Circuit design of a new type of digital display power meter for power grid

1 System working principle
In a DC circuit, active power reflects the product of the voltage and current of the measured circuit (P=UI); in an AC circuit, in addition to this product, it should also reflect the cosine of its phase difference, that is, the power factor cosψ of the circuit, and P=UI/cosψ. For industrial production and daily life, accurate and real-time measurement of power grid power is essential. This system uses a switching voltage source to rectify and stabilize the 220 V AC mains into analog and digital +5 V power supplies to power the entire meter circuit. The advanced single-phase bidirectional power/electric energy integrated chip CS5460A is used. The current signal detected by the current transformer and the voltage signal input by the resistor divider can be directly input into the integrated chip to complete the A/D conversion and instantaneous active power calculation on the chip. Its basic structure block diagram is shown in Figure 1.

2 Switching power supply circuit
The power supply is the core part of electronic equipment. Its quality directly affects the reliability of electronic equipment. Moreover, most of the failures of electronic equipment come from the power supply. Therefore, when designing electronic equipment, the design of the power supply is very important. The power supplies used in modern electronic equipment are roughly divided into two categories: linear regulated power supplies and switching power supplies. The so-called linear regulated power supply means that its adjustment tube works in the linear amplification area. The main disadvantage of this regulated power supply is low conversion efficiency, generally only 35% to 60%; the adjustment tube of the switching regulated power supply works in the switching state, and its main advantage is high conversion efficiency, which can reach 70% to 95%. In addition, the switching regulated power supply has the advantages of low power consumption, wide voltage regulation range, small size, light weight, safety and reliability. At present, the regulated power supply of electronic equipment is gradually replaced by the switching regulated power supply. In this system, since C55460A is used in the detection part, in order to avoid interference, two analog and digital +5 V power supplies are required. Therefore, two +5 V power supplies are also required in the switching regulated power supply part. The schematic diagram of this part is shown in Figure 2.

3 Power detection circuit
The power detection circuit is the main part of the power meter design. Its main function is to detect power and output power value by CS5460A under the control of single-chip microcomputer P89LPC916, among which X5045 stores standard correction value for single-chip microcomputer to calibrate power value at any time. Its circuit schematic is shown in Figure 3. The main components in the detection circuit are power transformer, CS5460A, single-chip microcomputer P89LPC916, X5045, etc.

3.1 CS5460A Functional Overview
CS5460A is a highly integrated Δ∑ analog-to-digital converter (ADC) that includes two Δ∑ analog-to-digital converters (ADCs), high-speed power calculation function and a serial interface. It can accurately measure and calculate active power, instantaneous power, IRMS and VRMS, and is used to develop single-phase 2-wire or 3-wire meters.

The CS5460A can measure current using low-cost shunts or current transformers, and measure voltage using voltage-dividing resistors or voltage transformers. The CS5460A has a bidirectional serial port for communicating with a microcontroller, and the chip's pulse output frequency is proportional to the active energy. The CS5460A has a convenient on-chip AC/DC system calibration function. The "self-boot" feature enables the CS5460A to work independently and automatically initialize after the system is powered on. In the self-boot mode, the CC5460A reads calibration data and startup instructions from an external EEPROM. When using this mode, the CS5460A 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.
The CS5460A is a CMOS monolithic power measurement chip with an active power calculation engine. It contains two gain programmable amplifiers, two △∑ modulators, and two high-speed filters. It has system calibration and effective value/power calculation functions to provide instantaneous voltage/current/power data sampling and periodic calculation results of active energy, IRMS, and VRMS. To accommodate low-cost measurement applications, the CS5460A can also output a pulse train on a given pin, with the number of pulses output proportional to the value of the active energy register. The CS5460A is optimized for power measurement. It is suitable for connecting to a shunt or current transformer to measure current; and connecting to a voltage divider resistor or voltage transformer to measure voltage. To accommodate input voltages of different levels, the current channel integrates a gain programmable amplifier (PGA) so that the full-scale input level can be selected as ±250 mVRMS or ±50 mVRMS. The PGA of the voltage channel can accommodate 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 to +5 V. In addition, the design can implement a double-ended differential input on one or both channels, in which case the common-mode voltage of the input signal is applied to AGND. Each channel of the CS5460A has a high-speed digital filter that attenuates the output of the two ∆∑ modulators by 10 times and integrates them. The filter outputs 24-bit data at a word output rate (OWR) of (MCLK/K)/1 024. To facilitate communication with an external microcontroller, the CS5460A integrates a simple three-wire serial interface that is compatible with the SPITM and Micro WireTM standards. The serial clock (SCLK) and RESET pins of the serial port contain a Schmitt trigger that allows the use of slower rising signals.
CS5460A features:
(1) Electric energy data linearity: ±0.1% within a 1 000:1 dynamic range;
(2) On-chip functions: can measure electric energy (active), I×V, IRMS and VRMS, and has electric energy-to-pulse conversion function;
(3) Can intelligently "self-boot" from serial EEPROM, no microcontroller required;
(4) AC or DC system calibration;
(5) With mechanical counter/stepper motor driver;
(6) Compliant with IEC687/1036, JIS industrial standards;
(7) Power consumption <12 mW;
(8) Optimized shunt interface;
(9) V to I phase compensation;
(10) Single power supply ground reference signal;
(11) On-chip 2.5 V reference voltage (maximum temperature drift 60 ppm/°C);
(12) Simple three-wire digital serial interface;
(13) Watchdog timer;
(14) Built-in power supply monitor;
(15) Power supply configuration is VA+ = +5 V; VA- = 0 V;
VD+ = +3.3 ~ +5 V.
As shown in Figure 4, the power measurement scheme of CS5460A in a single-power single-phase 2-wire system is shown. The resistor shunt used to monitor the power line current is connected to the live wire end of the power supply.

