1、 The function and classification of transformers 1. The function of transformers is to use the principle of electromagnetic induction to convert one AC voltage into another AC voltage with the same frequency. 2. According to the working frequency, transformers can be classified into various types, such as high-frequency transformers (also known as pulse transformers, switch transformers), low-frequency (power frequency) transformers, etc; According to their functions, transformers can be divided into step-down transformers and step-up transformers; According to the purpose, transformers can be divided into various types, such as power transformers, voltage regulating transformers, audio transformers, intermediate frequency transformers, isolation transformers, input transformers, output transformers, etc. The commonly used transformers in household appliances mainly include mains (power frequency) transformers and switch transformers. Their physical appearance is shown in Figures (a) and (b), and the circuit symbols are shown in Figure (c) 2. Transformer Testing 1. Power Frequency Transformer Testing (1) Insulation Performance Testing Place the Multimeter at "R × Measure the resistance values between the primary winding and each secondary winding, iron core, and electrostatic shielding layer at 10k "gear, and ensure that the resistance values are infinite. If the resistance value is too small, it indicates that there is leakage, which leads to poor insulation performance of the transformer. (2) The pins of the primary and secondary windings of a power frequency transformer are generally led out from both sides of the transformer, and the primary winding is often marked with the words "220V". The secondary winding is marked with the rated output voltage value, such as 6V, 9V, 12V, 15V, 24V, etc. By using these markings, the function of the winding can be identified. However, if some transformers are not marked or have unclear markings, it is necessary to use a multimeter to detect the primary and secondary windings of the transformer. Due to the fact that most power frequency transformers are step-down transformers, their primary winding has a high input voltage and low current, as well as a large number of turns and fine wire diameters of enameled wires, resulting in a high DC resistance. Although the output voltage of the secondary winding is low, the current is high, so the diameter of the enameled wire in the secondary winding is thicker and the number of turns is smaller, resulting in a smaller resistance value. In this way, different windings can be identified by measuring the resistance values of each winding. The typical transformer measurement is shown in the figure. If a transformer with the same output voltage and power values has a significant difference in resistance, it indicates that the transformer is damaged. However, this method is usually used to determine whether the primary and secondary windings are open or not, and when suspecting a short circuit in the winding, visual inspection, temperature method, and voltage detection method are often used for judgment. Tip: Many low-frequency power frequency transformers have temperature fuses installed between the primary winding and terminal blocks. Once the mains voltage rises or the load overcurrent causes the transformer to overheat, the fuse will overheat and blow, resulting in an open circuit fault in the primary winding. Carefully disassembling the winding at this time will reveal the fuse. After replacing it, the transformer can be repaired, and wires can also be short circuited during emergency repairs. A short circuit in the winding can cause the fuse of the mains input circuit to overheat and blow, or result in abnormal phenomena such as the primary winding of the transformer being burnt out or the winding being burnt out. (3) The detection of the voltage of each winding of the transformer is based on the input 220V mains voltage of the primary winding of the power transformer, as shown in Figure (a). The AC voltage output of the secondary winding of the transformer is 12.77V using a multimeter AC 20V voltage range, as shown in Figure (b). If the input voltage of the transformer is normal and the output voltage is abnormal, it indicates that the transformer or load is abnormal. The allowable error range between no-load voltage and nominal value is generally: the high-voltage winding should not exceed ± 10%, the low-voltage winding should not exceed ± 5%, and the voltage difference between two sets of symmetrical windings with central tap should not exceed ± 2%. (4) Temperature detection: Connect all secondary windings of the transformer and input 220V mains voltage for the primary winding. Generally, low-power power frequency transformers allow a temperature rise of 40-50 ℃. If the insulation material used is of good quality, the temperature rise is allowed to be higher. Reminder: If the temperature of the transformer rapidly increases shortly after being powered on, it indicates a short circuit in the winding or load. (5) The detection of no-load current disconnects all secondary windings of the transformer, and then places the multimeter in the AC "500mA" current range. The probe is connected in series to the primary winding circuit, and a 220V mains voltage is input to the primary winding. The value measured by the multimeter is the no-load current value. This value should be less than 10% to 20% of the transformer's full load current. If it exceeds too much, it indicates that the transformer has a short-circuit fault. Reminder: The normal no-load current of common electronic equipment power frequency transformers should be around 100mA.