Testing a transformer: how to accurately test a transformer
There are two types of transformers on the market: linear transformer and switched power transformer. The design of the transformers is different. It is sometimes referred to simply as an AC transformer. Linear transformer mainly used in radio, project kits, VCD, etc., while the switch mode transformer can be found in Monitor, Tv, Fax, etc.
A transformer consists of two or more coils wound on the same core. For power transformers, the core material is usually iron. For a radio frequency transformer, the core material is usually ferrite or air. The basic property of a transformer is to change the AC voltage. A transformer cannot change direct current voltage. A step-down transformer has a lower AC output voltage on its secondary winding than the AC input voltage on its primary winding. In contrast, a step-up transformer has a higher secondary voltage than the primary one. Transformer laminations or shellac, enamel or varnish coating are to isolate adjacent turns from shorts between windings. I will not go into too much detail on how transformers work and transformer calculation as you can read more details in most electrical and electronic transformer books.
How to test transformers: Transformer failures are relatively rare in most electronic equipment, compared to other components. If too much current is passed through a coil or winding, the winding becomes hot and may open completely, or the insulation between the turns of the wire may break, causing a partial or complete short circuit of the coil.
Like a coil, you can check the continuity of any primary or secondary transformer winding. The actual value of the DC resistor you read is nonsense, but at least you will know if one winding is open. Checking the linear transformer is pretty straightforward. Apply AC voltage to the primary winding and wait for an output voltage on the secondary side. Low or no output voltage means that the transformer winding has open or shorted winding. Look for any evidence of overheating, such as darkened or blackened areas or a burning smell.
About how to test a power transformer, it is a bit tricky. You cannot apply AC voltage to the equipment and wait for the desired output on the secondary side without connecting to the load; otherwise, a surge will occur (because you have removed all secondary load) and explode the power section consisting of power fet, pwm ic and etc. The switching power supply is designed to operate on load, while the linear transformer can be tested without load.
The best way to test a switched mode power transformer is to first remove it from the board. Check only the primary winding as secondary winding faults are extremely rare. How do we know which is the main winding? Trace the positive pin from the large filter cap to see where the circuit goes. It will go to one of the pins of the transformer. Then find the pin of the supply medium which is the drain and it will lead to another pin of the transformer. These two pins are the primary winding of the transformer. Checking the resistance of the primary winding can only reveal whether it is open, but cannot check whether it has developed a shorted winding. Only by using a buzzer test will you know if you have a short circuit or not. A short turn in the primary winding can cause the power section to explode.
The cheapest doorbell tester I found is the Dick Smith LOPT METER or flyback tester. Simply connect the probes to the primary winding and the result will be instantly visible on the LED bar graph. 4-8 bars represent a good winding, 1-2 bars represent a partial short and if the LED turns off it means there is a strong short in the winding. In addition to checking the transformer, it can be used to check B + COIL, horizontal fork coil and flyback primary winding and ballast (choke).
The expensive ring checker is from sencore equipment. Sencore LC102 and LC103C in addition to checking the capacitor, it also has the ability to check all types of windings. Whether it is low voltage transformer, high voltage transformer, current transformer, pulse transformer, electrical transformer, custom transformer, this equipment will be able to check and test it.
If you don’t want to invest in this tester, you can use a light bulb to check it. Remove one of the pins from the primary winding and plug in a 60 watt light bulb. One end to the circuit and the other to the transformer pin. If the bulb turns on and does not turn off, suspect a short circuit and if the light turns off, the winding is fine.
Transformer repair is not easy and requires good skill to do it. Most transformer repairers preferred to rewind larger windings such as the fan, car alternator, and starter motor. The switched mode power transformer is a bit difficult to rewind due to its small size and low price.