3.2 Working principle of power detection circuit
First, the mutual inductance current of the high-voltage circuit is obtained through the current transformer. The main advantage of using the current transformer is that it realizes the isolation of high voltage and low voltage. Since the power detection of CS5460A is actually the voltage detection after the current signal is converted into a voltage signal, the mutual inductance current is converted into a voltage signal by R48 and R49 in the circuit, and then input to the power detection pins 15 and 16 of CS5460A after the current limiting by R50 and R51 and the filtering by C21 and C22. In addition, the voltage is clamped with diodes in the two inputs to avoid damage to CS5460A when the voltage is too high. Pins 3 and 14 of CS5460A are analog and digital +5 V power supplies, respectively. At the same time, in order to filter out voltage fluctuations and increase the stability of the power supply, a small filter capacitor is added to the two power supply inputs. Since this is power detection, the two input pins 9 and 10 of the voltage detection should also be connected to the analog ground to avoid interference. Pin 13 VA- is connected to analog ground, pin 4 DGND is connected to digital ground, and the analog ground and digital ground are connected by inductance. Pins 1 and 24 (XOUT, XIN) are connected to a crystal oscillator with a frequency of 4.096 MHz. CS5460A automatically completes the detection of input signals and outputs data through pin 6 SDO; at the same time, the clock pulse, chip selection signal, reset signal, interrupt signal and control signal of the microcontroller to CS5460A are output by pin 5 SCLK, pin 7 CS, pin 19 R\E\S\, pin 20 I\N\T\, and pin 23 SDI. Since CS5460A completes the detection of analog signals and outputs the detection value as a digital signal, that is, a 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 great interference.

4 Control circuit
In the instrument, the single-chip microcomputer P89LPC916 controls CS5460A to detect power and display values. After the circuit is powered on, the circuit is initialized through the dip switch. The code of the power to be initialized by the single-chip microcomputer is input through the dip switch. The single-chip microcomputer sends the corresponding instruction to CS5460A according to the set coding rules. CS5460A transmits the standard calibration value of the corresponding power to be initialized to the single-chip microcomputer according to the instruction issued by the single-chip microcomputer, and the single-chip microcomputer transmits this data to X5045 and stores it in it, so that CS5460A can obtain the calibration value again when it is reset, and the single-chip microcomputer can calibrate the detection value at any time to improve the accuracy. The watchdog circuit in X5045 is used to reset the single-chip microcomputer, that is, 3 timing times can be selected through programming. When the single-chip microcomputer is caused by external interference and the program falls into an endless loop or runaway state, within the set timing time, if no access is made to: X5045, the reset signal pin of X5045 is RESET. After a delay of about 200 ms, RESET becomes low again. The RESET pin of X5045 then sends a reset signal to IST and X5045 to reset the microcontroller, thus avoiding the microcontroller from freezing and increasing the stability of the meter.

5 Display circuit
The instrument system uses a 7-segment LED display, as shown in Figure 5.

6 Conclusion

This paper gives the design of the new intelligent converter power meter, summarizes the working principle and design concept of the instrument system, and focuses on the principle and design of the power detection circuit. The actual use of this instrument has proved that it has the advantages of small size, low cost, high accuracy, good reliability, etc., and has a good market prospect. In addition, the software and hardware system can be further modified to increase functions for different users to meet the actual needs of users.