Testing methods for switching power supply transformers
1. Check for obvious abnormalities by observing the appearance of the transformer. For example, whether the coil leads are broken, desoldered, whether there are burnt marks on the insulation material, whether the fastening screws of the iron core are loose, whether the silicon steel sheet is rusted, and whether the winding coil is exposed.
2. Insulation test. Using a multimeter R × Measure the resistance values between the core and primary, primary and secondary, core and secondary, electrostatic shielding layer and secondary, and secondary windings at 10k gear. The multimeter pointer should be fixed at infinity. Otherwise, it indicates poor insulation performance of the transformer.
3. Detection of coil on/off. Place the multimeter in R × In gear 1, during the test, if the resistance value of a certain winding is infinite, it indicates that there is an open circuit fault in the winding.
4. Distinguish between 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 the word 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc. Then identify based on these markings.
5. Detection of no-load current.
a. Direct measurement method. Open all secondary windings and place the multimeter in the AC current range (500mA). Connect the multimeter to the primary winding in series. When the plug of the primary winding is inserted into a 220V AC mains supply, the multimeter indicates the no-load current value. This value should not exceed 10% to 20% of the transformer's full load current. The normal no-load current of a common electronic equipment power transformer should be around 100mA. If it exceeds too much, it indicates that the transformer has a short-circuit fault.
b. Indirect measurement method. Connect a 10 in series in the primary winding of the transformer/ A resistance of 5W, with all secondary circuits still empty. Set the multimeter to AC voltage. After powering on, use two probes to measure the voltage drop U at both ends of the resistance R, and then use Ohm's law to calculate the no-load current I empty, that is, I empty=U/R. F? Detection of no-load voltage. Connect the primary of the power transformer to 220V mains power, and use a multimeter to measure the no-load voltage values (U21, U22, U23, U24) of each winding in sequence. The required values should be met, and 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 central tap should be ≤ ± 2%.
6. The allowable temperature rise for general low-power power transformers is between 40 ℃ and 50 ℃. If the insulation material used is of good quality, the allowable temperature rise can also be increased.
7. Detect and distinguish the homonymous ends of each winding. When using a power transformer, sometimes two or more secondary windings can be connected in series to obtain the required secondary voltage. When using the series connection method to use a power transformer, the identical ends of each winding participating in the series connection must be correctly connected and cannot be mistaken. Otherwise, the transformer will not function properly.
8. Comprehensive detection and discrimination of short-circuit faults in power transformers. The main symptoms of short-circuit faults in power transformers are severe heating and abnormal output voltage of the secondary winding. Usually, the more inter turn short-circuit points inside the coil, the greater the short-circuit current, and the more severe the heating of the transformer. The simple method for detecting and judging whether a power transformer has a short-circuit fault is to measure the no-load current (the testing method has been introduced earlier). Transformers with short-circuit faults will have a no-load current value much greater than 10% of the full load current. When the short circuit is severe, the transformer will quickly heat up within a few seconds after no-load power on, and touching the iron core with your hand will feel hot. At this point, it can be concluded that there is a short-circuit point in the transformer without measuring the no-load current.
