Switching power supply transformer detection method
1. Check whether there are any obvious abnormalities by observing the appearance of the transformer. For example, whether the coil leads are broken or desoldered, whether the insulation material has scorch marks, whether the iron core fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, etc.
2. Insulation test. Use a multimeter R×10k scale to measure the resistance values between the core and the primary, the primary and each secondary, the core and each secondary, the electrostatic shielding layer and the secondary, and the secondary windings. The multimeter pointer should all point at the infinity position. move. Otherwise, the insulation performance of the transformer is poor.
3. Detection of coil continuity. Place the multimeter in the R×1 gear. During the test, if the resistance value of a certain winding is infinite, it means that this winding has an open circuit fault.
4. Distinguish the primary and secondary coils. The primary pin and secondary pin of the power transformer are generally drawn from both sides respectively, and the primary winding is often marked with 220V, while the secondary winding is marked with a rated voltage value, such as 15V, 24V, 35V, etc. Then identify based on these marks.
5. Detection of no-load current.
a. Direct measurement method. Open all secondary windings, put the multimeter in the AC current setting (500mA), and connect it in series to the primary winding. When the plug of the primary winding is inserted 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. The normal no-load current of a common electronic equipment power transformer should be around 100mA. If it exceeds too much, it means that the transformer has a short-circuit fault.
b. Indirect measurement method. A 10Ω/5W resistor is connected in series to the primary winding of the transformer, and the secondary is still completely unloaded. Set the multimeter to AC voltage. After powering 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 I, that is, I = U/R. F? Detection of no-load voltage. Connect the primary of the power transformer to the 220V mains power, and use a multimeter to connect the AC voltage to measure the no-load voltage values of each winding (U21, U22, U23, U24) in sequence, which should meet the required values. The allowable error range is generally: high voltage winding ≤ ±10 %, low-voltage winding ≤ ±5%, the voltage difference of two sets of symmetrical windings with center tap should be ≤ ±2%.
6. Generally, the allowable temperature rise of small power power transformers is 40℃~50℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
7. Detect and identify the identical ends of each winding. 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 same terminals of each winding participating in the series must be connected correctly and cannot be mistaken. Otherwise, the transformer will not work properly.
8. Comprehensive detection and identification of short-circuit faults in power transformers. The main symptoms after a short-circuit fault occurs in a power transformer are severe heating and abnormal secondary winding output voltage. Generally, the more inter-turn short-circuit points inside the coil, the greater the short-circuit current and the more serious the transformer heating. A simple way to detect and determine whether the power transformer has a short-circuit fault is to measure the no-load current (the test method has been introduced previously). The no-load current value of a transformer with a short-circuit fault 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 no-load power-up, and the iron core will feel hot when touching it with your hands. At this time, it can be concluded that there is a short circuit point in the transformer without measuring the no-load current.
