Analysis of Circuit Principle of Automatic Washing Machine

2020-12-11 16:03:56

A fully automatic washing machine is to pre-set N programs for the whole washing process (soaking-washing-rinsing-dehydration), select one of the programs when washing, turn on the faucet and start the washing machine, and the whole washing process will be automatically completed. The buzzer will sound when washing is completed.


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Analysis of Circuit Principle of Automatic Washing Machine



The fully automatic washing machine consists of a washing system and a control circuit. The control circuit is divided into mechanical and computer type, and the computer type control circuit uses a single-chip microcomputer as the core of the control circuit. Figure 1 shows the control circuit of the fully automatic washing machine composed of the single-chip Z86C09.


Ⅰ. Washing procedure of automatic washing machine

There are 4 buttons K1, K2, K5 and K6 on the washing machine panel.

K1 is used for water flow selection, divided into two levels: ordinary water flow and soft water flow;

K2 is used for the selection of washing cycle, you can choose three processes of washing, rinsing and dehydration;

K5 is a pause switch;

K6 is the washing program selection key. The laundry program is divided into a standard program and an economic program.


The standard laundry procedure of the washing machine is: washing-dehydration-dehydration-rinsing-dehydration-rinsing-dehydration. The economical laundry program saves one rinse and dehydration process.


1. Washing process

After the power is turned on, the washing machine enters a pause state to put the clothes in. If the washing cycle is not selected, the washing machine starts from the washing process. When the pause button K5 is pressed, the washing process is entered. Firstly, the water inlet valve FV is energized, the water inlet switch is turned on, and water is supplied to the laundry Yang; when the predetermined water level is reached, the water level switch K4 is turned on, and the water inlet valve is de-energized and closed to stop the water inlet; the motor MO is connected to the power supply to drive the pulsator to rotate , The formation of washing water flow. The motor MO is a forward and reverse motor, which can form a round-trip water flow, which is beneficial to washing clothes.


2. Dehydration process

After the washing or rinsing process is finished, the motor MO stops rotating, and the drain valve MG is energized to start draining. When the drain valve is activated, the clutch is driven, so that the motor can drive the inner barrel to rotate. When the water level is low to a certain value, the water level switch K4 is disconnected, and after a period of time, the motor starts to rotate forward, driving the inner tub to rotate at a high speed and drying the clothes.

3. Rinsing process

The operation is the same as the washing process, but the time is shorter.

After all the laundry work is completed, the buzzer will sound to indicate that the laundry has been cleaned.


Ⅱ. Hardware composition principle of washing machine controller

The washing machine controller is composed of the single-chip Z86C09 as the core of the controller. The controller has the following characteristics:

(1) It has strong anti-interference ability. When the program error occurs due to strong external interference, it can automatically reset the system and execute the program again.

(2) Adopt bidirectional thyristor without noise and electromagnetic interference as the control element to control solenoid valve and motor.

(3) With undervoltage and overvoltage protection, when undervoltage, the controller does not work; when overvoltage, the protection circuit works.

(4) It has an instantaneous power-off protection function. After a short-term power failure, when the voltage is restored, it can maintain the working state of the original running program and continue to complete the washing program.

(5) All operations and the running status of the washing machine are displayed by LEDs.


The characteristics and composition principles of each part are introduced below.

1. MCU Z86C09

Z86C09 is the simplest one in the Z8 series of single-chip microcomputers, with lower cost. Using CMOS structure, it has the characteristics of low power consumption, strong anti-interference ability and wide working voltage, and it can work in the voltage range of 2.5~5.5V. Z86C09 has 14 I/O lines, P2.0~P2.7 are bidirectional I/O ports, which can be set to input or output by bit. P3.1~3.3 of P3 port are defined as input ports, which can be used as input ports or interrupt request ports. P3.4~P3.6 are defined as output ports. Z86C09 contains 2 multi-function timer/counters, 2K bytes of ROM and 144 bytes of register array.


