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(86 class hours ) Fixed frequency air conditioning maintenance video guidance complete set
Home air conditioner microcomputer control system is commonly known as The computer board is the nerve center of the air conditioner. When a fault occurs, the entire air conditioner will be paralyzed, resulting in no cooling or poor cooling effect. At this time, it must be repaired to restore it to normal.
However, because the microcomputer control system has a very high technical content, which involves single-chip microcomputer technology, electronic control technology, sensing technology, photoelectric conversion technology, etc., for students majoring in refrigeration in school and general refrigeration practitioners, it is generally believed that repairing computer boards It’s difficult, and I always feel like I don’t know what to do when I encounter a problem. The following introduces the working principle of the air conditioner computer board and its detection and maintenance techniques from a maintenance perspective.
1. Introduction to the principle of air conditioner computer board
Although the computer control system is complicated, looking at the computer boards of various manufacturers, without exception they are composed of:
microprocessor (CPU), detection circuit (thermistor), receiving circuit ( It consists of receiving head ), controlled circuit ( relay ), display circuit ( light-emitting tube ), etc. The actual control panel of
is shown in Figure 1. The basic principle block diagram is shown in Figure 2. The working principle is that the CPU issues control instructions based on the operating instructions and the detection and judgment of the ambient temperature and internal working status of the machine, so that the relevant circuits, compressors, fans, etc. work according to the pre-designed program. At the same time, the various working statuses are displayed through the display. display.
2. The functions and maintenance skills of each part of the circuit of the computer board
(1) power circuit
AC voltage 220V through fuse , varistor, transformer, bridge rectifier, three-terminal voltage stabilization integration (7812, 7805) , filter capacitor composition, as shown in Figure 3. Its function is to provide +5V and +12V DC voltage to the CPU and relays. The fault phenomenon caused by the power circuit is that the indicator light does not light up and the whole machine does not work.
maintenance method:
The characteristic of power circuit failure is that the fuse is intact and the fuse burns as soon as the power is turned on. For the former fault, you can use the multimeter AC gear to measure whether there are 220V and 13V voltages on the primary and secondary sides of the transformer. If so, use the DC gear of the multimeter to measure whether there are +12V and +5V voltages on 7812 and 7805, so that the fault location can be distinguished. .
For the latter, it means that there is a short circuit in the circuit. Use the ohm block of a multimeter to detect the resistance to determine the short circuit part of the circuit. At the same time, the segmentation method can also be used to check. For example, you can disconnect the primary winding of the transformer and power on the test machine. If the fuse is still burned, it means that the burned fuse is due to a short circuit in the varistor or ceramic capacitor. Otherwise, it is due to a short circuit in the transformer. Or the rectifier tube has a short circuit.
(2) Temperature sensing circuit
It converts temperature changes such as ambient temperature and air conditioner evaporator temperature into a certain numerical electrical signal through the thermistor and transmits it to the CPU, so that the air conditioner operates according to the state set by the person, creating a Comfortable space environment. The core component of the temperature sensing circuit is the thermistor. The main fault of the thermistor is that the resistance value becomes larger or smaller, causing the CPU to malfunction, causing no shutdown or operation, and abnormal cooling.
maintenance method:
thermistor is a thermistor with a negative temperature coefficient, that is, the higher the temperature, the smaller the resistance, the lower the temperature, the greater the resistance. At 250C, the resistance is about 15KΩ (varies by model) . Therefore, you can use a multimeter with ohm block to measure its resistance value to judge whether it is good or bad. If the measured resistance value is infinite or very small, it means that the thermistor is damaged.
