Switch mode power supply transformer detection method
1. Check for any obvious abnormalities by observing the appearance of the transformer. Whether the coil leads are broken, desoldered, whether there are burnt marks on the insulation material, whether the iron core fastening screws are loose, whether the silicon steel sheets are corroded, whether the winding coils are exposed, etc.
2. Insulation test. Measure the resistance values between the iron core and the primary, between the primary and each secondary, between the iron core and each secondary, between the electrostatic shielding layer and the secondary winding, and between each winding of the secondary using a multimeter R × 10k. The multimeter pointer should all point to infinity and remain stationary. Otherwise, it indicates poor insulation performance of the transformer.
3. Detection of coil continuity. Place the multimeter in the R × 1 position. During testing, if the resistance value of a winding is infinite, it indicates that the winding has an open circuit fault.
4. Distinguish between primary and secondary coils. The primary and secondary pins of a power transformer are generally led out from both sides, and the primary winding is often labeled with the word 220V, while the secondary winding is labeled with the rated voltage value, such as 15V, 24V, 35V, etc. Identify based on these markings.
5. Detection of no-load current.
a, Direct measurement method. Open all secondary windings and place the multimeter in AC current mode (500mA). Connect it in series with the primary winding. When the plug of the primary winding is plugged into 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 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 series in the primary winding of the transformer/ 5W resistor, the secondary is still completely unloaded. Set the multimeter to AC voltage mode. After power on, use two probes to measure the voltage drop U across resistor R, and then use Ohm's law to calculate the no-load current I, which is I=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 allowable error range is generally: high voltage winding ≤± 10%, low voltage winding ≤± 5%, and the voltage difference between two symmetrical windings with center tap should be ≤± 2%.
6. Generally, low-power power transformers allow a temperature rise of 40 ℃ to 50 ℃. If the insulation material used is of good quality, the allowable temperature rise can be increased.
7. Detect and distinguish the same named terminals 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 a power transformer in series, the same named terminals of each winding participating in the series connection must be connected correctly 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 a short-circuit fault in a power transformer 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 transformer's heating. A simple method to detect and determine 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 no-load current values much greater than 10% of full load current. When the short circuit is severe, the transformer will quickly heat up within a few tens of 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.
