Abstract: With the increasing market share of water heaters, research on water heaters has become an important topic. This paper combines the advantages of boiling water heaters in terms of structure and control, analyzes their working principle, and completes the system construction of a boiling water heater, including the design of hardware circuits and related software programs. The water heater studied in this paper realizes functions such as water level signal detection, bidirectional thyristor heating control, and water inlet control.
Keywords : water level detection; bidirectional thyristor; heating control
Chinese Library Classification Number : Document Identification Code :
The Research and Implementation of Control System with Boiling Electric Water Boiler
Wang Yan Hong Meng Wen
Abstract : As the increasing market share of water boiler, the research on water boiler has become an important subject. Combining with boiling water boiler in structure and control, the paper analyzed its working principle, completed with the system structure, including hardware circuit design and related software programming. The paper has realized the functions on detection circuit for water-level signals, heating control circuit with two-way SCR drive, and feed-water control circuit, etc.
Key Words: Water Level Detection; Two-way SCR; Heating Control
1 Introduction
Automatic water boilers were first introduced in the 1980s, featuring automated control characteristics such as automatic cold water replenishment, automatic boiling, and automatic shutdown. While their operating principle remains unchanged, many water boilers still suffer from the following problems: water level control often uses float-type switches, which allow harmful substances to enter the drinking water as the float remains submerged, posing a health risk; the hot and cold water tanks are not completely isolated, leading to the formation of "mixed water" and "repeatedly boiled water"; they cannot provide hot water immediately upon use, as large amounts of boiling water cool down and turn cold while waiting to be collected; and the large amount of steam generated after boiling cannot be recovered promptly, drastically increasing the heat load on the control system, creating an unfavorable working environment for the boiler's operation.
In response to the numerous problems existing in current water heater control systems, this paper studies a water heater based on the AT89S52 microcontroller, which achieves a combination of hardware and software, and is characterized by low cost, energy saving and hygiene, simple structure, stable performance and strong practicality.
2 System Overall Design
2.1 Box Structure Design
According to thermodynamic theories, a large amount of heat energy is released when steam condenses into a liquid state. If this heat can be recovered and utilized to preheat cold water before it is heated, the energy consumption of heating the cold water will be greatly reduced. Therefore, this paper designs a steam recovery channel.
This channel connects to the bottom cold water tank, thus preheating the cold water. Its structural diagram is shown in Figure 1.
In the structural design, the cold water storage tank and the hot water storage tank are completely isolated. The hot water tank is a container for overflowing boiling water, which ensures that the water received from the outlet is 100% boiling water, without worrying about "mixed water" or "repeatedly boiled water".
Figure 1. Water tank structure diagram
As shown in Figure 1, the heating chamber and the cold water storage tank are connected by the principle of communicating vessels. This allows the control of the cold water to be transferred to the heating chamber, and the change in the water level in the cold water storage tank is also the change in the water level in the heating chamber, which facilitates the implementation of the software.
1.2 Hardware Circuit Design
The hardware circuit includes the following parts: 1. Water level detection circuit; 2. Heating control circuit; 3. Water inlet and replenishment control circuit; 4. Water level indicator circuit; 5. Power supply filtering and rectification circuit; 6. Microcontroller minimum system. Its overall hardware control block diagram is shown in Figure 2.
Figure 2 Overall Hardware Control Block Diagram
2.2.1 Water Level Detection Circuit Design
This paper uses an electrode-type sensor to acquire water level signals. The electrode rods of the electrode-type sensor are made of conductive metal, which does not produce harmful substances in either cold or hot water, thus meeting the requirements for safe drinking water. Three water level sensors acquire the low cold water level signal CWL, the high cold water level signal CWH, and the boiling water level signal HW, respectively. Figure 3 shows the circuit diagram for acquiring the water level signals.
Figure 3 Water level signal acquisition circuit
As shown in the circuit diagram, a 15V AC power supply is connected to the circuit via terminals. One end is directly connected to one input of the bridge rectifier, and the other end is connected to the water box. The electrodes of the water level sensor are suspended inside the water box. When the water level reaches the electrode rod, due to the conductivity of water, the electrode rod and the water box are connected, meaning the 15V AC power is transmitted to the electrode rod through the water. The electrode rod is the other input of the bridge rectifier; if the water level reaches the electrode rod, it means that 15VAC is connected to the bridge rectifier. After receiving the 15V AC power, the bridge rectifier rectifies and filters it, turning on the optocoupler. One end of the optocoupler's output is connected to the P2 port of the control chip, and the other end is grounded. When the optocoupler is on, the voltage at the P2 port is pulled low. At this time, the control chip detects the level change and executes the corresponding program based on the determined water level signal.
Several issues need to be considered when designing the circuit: First, the sensitivity of the water level signal detection must be ensured. To this end, when selecting an optocoupler, optocouplers with low conduction voltage should be chosen as much as possible while still meeting the voltage requirements. Second, the voltage divider resistors in the circuit can be selected based on the actual situation and are flexible components. Finally, for safety reasons, the enclosure should be connected to a common ground.
