Power transformer home » Transformer Articles » Examination of Electrical Transformers

Examination of Electrical Transformers

Electrical markets are flooded with two variations of transformers.- linear transformer & switch-mode power transformer. The basic designs of both the varieties are different. They are referred to as AC transformers. Linear transformers are generally used in radio, project kits, VCD, etc,. And switch-mode transformers are employed in monitor, TV, fax and so on.

A transformer is a core on which two or more coils are wound. In case of power transformers, the core material is normally iron. For a radio-frequency transformer, the core material is generally ferrite.

A transformer has a fundamental function: to change AC Voltage. A transformer cannot make changes in direct current voltage. A step down transformer has a lower AC output voltage at its secondary winding than the AC input voltage to its primary winding. Additionally, a step up transformer has a higher secondary than primary voltage. To insulate adjacent turns from shorts between winding, laminations, or coating of shellac, enamel or varnish is used.

How to Test Transformers:

Transformer failures are generally a rare phenomenon. If unlimited current passes through a coil or winding, it heats up, resulting in opening up fully. Otherwise, the insulation between turns of wire can break down, damaging the coil. Similar to a coil, it is possible to check continuity of any primary or secondary transformer winding. The actual value of DC resistance is unacceptable. But you can ascertain whether a winding is open or not. If you want to check a linear transformer, it is completely direct & straight-forward. You have top apply the AC voltage to the primary winding & you can receive output voltage at the secondary side. If you notice no or low output voltage, the transformer winding has open or shorted winding. You have to look for any indication of overheating like blackened spaces, darkened points in addition to burnt smell. It is not advisable to apply AC voltage to the equipment & expect the desired output at the secondary side without connecting to load or else, over voltage will occur & blow the power section. Switch-mode power distribution is designed to run with load whereas linear transformers are tested without load.

The most appropriate method to test a switch-mode power transformer has many steps to be followed. First remove it from the board. Check only primary windings. To find out the primary winding, you have to trace from the big filter cap, the positive pin to ascertain where the circuit goes. It will go to on of the pins of the transformers. You will find the power fet middle pin (drain) to lead to another pin. These two pins constitute the primary wining of the transformer. Checking the resistance of the primary winding will indicate whether it is open or not. By using a ringer test, we can judge whether it is shorted or not. A shorted turn in the primary winding can trigger off a blast.

Dick Smith Lopt Meter or flyback tester is the cheapest ringer tester. By connecting the probes to the primary winding, you can know the result. A good winding is represented by 4-8 bar, partial short is indicated by 1-2 bar. If the lid goes off, it shows there is a heavy short in the winding. In addition to checking the transformer, B+ coil, horizontal yoke coil, flyback primary winding & ballast can also be checked. Sencore LC 102, LC 103C are the most useful instruments in this regard. Light voltage transformer (LVT),High voltage transformer (HVT), Current Transformer, Pulse Transformer, Electric Transformer, Custom Transformer - this sencore equipment will be able to test all these above transformers.

A light bulb can also be used for testing. Remove one of the primary winding pins & connect a 60 watt bright bulb - one end to the circuit & the other end to the transformer pin. If the bulb light does not go off, it is a shorted winding. If the light goes of, it is an indication for okay winding.