Design of metering socket based on electric energy metering chip CSE7759

The concepts of smart home and Internet of Things are gradually gaining popularity, and more and more users hope that they can use electricity clearly and safely. Energy-saving and environmentally friendly smart sockets that can measure parameters such as electrical energy consumption are becoming a new favorite in families. This article introduces a design scheme for metering sockets based on the CSE7759 power metering chip of Xinhai Technology Co., Ltd. The scheme can measure power parameters such as voltage, current, power, and electricity of electrical equipment, and is equipped with an LCD display. The power parameters displayed on the LCD can be switched by pressing buttons. The scheme also has the function of automatically cutting off the power supply when the power is exceeded.

1. System design

The hardware circuit of the metering socket mainly includes: power metering, MCU control, LCD drive, relay control, and power supply voltage stabilization circuit, as shown in the figure below.



2. Detailed design

2.1. Power metering circuit

The voltage sampling collects the signal on the neutral line. The maximum input signal of the CSE7759 voltage sampling input pin is ±700mV. The input signal is about 220mV through the 1M and 1K resistor network voltage division, which meets the input requirements. Current sampling is completed by collecting the current on the live wire. The current is converted into a voltage signal through a manganese copper resistor. The maximum input differential signal of the current input pin of CSE7759 is ±43.75mV. Theoretically, the larger the sampling signal, the higher the measurement accuracy. However, if the manganese copper resistor is too large, it will cause excessive power consumption. For specific selection values, please refer to the table below.



2.2. Pulse counting

The voltage and current signals are converted into pulse signals through CSE7759 and output. The CF pin outputs the pulse frequency to indicate the active power value. The CF pin outputs the pulse frequency. When the SEL pin is high, it indicates the effective value of the voltage. When the SEL pin is low, it indicates the effective value of the current. The MCU counts the CF and CF1 output pulses for a certain period of time, calculates the pulse frequency, and then calculates the actual value through formulas (1), (2), and (3). Taking voltage calculation as an example, assuming that the measured voltage value is U, and the measured pulse frequency FU=1260Hz, then: 1260Hz=（U/1001×2）/2.43V×3579000Hz/512, and U=219.2V. In addition, the accuracy of CSE7759 voltage and current effective values ​​is ±0.5% within the range of 500:1, and the accuracy of active power is ±0.2% within the range of 1000:1, which can fully meet the measurement of general electrical equipment parameters.


Note: V1 and V2 are voltage and current sampling signals, respectively, Fsoc is the built-in crystal oscillator, 3.579MHz, and Vref is the built-in reference voltage, 2.43V.

2.3. Measurement calibration

Due to the differences between devices, the parameters of each metering socket cannot be exactly the same, so the measurement calibration problem is involved. The following introduces the calibration method using voltage calibration as an example.

① Add the standard voltage U=220V, and according to the formula Fu=（V1×2）/Vref×Fosc/512, the theoretical value of the output pulse is Fu=1264.5Hz.

② Measure the output pulse frequency of the metering circuit CF1. If Fu'=1000Hz, the ratio to the theoretical value is L≡1264.5/1000=1.2645.

③ Store L in the EEPROM. In the actual test process, the voltage pulse frequency is measured to be F, then after correction, it should be F'=F*L, and then the actual voltage value can be calculated according to formula (1).

3. Conclusion

The metering socket based on CSE7759 can meet the needs of measuring the power parameters of daily household appliances. In addition, CSE7759 has the advantages of small size (SOP8 package), built-in oscillator, simple peripheral circuit, high accuracy, low power consumption (working current <3mA), etc. It is a good choice for energy metering chips in smart sockets and household appliance metering modules.