Testing methods and maintenance skills of air-conditioning computer boards

Testing methods and maintenance skills of air-conditioning computer boards

The microcomputer control system of the household air conditioner is commonly known as the computer board. It is the nerve center of the air conditioner. When a failure occurs, the entire air conditioner will be in a paralyzed state, resulting in no cooling or poor cooling effect. At this time, it must be repaired. Bring it back to normal. However, due to the very high technical content of the microcomputer control system, which involves single-chip technology, electronic control technology, sensor technology, photoelectric conversion technology, etc., for school refrigeration students and general refrigeration practitioners, it is generally believed that repairing computer boards is very Difficult, always feel unable to start when encountering a failure The following describes the working principle of the air-conditioning computer board and its detection and maintenance skills from the perspective of maintenance.

　　 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 a microprocessor (CPU), a detection circuit (thermistor), a receiving circuit (receiving head), a controlled circuit (relay), and a display Circuit (light emitting tube) and other components. The actual control board is shown in Figure 1. The basic principle block diagram is shown as in Fig. 2. The working principle is that the CPU sends out control instructions according to the operating instructions and the detection and judgment of the ambient temperature and the working state of the machine, so that the relevant circuits, compressors, fans, etc. work according to the pre-designed program, and at the same time, various working states are displayed through the display show.

　　 2. The function of each part of the computer board and the maintenance skills

　　 (1) Power circuit

　　 AC voltage 220V is composed of fuse, varistor, transformer, bridge rectifier, three-terminal voltage regulator integration (7812, 7805), and filter capacitor, as shown in Figure 3. Its function is to provide +5V and +12V DC voltage to CPU and relay. The fault phenomenon caused by the power circuit is that the indicator light does not light up and the whole machine does not work.

　　 Inspection method:

　　 The characteristic of the power circuit failure is that the fuse is intact and burns as soon as it is turned on. For the former fault, you can use a multimeter to measure whether the transformer primary and secondary have 220V and 13V voltage. If so, use the multimeter to measure whether the 7812 and 7805 have +12V and +5V voltages, so that you can distinguish the fault location. For the latter, it means that there is a short circuit in the circuit. Use a multimeter ohm block to detect the resistance value to determine the short circuit position of the circuit. At the same time, the division method can also be used to check, such as by disconnecting the primary winding of the transformer, powering on and testing the machine, if the fuse is still burned, it means that the burning of the fuse is due to a short circuit in the varistor or ceramic capacitor, otherwise, it is due to the transformer Or the rectifier tube is short-circuited.

　　 (two) temperature sensing circuit

　　 It uses the thermistor to convert the ambient temperature, air conditioner evaporator temperature and other temperature changes into certain numerical electrical signals and send them to the CPU, so that the air conditioner can operate in a state set by people, creating a comfortable space environment. The core component of the temperature sensing circuit is the thermistor. The thermistor's failure is mainly due to the increase or decrease of the resistance value, which causes the CPU to malfunction, non-stop or non-operation, and abnormal cooling.

　　 Inspection method:

　　 Thermistor is a thermistor with a negative temperature coefficient, that is, the higher the temperature, the smaller the resistance, and the lower the temperature, the greater the resistance. The resistance is about 15KΩ at 250C (depending on the model). Therefore, the resistance value can be measured with a multimeter ohm block to judge whether it is good or bad. If the measured resistance value is infinite or very small, the thermistor has been damaged.

　　 (three) relay circuit

　　The relay circuit is a circuit that converts the instructions issued by the CPU into a circuit that controls the on and off of the compressor, fan, four-way valve and other strong electric components. It is generally composed of integrated power drive modules, relays and related components, as shown in Figure 4. Most of the circuit failures are damage to the integrated power drive module, burnout of the relay coil, adhesion of contacts, etc., resulting in uncooling or abnormal cooling of the air conditioner.

　　 Inspection method:

The first distinction is that the integrated power drive module is damaged or the relay is damaged. If the buzzer makes a sound after turning on the remote control, but the whole machine does not work, the integrated power drive module is generally damaged; if only part of the function is abnormal after turning on It is possible that the relay is damaged. At this time, you can continue to listen to whether the relay is picking up sound to determine whether the relay is coil burned or contact sticking. There is no picking sound when the relay coil burns out. The relay can also be judged by the multimeter ohm block. Disconnect the power supply and first measure the resistance of the coil. The normal resistance is several hundred ohms. If infinity or zero, the relay is damaged; then measure the contacts. If the resistance is zero, then Indicates that the contacts are stuck.

　　 (four) receiving circuit

The control signal from becomes a signal that the CPU can recognize and is sent 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 triode, and the outside is generally three pins, which are supply electrodes, ground electrodes, and signal electrodes. The failure phenomenon of the receiving circuit is: manual power on is normal, when the remote control is pressed, the whole machine does not respond, and the buzzer does not sound.

　　 Inspection method:

　　 Power on and start, use a multimeter to measure the voltage between the power supply terminal and the signal terminal of the receiving head to the ground. The normal value should be the voltage of the supply electrode +5V and the voltage of the signal electrode +2.5V. Otherwise it is not normal. Common faults include damage to the receiving head or capacitor breakdown.

　　(5) Reset circuit

　　 Fluctuations or external electromagnetic waves will cause the CPU to crash. The reset circuit resets the CPU chip and works normally when the power is turned on or abnormal. Generally, it is low level reset, and high level is normal workpiece state. The fault phenomenon caused by this circuit: the indicator light is on, the buzzer does not sound by pressing the remote control, and the whole machine does not work.

　　 Inspection method:

The reset voltage is a delayed rising voltage. It can be observed with a multimeter's DC voltage file. If the observation is not clear, it can also be tested with an oscilloscope. When testing with an oscilloscope, a baseline may be jittered and then changed to a high level. This is the start of the reset voltage. process. If you do not see the jitter of the baseline, it means that the reset circuit is faulty.

　　(6) Crystal oscillator circuit

　　 It is a reference clock signal for the chip CPU 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 legs of the quartz crystal tube at about +2.2V. If it is less than 1.5V, the circuit stops vibration. Elsewhere, it can be judged by removing the quartz crystal tube and using the multimeter ohm block to judge. A good quartz crystal tube should be open circuit when measured with a multimeter. If a short circuit is found, it means that the crystal is damaged. For open-circuit faults (disconnected or shattered), it is impossible to judge with a multimeter. In this case, alternative methods can be used to check.

　　 (seven) CPU

　　The CPU is the command center of the entire control circuit. It distinguishes the various instructions received and the detected data and sends out corresponding command plans to control the operation of various circuits 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 normally. Therefore, a multimeter can be used to detect the voltage of 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.

　　 (eight) display circuit

　　The display circuit generally has luminous tube and fluorescent tube display, which is driven by the system control circuit to display various working status of the system. This circuit generally does not malfunction.

　　 Three, the maintenance process of the air conditioner computer board

　　 In the fault maintenance of the actual air conditioner computer board, the principle of easy first and difficult should be followed. First check the power circuit and reset circuit, then replace the crystal oscillator, then check the remote control receiving circuit, and finally replace the CPU. Because the replacement of CPU pins is the most difficult and expensive, more importantly, the probability of CPU damage is quite low, so do not easily replace the CPU in the microcomputer control circuit.