Through these simple checks below you can tell if the small transformer is bad. Remember to turn off the power before these checks!
1. Visual Inspection
Look at the outside first to check if there are burn marks, bulging, melted glue, cracks, exposed windings, cold-soldered or deformed terminals. If there is obvious burning damage, the transformer is basically considered scrap.
2. Smell Test
Get it close and see if there’s a sharp burnt smell, which often indicates that the windings or insulation have been badly overheated.
3. Basic Test with a Multimeter
A) Check if the windings are still connected
After powering off and disassembling, use the multimeter to measure the resistance of the primary and secondary windings. An “infinite” reading indicates an open circuit, while a reading close to zero may indicate a severe short circuit.
B) Insulation test:
Use the high-resistance range to measure resistance between the core and each winding, and between windings. If it reads “infinite,” the winding is broken. If it’s almost zero, there may be a serious short inside.
4. Functional testing under power (Mains Voltage Hazard)
A) No-load output test:
After confirming correct wiring and ensuring safe isolation, apply power only to the primary with no load on the secondary. Check whether the output voltages are close to the rated values on the nameplate. Large deviations or zero output usually indicate winding problems.
B) Observe heating and strange noise:
Let it run with no load or a light load for a few minutes. If there is obvious overheating, abnormal humming, or a burning smell, there’s likely an internal fault such as inter-turn short circuits. Turn off the power right away and replace it.
Safety note:
If you’re not experienced, don’t test anything connected to mains power yourself. Performing only visual inspection and resistance checks in a powered-off state is much safer.
