Switching power supply transformer detection technique
1. Examine the transformer's look to see if there are any visible problems. For instance, whether the iron core's fastening screw is loose, whether the silicon steel sheet is rusted, whether the winding coil is exposed, whether the coil lead wire is broken or desoldered, whether the insulating material has scorch marks, etc.
Insulation evaluation. Using a multimeter R10k, transfer the multimeter between the iron core and the primary, secondary, iron core and secondary, electrostatic shielding layer and secondary, and secondary windings. If not, it indicates that the transformer's insulating performance is subpar.
3. Coil on/off detection. A multimeter should be set to R1. If a winding's resistance value is infinite during the test, the winding has an open circuit defect.
4. Differentiate between primary and secondary coils. The primary and secondary pins of a power transformer are often drawn from both sides, and the secondary winding is labelled with the rated voltage, such as 15V, 24V, or 35V. The primary winding of a power transformer is typically marked with 220V. Then, based on these markings, identify them.
5. Detection of no-load current.
Direct measuring technique (a). Put the multimeter in the AC current equipment (500mA) with all of the secondary windings open, and link it in series with the primary winding. The multimeter displays the no-load current figure when the primary winding's plug is hooked into the 220V AC mains. There shouldn't be this value It exceeds 10% to 20% of the transformer's full load current. Typically, the power transformer of typical electronic equipment should have a no-load current of around 100 mA. It indicates a short-circuit issue in the transformer if it surpasses too much.
b. Indirect measurement method. A 10?/5W resistor is connected in series with the primary winding of the transformer, and the secondary is still completely unloaded. Turn the multimeter to AC voltage. 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 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 the 220V mains, and use a multimeter to measure the no-load voltage value of each winding (U21, U22, U23, U24) in turn, 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%.
6. 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.
7. 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.
8. Comprehensive detection and judgment of short-circuit faults of 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 a transformer with a short-circuit fault, its 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.