2.2.1 Design of Water Inlet and Water Replenishment Control Circuit
In the tank structure design, the water supply pipe is directly connected to the faucet, and a normally closed solenoid valve is installed between the water pipe and the water storage tank. Based on the detected water level signal, the circuit controls the opening and closing of the solenoid valve to realize the action of water supply and stopping. Its control circuit diagram is shown in Figure 4.
Figure 4 Water inlet and water replenishment control circuit
Since water intake and replenishment is a frequent operation, the performance and reliability of components should be considered in advance. A relay is used as a switch to control the water inlet, with a diode D1 connected in reverse at the relay input. Because the relay uses electromagnetic induction to open and close the movable contact to switch circuits, current changes are impeded during this process due to inductance characteristics. When the current to the inductor coil is suddenly cut off, a very strong back electromotive force is generated. The serious consequence could be the breakdown or burning of the amplifying diode N1, thus affecting the operation of the entire circuit. Therefore, a diode D1 is connected in reverse to provide a buffer and discharge path for the back electromotive force, thus protecting the circuit. In addition, a 24VAC normally closed solenoid valve is used, with J1 being a jumper terminal connected to the 24VAC circuit.
2.23 Heating Control Circuit Design
Heating is achieved using a high-power 380V three-phase AC power supply, which ensures that cold water can be boiled quickly in a short time, providing the objective conditions for boiling water and using it immediately.
In the design of the heating control circuit, due to the high power and frequent switching during heating, a three-quadrant bidirectional thyristor is used as the AC control switch to prevent false pulse triggering of the thyristor. The bidirectional thyristor only requires one trigger circuit to operate, and its internal buffer circuit consists of a series capacitor and a carbon resistor. By appropriately selecting the buffer circuit components according to the power supply voltage, stable triggering of the bidirectional thyristor can be guaranteed.
The trigger circuit of a bidirectional thyristor has a significant impact on the main control circuit. Common trigger circuits and main control circuits are electrically connected, making them susceptible to electromagnetic voltage fluctuations and power supply waveform distortion. Therefore, opto-isolation technology should be used to separate the heating element from the main control circuit. This design uses the MOC3061 optocoupler manufactured by Motorola to achieve zero-crossing triggering of the bidirectional thyristor. The internal structure and pin arrangement of the MOC3061 are shown in Figure 5.
Figure 5 Internal structure diagram of MOC3061
Figure 6 shows a heating control circuit diagram triggered by an MOC3061 bidirectional thyristor. This circuit achieves complete electrical isolation between the heating circuit and the main control circuit. During control, the chip starts or stops the heating control circuit by detecting the boiling water level signals HW and CWL. The control pin is P1.5. The pulse signal is output from P1.5, amplified, and then sent to the input of the optocoupler MOC3061. The MOC3061 triggers the bidirectional thyristor at zero crossing, ultimately connecting the three-phase power supply (U/V/W) to the heater (A/B/C) to achieve heating.
Figure 6 Heating control circuit
2.2 Control Software Design
Based on the hardware circuit platform, analysis of the water heater's operating characteristics reveals that boiling water overflows from the heating chamber and slowly reaches the boiling water level HW. At this point, the cold water level is decreasing. Heating should continue until the boiling water level reaches HW, provided it hasn't reached HW and is above CWL. It is crucial that heating ceases and water is replenished once the cold water level falls below CWL, regardless of whether HW has been reached. As long as the cold water level remains above CWL, heating can continue, preventing dry burning. This is a particularly important consideration in program design.
Based on this working principle, its control software was developed. Among the three water level signals, the cold water low level is responsible for controlling the lower limit of the water level, ensuring that dry burning does not occur when the water level is below the lower limit, and the low water level signal determines the water supply control; if the water level is below CWL, water must be added immediately. The high water level signal is responsible for controlling the upper limit of the water level; when the water level reaches CWH, water supply should be stopped. The boiling water level HW determines whether heating is needed; if the water level has not reached HW, heating should begin while ensuring that the cold water level is above CWL, and heating should be stopped when the water level reaches HW. The water level control flowchart is shown in Figure 7.
Figure 7. Flowchart of main circulation control for water level detection and control
Under normal operating conditions, the control chip continuously scans the water level signal input pin. A low level serves as the enable signal, and the chip detects various control pins to perform water intake and heating operations, with water level indicator lights representing changes in water level. However, in the event of a power outage and subsequent restart, the program design must include a judgment on the current state after power-on reset to facilitate user identification of the water heater's internal status.
This design uses Keiluvision4 programming software, and its programming interface is shown in Figure 8:
Figure 8 Keil uVision4 programming interface
3. Conclusion
With the improvement of people's living standards and awareness of environmental protection and energy conservation, the development of high-efficiency electric water heaters will be an important direction. This article is both a comprehensive analysis of the current water heater market and a new exploration. With the continuous popularization of intelligent control technology, the performance of the control system of electric water heaters will be further improved, and it will move towards intelligence, humanization, and greater environmental protection.
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