Introduction to Testing Methods for Switching Power Supply Transformers
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 the 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. Further identify based on these markers
5. Detection of no-load current.
a, Direct measurement method. Open all secondary windings, set the multimeter to AC current mode (500mA), and connect it in series with 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 exceed 10% to 20% of the transformer's full load current. The normal no-load current of power transformers for common electronic devices 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 /5W resistor in series in the primary winding of the transformer, while 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%.
