Switching power supply - detection method of power transformer
A. Check whether there are obvious abnormalities by observing the appearance of the transformer. Such as whether the coil lead wire is broken, desoldered, whether the insulating material has scorched marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.
B. Insulation test. Measure the resistance values between the iron core and the primary, the primary and each secondary, the iron core and each secondary, the electrostatic shielding layer and the secondary, and the secondary windings with a multimeter R×10k. The pointer of the multimeter should point to infinity. The position does not move. Otherwise, it means that the insulation performance of the transformer is poor.
C. Detection of coil on-off. Put the multimeter in the R×1 position. During the test, if the resistance value of a winding is infinite, it means that the winding has an open circuit fault.
D. Distinguish primary and secondary coils. The primary pins and secondary pins of the power transformer are generally drawn from both sides, and the primary winding is usually marked with 220V, and the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Then identify them based on these marks.
E. Detection of no-load current. (1) Direct measurement method. Open all the secondary windings, put the multimeter in the AC current gear (500mA, and connect it in series with the primary winding. 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 It is greater than 10% to 20% of the full load current of the transformer. Generally, the normal no-load current of the power transformer of common electronic equipment should be about 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault. (2) Indirect measurement method. In the transformer A 10?/5W resistor is connected in series in the primary winding, and the secondary is still completely unloaded. Turn the multimeter to the AC voltage gear. 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 empty voltage. The carrying current I is empty, that is, I is empty=U/R.
F. Detection of no-load voltage. Connect the primary of the power transformer to the 220V mains, and use a multimeter to measure the no-load voltage value of each winding (U21, U22, U23, U24) in sequence, which should meet the required value. The allowable error range is generally: high voltage winding≤±10 %, low-voltage windings ≤±5%, and the voltage difference between two sets of symmetrical windings with center taps should be ≤±2%.
G. Generally, the allowable temperature rise of small-power power transformers is 40°C to 50°C. If the quality of the insulating material used is better, the allowable temperature rise can also be increased.
H. Detect and judge the end of each winding with the same name. When using a power transformer, sometimes two or more secondary windings can be connected in series in order to obtain the required secondary voltage. When using the power transformer in series, the terminals with the same names of the windings participating in the series must be connected correctly and cannot be mistaken. Otherwise, the transformer cannot work properly.
I. Comprehensive detection and judgment of short-circuit faults in power transformers. The main symptoms after the short-circuit fault of the power transformer are severe heating and abnormal output voltage of the secondary winding. Generally, the more inter-turn short-circuit points inside the coil, the greater the short-circuit current and the more serious the heat of the transformer. The simple way to detect and judge whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced earlier). For transformers with short-circuit faults, the no-load current value will be much greater than 10% of the full-load current. When the short circuit is severe, the transformer will heat up rapidly within tens of seconds after being powered on without load, and the iron core will feel hot when touched by hand. At this time, it can be concluded that there is a short circuit point in the transformer without measuring the no-load current.
