Eight detection methods for transformers in switching power supplies

　　1. Check the transformer for any obvious abnormalities by observing its appearance, such as whether the coil lead is broken or desoldering, whether the insulating material has burnt marks, whether the core fastening screw is loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.

　　2. Insulation test. Use the multimeter R×10k to measure the resistance between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary, and each secondary winding. The multimeter pointer should point to the infinite position and not move. Otherwise, it means that the insulation performance of the transformer is poor.

　　3. Detection of the continuity of the coil. Set the multimeter to the R×1 position. During the test, if the resistance value of a winding is infinite, it means that the winding has a circuit breaker fault.

　　4. Identify the primary and secondary coils. The primary and secondary pins of the power transformer are generally led out from both sides, and the primary winding is often marked with 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Then identify according to these marks.

　　5. Detection of no-load current.

　　a. Direct measurement method. Open all secondary windings, set the multimeter to the AC current range (500mA, and connect the primary winding in series. When the plug of the primary winding is plugged into the 220V AC mains, the multimeter indicates the no-load current value. This value should not be greater than 10% to 20% of the full-load current of the transformer. The normal no-load current of the power transformer of common electronic equipment should be around 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.

　　b. Indirect measurement method. Connect a 10Ω/5W resistor in series in the primary winding of the transformer, and the secondary winding is still completely unloaded. Set the multimeter to the AC voltage range. After power on, use two test leads to measure the voltage drop U across the resistor R, and then use Ohm's law to calculate the no-load current Iempty, that is, Iempty=U/R. F? No-load voltage detection. Connect the primary of the power transformer to 220V AC power, and use a multimeter to measure the no-load voltage value of each winding in turn (U21, U22, U23, U24). It should meet the required value. The allowable error range is generally: high-voltage winding ≤±10%, low-voltage winding ≤±5%, and the voltage difference between two sets of symmetrical windings with center taps should be ≤±2%.

　　6. Generally, the allowable temperature rise of small-power power transformers is 40℃~50℃. If the quality of the insulating material used is good, the allowable temperature rise can be further increased.

　　7. Detect and identify the same-named ends of each winding. When using a power transformer, sometimes in order to obtain the required secondary voltage, two or more secondary windings can be connected in series. When using a power transformer in series, the same-named ends of each winding in series must be connected correctly and cannot be mistaken. Otherwise, the transformer cannot work properly.

　　8. Comprehensive detection and identification of short-circuit faults in power transformers. The main symptoms of a short-circuit fault in a power transformer are severe heating and abnormal output voltage of the secondary winding. Generally, the more short-circuit points there are between turns in the coil, the greater the short-circuit current, and the more severe the transformer heating. A simple way to detect and determine whether a power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced earlier). For a transformer with a short-circuit fault, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is serious, the transformer will heat up rapidly within tens of seconds after power is applied at no load, and the core will feel hot when touched by hand. At this time, it is not necessary to measure the no-load current to determine that the transformer has a short-circuit point.