(3) Relay circuit
The relay circuit is a circuit that converts the instructions issued by the CPU into controlling the opening and stopping of strong current components such as compressors, fans, four-way valves, etc. It generally consists of integrated power drive modules, relays and related components, as shown in Figure 4. Most of the circuit faults are damage to the integrated power drive module, burnt relay coil , contact adhesion, etc., resulting in the air conditioner not cooling or cooling abnormally.
maintenance method:
First of all, distinguish whether the integrated power drive module is damaged or the relay is damaged. If the buzzer sounds after pressing the remote control after turning on the computer, but the whole machine does not work, it is generally because the integrated power driver module is damaged; if after turning on the computer, If only part of the function is not normal, the relay may be damaged. At this time, you can continue to listen to whether the relay makes a closing sound to determine whether the relay coil is burned out or the contacts are stuck. There is no closing sound when the relay coil is burned out. You can also use the ohm block of a multimeter to judge whether the relay is good or bad. Disconnect the power supply and measure the coil resistance first. The normal resistance value is several hundred ohms. If it is infinite or zero, it means the relay is damaged. Then measure the contacts. If the resistance value is zero, then Indicates stuck contacts.
( four) receives the control signal from the circuit
and turns it into a signal that the CPU can recognize and transmits it to the CPU, so that the CPU controls the operation of various components according to human instructions. The infrared receiver is the core component of the receiving circuit. The inside is a photosensitive transistor, and the outside is usually three pins, which are the power supply electrode, the ground electrode and the signal electrode. The phenomenon of receiving circuit failure is: manual power-on is normal, but when pressing the remote control, the whole machine does not respond and the buzzer does not sound.
maintenance method: Turn on the power of
, and use the DC block of a multimeter to measure the voltage of the power supply terminal and the signal terminal of the receiving head to the ground. The normal value should be +5V for the power supply pole and +2.5V for the signal pole. Otherwise it is abnormal. Common faults include damage to the receiver or capacitor breakdown.
(5) reset circuit
fluctuation or interference of external electromagnetic waves, the CPU will crash. The reset circuit is to reset the CPU chip and work normally when the power is turned on or abnormal. Generally, it is a low-level reset, and when is high-level and is a normal workpiece state. Fault phenomena caused by this circuit: the indicator light is on, the buzzer does not sound when the remote control is pressed, and the whole machine does not work.
maintenance method:
reset voltage is a voltage that rises with delay. You can use a multimeter to observe the DC voltage range. If the observation is not clear, you can also use an oscilloscope to detect it. When you use an oscilloscope to detect it, you can have a baseline that is shaking and then changes to a high level. This It is the startup process of reset voltage. If you don't see jitter in the baseline, the reset circuit is faulty.
(6) crystal oscillator circuit
It gives the chip CPU a reference working clock signal to make the chip CPU work normally. When the crystal oscillator circuit fails, the whole machine will not work.
maintenance method
Turn on the power and use a multimeter to measure the voltage of the two pins of the quartz crystal oscillator when it is normal, which is about +2.2V. If it is less than 1.5V, the circuit stops oscillating. Elsewhere, you can also judge by removing the quartz crystal oscillator and using the ohm block of a multimeter. A good quartz crystal oscillator should be open circuit when measured with a multimeter. If a short circuit is found, the crystal is damaged. For open circuit faults (broken wires or cracks), it is impossible to judge with a multimeter. In this case, alternative methods can be used to check.
7) CPU
CPU is the command center of the entire control circuit. It judges the various instructions received and the detected data and then issues corresponding command plans to control the work of each circuit and equipment and output display signals. If the CPU is damaged, the whole machine will not work.
maintenance method
Because the necessary conditions for the normal operation of the CPU are: power supply voltage +5V, reset voltage, and clock pulse signal. These three conditions are indispensable, otherwise the CPU will not work properly. Therefore, a multimeter can be used to detect the voltage under its working conditions. If the voltage value is normal and the whole machine does not work, it can be judged that the CPU chip is damaged.
(8) Display circuit
display circuit generally has light-emitting tube and fluorescent tube displays, which are driven by the system control circuit to display various working states of the system. This circuit generally does not malfunction.
3. Maintenance process of air conditioner computer board
In the actual fault repair of air conditioner computer board, the principle of easy first and then difficult should be followed, and the inspection should be carried out according to the following steps:
First check the power circuit and reset circuit, and then replace the crystal oscillator , then check the remote control receiving circuit, and finally replace the CPU.Because the CPU has many pins and is most difficult to replace and the price is high. More importantly, the probability of CPU damage is very low, so do not easily replace the CPU in the microcomputer control circuit. Free open class, come in and take a look
