Application Analysis of CAWS600B Weather Station Power Control System

With the completion of the construction of the atmospheric monitoring automation system, more and more automatic weather stations have been put into operation. At present, a large part of the automatic weather stations used in the meteorological system are CAWS600B automatic weather stations. This type of automatic weather station has been put into operation for nearly 10 years and is running relatively stably. A large part of the faults are caused by its power control system. Therefore, this article introduces the working principle and typical fault analysis of the power control system of the CAWS600B automatic weather station based on years of practical experience.

1 CAWS-DY01 circuit principle
The power supply system of CAWS600B is the CAWS-DY01 module. Its main components are the single-chip microcomputer PIC16C711-041/P module, and an intelligent control power supply is formed through the LM integrated voltage regulator and field effect tube. Its circuit schematic diagram is shown in Figure 1. Since the CAWS600B automatic weather station is powered by 220 V mains electricity, the power supply module DY01 of the power system rectifies and stabilizes the transformed AC power, and charges the 12 V battery with DC. The intelligent function of the power system microcontroller PIC16C711-041/P module can monitor the energy storage status of the battery at any time and automatically control whether to charge the battery, which can solve the problem of the battery's virtual fullness. At the same time, it can also directly power the collector DT50 unit and the sensor through the rectifier and voltage regulator circuit. It mainly consists of four parts: rectifier and filter system, intelligent control system, charge and discharge system, and power supply detection system.

