Abstract : Two kinds of three-phase intelligent electric meter power failure detection and its realization method are ana- l yzed. According to the re quirements of power failure detection of intelligent electricity meter , a circuit which can detect power failure efficiently and the corresponding software design scheme , and a power down detection circuit with simple function and high cost performance of ordinary electricity meter are designed.
Key words : power-downdetection ; smart meter ; ordinary electricity meter
Content：
2. Analysis of the smart meter power failure detection circuit
2.1 Overall framework of the smart meter power supply system
2.3 Power-off detection circuit
3. Analysis of power-off detection circuit of common electric meter
3.1 Ordinary electric meter power failure detection circuit
3.2 Ordinary meter split-phase power supply
3.3 Power-off signal software processing
1. Introduction
The existing three-phase smart meter power failure detection circuit may misjudge, resulting in the failure to power on and off normally and the power not being saved in time. This paper proposes two hardware and software solutions to perfectly solve the power failure detection problem for two different three-phase meters. The solutions are applied in two representative actual products to verify that the solutions can meet the design requirements.
2. Analysis of the smart meter power failure detection circuit
2.1 Overall framework of the smart meter power supply system
(1) Power-on detection: When the DC IN voltage is greater than 5.8V (AC input voltage is greater than 128V), the transistor Q9 is saturated, and Q9 outputs a low level and is sent to the power detection pin of the MCU through R55, informing that the power supply is normal and can be initialized or exit the low power state.
(2) Power-off detection: When the DC IN voltage is less than 5.8V (AC input voltage is less than 128V), the transistor Q9 is cut off, and Q9 outputs a high level and is sent to the power detection pin of the MCU through R55, informing that the power supply is abnormal, exiting the normal working mode, saving data and entering the low power state.
The switching characteristics of this circuit are not good, there is no hysteresis characteristic, and output jitter is easy to occur around the critical value. If the software has no relevant processing, the meter is prone to abnormality. If there is no battery inside the meter, the Q9 pin is also at a low level in the power-off state, which is the same as the state when the power is normal.
AC __N is the N line, GNDC is the C phase live wire, DC1, RC7, CC4, and DC2 form a resistor-capacitor voltage drop circuit. Most of the voltage drop of the AC power acts on RC7 and CC4. The voltage AC CN and AC N clamped by DC1 are connected to Figure 1 for power supply of the control circuit; the voltage clamped by DC2 is passed through DC3, RC8, QC2, CC5, and DC4 to form a full-bridge rectification circuit, and is filtered by CC6 voltage stabilization input, VC1 voltage stabilization, and CC13, CC14, and CC16 voltage stabilization output filtering to obtain VC 5V power supply to supply the metering power supply.
The RC power supply commonly used in this circuit can only output one way, so the three-phase current sampling must be isolated by a transformer, otherwise it will cause a short circuit between the neutral wire and the live wire or between the live wires of different phases; direct sampling of AC current can be achieved (current flows through the resistor to produce a voltage drop), reducing the overall cost.
2.3 Power-off detection circuit
R11 is the current-limiting resistor driving Q9, and Q9 is a switch tube. When the voltage between BC of Q9 (i.e., point b) is lower than 0.7V, Q9 is cut off, and the AC OFF input to the main chip is high after R50 pulls up, R55 limits the current, and C1 eliminates the jitter. When the voltage between BC of Q9 (i.e., point b) is higher than 0.7V, Q9 is turned on, and the AC OFF input to the main chip is low. Z1 is a voltage regulator tube, R48 and R49 are voltage-dividing resistors, and the power-off detection value at point b is 0.7V, and the voltage at point a is 0.7× ()V. That is, the power-off detection value of DC __IN generated in Figure 1 is 0.7× () + Z1. When DC __IN is greater than this value, the AC __OFF input to the main chip is low; when DC __IN is less than this value, the AC __OFF input to the main chip is high.
3. Analysis of power-off detection circuit of common electric meter
3.1 Ordinary electric meter power failure detection circuit
VCC is the DC voltage after the mains voltage is stepped down and rectified and filtered by E1. The voltage here is relatively high and cannot be directly sampled through the AD port of the main chip. R11, R51 and C8 can be directly sampled by AD after voltage division. R7, D16 and C38 form the sampling circuit of the voltage stabilization circuit, driving Q4 to achieve the purpose of controlling the output voltage. The output voltage of the +5V point depends on the parameters of D16, and C7 and C6 filter the output voltage. When power failure occurs, the voltage of the load connected to the back end of the +5V point slowly decreases, and the voltage of the VCC point also slowly decreases at the same time. The voltage from PWRDN to the AD sampling port of the main chip decreases in proportion to the VCC point, and the main chip can detect the power failure.
3.2 Ordinary meter split-phase power supply
GND is the N line, GNDC is the C phase live line, D6, C3, R3, D5 form a resistor-capacitor voltage drop circuit, most of the voltage drop of the mains acts on C3 and R3, the voltage clamped by D6 is half-wave rectified by D12 and converged with the voltage after half-wave rectification of phase A and phase B at VCC, which is used to power the control circuit, the voltage clamped by D5 is half-wave rectified by D11 and filtered by E4 to obtain VCC DC power supply, the sampling circuit of the voltage stabilization circuit composed of R6, D15, C37 drives Q3 to control the output voltage C+5V, and C15 is the output filter capacitor.
3.3 Power-off signal software processing
4. Conclusion
Through the coordination of software and hardware, this paper realizes the stability and reliability of power-on and power-off detection of three-phase smart meters and three-phase ordinary meters for different design schemes and requirements, laying a solid foundation for the normal operation of other functions of smart meters and ordinary meters. Through the joint efforts of the project team's technical personnel, the three-phase meter power-off detection circuit has been applied to the three-phase smart meter platform and the three-phase ordinary meter platform, and has achieved good practical results.