faster than the standard 80C51 device. Since P89LPC916 integrates many system-level functions, it can greatly reduce the number of components and circuit board area, improve circuit reliability, and reduce system costs.

This instrument controls the detection process and numerical display of CS5463 by P89LPC916. After the circuit is powered on, the circuit can be initialized through the dip switch. After the power code to be initialized by the microcontroller is input through the dip switch, the microcontroller sends the corresponding instruction to CS5463 according to the set coding rules. CS5463 transmits the corresponding standard calibration value of the power to be initialized to the microcontroller according to the instruction sent by the microcontroller, and the microcontroller then transmits this data to X5045 and stores it in it, so that the calibration value can be obtained again when CS5463 is reset, and the microcontroller can be used to calibrate the detection value at any time, thereby improving the accuracy. That is to say, under the control of LPC916, CS5463 detects the electrical signal and outputs the measured value to the microcontroller, where X5045 can store the standard correction value. The control module in the metering circuit is shown in Figure 3.

The voltage sampling circuit consists of a resistor network, overvoltage protection and a de-jitter capacitor. To ensure the sampling accuracy, all resistors should use high-stability precision resistors.

[page] The current sampling circuit in this system consists of a current transformer, overvoltage protection, a resistor network and a de-jitter capacitor. The main advantage of using a current transformer is that it can achieve isolation between high voltage and low voltage. Since the current detection of CS5463 is actually the voltage detection after the current signal is converted into a voltage signal, in the circuit, the mutual inductance current is first converted into a voltage signal by the sampling resistor, and then filtered by the current limiting resistor and capacitor and input to the current detection pin (pin 16, 15) of CS5463. In addition, in the two inputs, diodes are used for voltage clamping to avoid excessive voltage from damaging CS5463. Pins 3 and 14 of CS5463 are analog and digital +5V power supplies respectively. At the same time, in order to filter out voltage fluctuations and increase the stability of the power supply, filter capacitors are added to the two power supply inputs. Since this is current detection, the two input pins 9 and 10 of voltage detection should also be connected to the analog ground to avoid interference. In addition, pin 13 VA should be connected to the analog ground, pin 4 DGND should be connected to the digital ground, and the analog ground and digital ground should be connected with an inductor. Pins 1 and 24 (XOUT, XIN) are connected to a 4.096 NHz crystal oscillator. The CS5463 is controlled by a program to automatically complete the detection of the input signal, and the data is output through pin 6 SDO; the clock pulse, chip select signal, reset signal, interrupt signal of the microcontroller and the control signal of the microcontroller to CS5463 are output through pin 5 SCLK, pin 7, pin 19 RES, pin 20 INT, and pin 23 SDI respectively. Since CS5463 needs to complete the detection of analog signals and output the detection value as a digital signal, that is, one chip has both analog and digital signals, it needs two analog and digital power supplies; at the same time, the pins that need to be grounded must be correctly connected to the corresponding ground, otherwise it will introduce a lot of interference.

5 Display circuit

This instrument system uses a seven-segment LED display. The clock signal of the shift register 74HC164 is provided by the single-chip microcomputer. When the data signal is output from the single-chip microcomputer, the low bit is output first and the high bit is output later. The data is first transmitted to the 74HC164 corresponding to the high-bit digital tube. Every time a clock pulse arrives, the data is shifted once. After a cycle, the data will be fully transmitted to the 74HC164 corresponding to each bit and displayed by the digital tube.

6 Conclusion

This paper gives a design method for a multifunctional electric energy meter, explains the working principle and design ideas of the electric energy meter in detail, and focuses on the principle, implementation and precautions of the power supply design and electric energy metering module. It has been proved that the electric energy meter has the advantages of stable operation, high reliability, high accuracy, low cost, and strong practicality. In addition, the functions can be improved according to different needs, such as adding communication modules.

Keywords：Sensors Reference address：Circuit design of a new multifunctional electric energy meter

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