(1) Rectification and filtering system working process. The CAWS600B automatic weather station is mainly powered by the CAWS-DY01 module, which converts the 220 V AC mains power supply. As shown in the upper left of Figure 1, the CAWS-DY01 module can transform the 220 V AC mains power from the duty room into (16-18 V) AC voltage through the AC transformer T1, and then rectifies and filters the 18 V AC power through the bridge BD1 and C5 full-wave rectifier circuit, which can transform the AC power into about 20 V DC power. At this time, the 220 V AC mains power becomes unstable DC power with slight pulsation. By using the DC voltage range of the multimeter, it can be measured that the voltage at this time fluctuates between 18 and 22 V DC. At this time, the large-capacity electrolytic capacitor C5 plays the role of energy storage and voltage stabilization. The pulsating DC current from C5 supplies power to the DC output circuit unit, battery charging circuit unit, and central control circuit unit through point A as shown in Figure 1. It can also be said to be a DC power supply point.
(2) Circuit principle of intelligent control system. The intelligent control system is mainly composed of a single-chip microcomputer PIC16C711-041/P module. The power supply of the intelligent control system is mainly composed of two circuits: the pulsating DC power of about 20 V at point A is stabilized by the U6 three-terminal voltage regulator integrated circuit LM7809 and then outputs 9 V DC to the position of point B as shown in Figure 1. The second part of the intelligent control system power supply circuit is the 12 V DC directly provided by the battery. As shown in Figure 1, point E is the positive pole of the battery. The DC power supplied by the battery outputs about 12 V DC to point B through diode D7; after the two circuits meet, due to the voltage difference, the power supply voltage of the battery is higher than the power supply voltage of part 1 of the circuit. At this time, according to the unidirectional conduction law of the diode, it can be determined that the battery is powering the single-chip microcomputer PIC16C711-041/P module. Therefore, under normal circumstances, the collector power supply system is mainly powered by the battery to the DT50 acquisition unit. If the battery is overused or damaged, causing the battery supply voltage to be too low, below 10 V, then the 220 V mains power is rectified and transformed by the rectifier and filter system, and then the system is supplied with DC power from point A. At this time, the 8 V DC power reaching point B is stabilized by the three-terminal voltage regulator integrated circuit LM7805 and filtered by the filter capacitor C4, and then outputs a stable 5 V DC power to the control unit microcontroller PIC16C711-041/P module. (3) Charging and voltage stabilization output circuit
The microcontroller PIC16C711-041/P module is an 8-bit microcontroller with an analog/digital converter. When the input voltage is near its threshold value, the CMOS input buffer will absorb current. According to this principle, the battery voltage is regulated by the potential modulator W1 and reaches the 18th pin of the single-chip microcomputer module. At this time, it can be regarded as the threshold voltage of the control circuit charging. When the voltage is not greater than 2.2 V, the CMOS input buffer will start the internal program of the single-chip microcomputer to charge the battery until the battery voltage rises to about 3.8 V. At this time, the charging circuit is turned off and the charging process ends. Similarly
, when the battery voltage is greater than or equal to 3.8 V, the battery discharges, and the voltage slowly decreases until it reaches 2.2 V. The discharge process ends, and the charging circuit restarts to charge the battery. The entire charging circuit is mainly composed of the transistor Q2, field effect transistor Q4 (same principle as Q5, Q6) and adjustable 3-terminal positive voltage regulator U3, R20 as shown in the lower right corner of Figure 1. The adjustable 3-terminal positive voltage regulator U3 ( LM317 ) is an integrated voltage regulator with adjustable output voltage. The output voltage of the voltage regulator can be calculated by the following formula: Vo = 1.25 (1 + R2 / R1). Judging from the formula itself, the resistance values ​​of R1 and R2 can be set arbitrarily. However, as the output voltage calculation formula of the voltage regulator, the resistance values ​​of R1 and R2 cannot be set arbitrarily. First of all, the output voltage range of the LM317 voltage regulator is 1.25 ~ 37 V (the output voltage range of the high output voltage LM317 voltage regulator such as LM317HVA, LM317HVK, etc. is 1.25 ~ 45 V), so the ratio range of R2 / R1 can only be 0 ~ 28.6. Therefore, this circuit constitutes a constant current source circuit to charge the battery with a constant current, the purpose is to extend the battery life and maintain the stability of the instrument operation. (4) Power supply detection system circuit. According to the working principle of the microcontroller PIC16C711-041/P, the first pin of the microcontroller can detect whether there is voltage at point A through the signal from resistor R15. If there is voltage, a 4.8 V DC voltage is output at pin 9 through the regulation inside the module. After being divided by the two resistors R21 and R22 in the upper right corner of Figure 1, the transistor Q5 is turned on. At this time, the source voltage Vs of the field effect transistor Q6 is the DC voltage of point A 22 V, and a 9.8 VDC gate voltage Vg is generated after being divided by three resistors R23, R24, and R25. In summary, at this time, the gate voltage Vg is 9.8 VDC, and the source voltage Vs of the field effect tube Q6 is 22 VDC. It can be concluded that the two-stage voltage difference Vgs is -12.2 VDC, and the absolute voltage value is much larger than the turn-on voltage V1 of the P-channel MOS field effect tube RF9530, so the field effect tube Q6 is in the on state, and the internal resistance is extremely small and far close to zero. At this time, the drain voltage of the field effect tube Q6 should be 22 VDC, and after being stabilized by the three-terminal regulator U7 (7812) and filtered by the filter capacitor C10 (filtering noise to make the output more stable; providing reserve current to make up for the shortage when a sudden large current is needed), the output is a 12 V DC voltage. When the 1st pin of the MCU PIC16C711-041/P module does not receive the voltage at point A, the transistor Q5 is in the cut-off state. At this time, the gate voltage Vg of the field effect tube Q6 is the same as the source voltage Vs, both of which are 22 VDC. Then the two-level voltage Vgs is 0, which does not meet the field effect tube conduction condition and is in the cut-off state. This circuit has no current and does not power the system. At the same time, the 7th pin of the MCU PIC16C711-041/P outputs a 4.8 V DC voltage, which turns on the transistor Q1 and the field effect tube Q3 (the working principle is the same as Q5 and Q6). The DC voltage output by the battery provides a 12 V DC output voltage through the field effect tube Q3 and the diode D8 through point E. The output 12 V DC voltage is stabilized by the three-terminal voltage regulator integrator U4 (7805) and filtered by the filter capacitor C8 to provide a 5 V DC voltage. At this time, the system displays whether the 12 V DC output voltage is normal through the light-emitting diode, and it is on if it is normal. It should be noted that the output of the entire power supply system is output through the 12 VDC output on the right side. The 5 V output function marked on the right side is idle and not used by the system. 2 Common fault analysis and processing Common fault analysis can be started from three parts: AC input part, intelligent control system circuit fault, charging and DC output circuit: (1) AC input part fault. This part of the troubleshooting is relatively simple. First, use a multimeter to measure whether the AC 220 V voltage at the input end of transformer T1 is normal, and then measure whether the output end of T1 is normal. Sometimes, voltage attenuation is prone to occur due to transformer failure. Then measure whether the AC input voltage of the power board is 18 V AC, and then measure whether the DC voltage at point A is 22 V. Through the above measurements, the faulty component can be determined. (2) Intelligent control system circuit fault. There are generally two situations when this part of the circuit fails: the microcontroller PIC16C711-041/P is damaged or cannot work due to power supply failure. This fault phenomenon is generally manifested in the power supply system having no 12 V DC output and cannot charge the battery, causing the collector to be unable to collect data. Measure the 4th and 14th pins of the MCU to see if there is a 5 V DC voltage, and whether there is a DC voltage of about 11 V at point B, and then test in two ways to points A and E to find out the cause of the fault. There was once a site where the collector worked abnormally after lightning. After on-site measurement, the voltage of the 4th pin of the MCU was 4.8 VDC, and the voltage at point B was zero. At this time, it can be determined that the MCU chip is damaged. After replacing the MCU chip, the collector resumed normal operation.







(3) Faults in the charging and DC output circuits. As can be seen from the schematic diagram, the charging and discharging circuits include three circuits, the main parts of which are exactly the same and have the same principles. Taking the AC 220 V power supply circuit as an example, first determine whether the voltage at the 9th pin of the microcontroller is 4.8 VDC, and then measure whether the source voltage Vs of the source of the field effect tube Q6 is DC 22 V, whether the gate voltage Vg is 9.8 VDC, and whether the drain voltage should be 22 VDC. Otherwise, the field effect tube Q6 is damaged. If the gate voltage Vg of the field effect tube Q6 is basically the same as the source voltage Vs, then the transistor Q5 is not connected, and it can be determined that the transistor Q5 is damaged or the related resistor is faulty. If the gate voltage Vg of the field effect tube Q6 has no voltage indication, it can be determined that the resistor R25 is faulty. The faults of the three-terminal voltage regulator integrated circuit U7 and the filter capacitor can be directly determined by measuring the voltage at their terminal pins.

3 Conclusion
By studying and understanding the working principles of the circuit system, maintenance and repair work can be carried out well, and when facing some sudden faults, detours can be avoided, saving time and cost. Therefore, when faced with some new products, understanding and mastering their internal working principles and operating mechanisms has far-reaching significance for future maintenance and support work.