2. Power circuit part

The power supply of the controller consists of transformer B, rectifier diodes D14~D19, filter capacitor C1 and voltage regulator integrated circuit 7806. The voltage output by the 7806 is divided into three channels for thyristor triggering, keyboard input and LED display, and power supply for the single-chip microcomputer. The last two paths each pass through a diode and a capacitor. When the output voltage of 7806 drops, it can also rely on the energy held by the capacitor to keep the circuit working for a while.

Transistor T11, T10 and voltage regulator tube DW form an under-voltage protection circuit. When the power supply voltage is insufficient and the base voltage of T11 is less than 3.9V, T11 is cut off and T10 is also cut off. There is no voltage at P3.1 terminal of Z86C09, which is often low level. T12 is cut off, causing the emitters of T5 to T9 to be suspended, so T5 to T9 are cut off and are not controlled by Z86C09. At this time, although Z86C09 can work normally, all peripheral control components are turned off and the washing machine does not work. The P3.1 end of the single-chip microcomputer inputs a signal for judging the working state of the undervoltage protection circuit, and only when the voltage is normal, the single-chip microcomputer starts to execute the washing program.

When the power supply voltage exceeds the operating voltage, the resistance value of the varistor MR will suddenly decrease, so that the voltage cannot exceed the protection voltage value. When the overvoltage time is longer, the fuse RD will be blown.




3. Zero-crossing detection circuit

The zero-crossing detection circuit is composed of transistor T14, transformer B and diodes D17~D19. D17 acts as isolation. When the voltage is zero, the pulsating voltage is zero, and T14 is cut off. Since the collector resistance of T14 is connected to the collector of T10, only when T10 is turned on, that is, the power supply voltage is normal, can T14 output a high level when the power supply voltage crosses zero. The P3.1 terminal of Z86C09 detects the zero-crossing signal.


4. Keyboard and display circuit

The keyboard is composed of K1~K6, among which K3 and K4 are detection switches, and the button state detection adopts the scanning method. The P3.4~P3.6 of the single-chip microcomputer outputs the scanning signal, so that the transistors T1~T3 are turned on in turn, and the T1~T3 output Scan each key after passing the diode D1~D6. The 6 keys are divided into two groups, and the key signal is input by P3.2 and P3.3. P3.2 and P3.3 are normally low level. When a key is pressed and the high level scans to this key, the input of P3.2 or P3.3 will become high level. Z86C09 detects this high level, and then according to which bit is currently scanned, it can determine which key is pressed. The function of D1~D6 is to prevent short-circuits to three scan lines when multiple keys are pressed at the same time.

The display circuit is composed of LED1~LED7. The display mode adopts the dynamic scanning mode, the column broom signal line is shared with the row broom line, and the row display signal is directly driven by P2.4~P2.6 of Z86C09. Since the brightness required by the LED is low, the driving current is not large, about 9mA. The display time of each LED is 1/3 of the total display time, and the average current is about 3mA.


5. Trigger circuit of bidirectional thyristor

The bidirectional thyristor adopts DC triggering, and the gate of the thyristor is controlled by transistors T5~T8. When the transistor is turned on, the bidirectional thyristor is triggered to turn on, the second to third quadrants are triggered, and the collector resistance of T5 to T8 is used for current limiting. Because 1A and 3A bidirectional thyristors require relatively small trigger currents, they are susceptible to external interference. In order to improve the anti-interference ability of the system, a 0.01uF capacitor is connected in parallel in the 1A and 3A bidirectional thyristor trigger circuits to suppress instantaneous interference signals. Two 8A bidirectional thyristors are used to control the forward and reverse rotation of the motor MO. Only one of the two thyristors is allowed to be turned on at any time. If two are turned on at the same time, the thyristor will be damaged. The two main electrodes of the two 8A bidirectional thyristors are connected in parallel with a 100Ω resistor and 0.01uF capacitor to form a resistance-capacitance circuit, which is used to absorb the instantaneous voltage pulse between the two main electrodes of the bidirectional thyristor and protect the bidirectional thyristor.


Attachment: The single-chip microcomputer in the article can also be composed of AT89S51.