The method of testing a power transformer with a multimeter includes the following steps:
1. **Insulation test**: use the multimeter R × 10k stop, respectively, to measure the core and the primary, primary and each secondary, core and each secondary, secondary windings between the resistance value. Under normal circumstances, the multimeter pointer should point to infinity position. If the indication is abnormal, it means that the transformer insulation performance is poor.
2. ** coil on-off detection **: the multimeter will be placed in the R × 1 block, test the resistance value of each winding. If the resistance value of a winding is infinite, it indicates that there is a circuit-breaking fault in this winding.
3. ** primary and secondary coil discrimination **: usually, the primary and secondary pins of the power transformer pins from both sides. Primary winding more marked with 220V, while the secondary winding is marked with the rated voltage value, such as 15V, 24V, 35V, etc.. Identify according to these markings.
4. **No-load current detection**:
* Direct measurement method: all the secondary windings will be open circuit, the multimeter will be placed in the AC current block position, series into the primary winding. When the primary winding is connected to the 220V AC mains, the multimeter indicates the value of no-load current. This value should not be greater than 10% to 20% of the full load current of the transformer.
* Indirect measurement method: in the primary winding of the transformer in series with a 10/5W resistor, the secondary is still all no-load. The multimeter will be dialed to the AC voltage block for measurement.
5. **Resistance Measurement**: Use the resistance gear of the multimeter, such as RX1 or RX10, to measure both sides of the transformer coil. Usually, the primary resistance is larger and the secondary resistance is smaller (step-down transformer). Re-measure between the primary stages, which should not be energized.
6. **induced current and mutual inductance voltage test **: the transformer's primary coil ends into the multimeter AC voltage 50-100 volts gear. With a dry cell to touch the transformer secondary coil ends. If at this time the multimeter needle has a strong swing, which indicates that the induced current and mutual inductance voltage is normal, proving that the transformer does not have a short circuit between the turns.
By following these steps, the performance of a power transformer can be more fully evaluated to determine its suitability for use. When performing the test, it is important to pay attention to safety to avoid electric shock and other accidents.
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