Coil testing is very easy compared to testing three-wire components, such as SCR, FET, etc. In general, a coil consists of many turns or wire wound around a common core. The core could be made of iron or even air. When an electrical current passes through the coil, a magnetic field is produced. A coil in some respect s acts just opposite a capacitor. A capacitor blocks DC while allowing AC to flow through it; a coil allows DC to flow through it while restricting the flow of AC current. Another name for a coil is an inductor.

The coil or inductor can be tested with an analog, inductance, or coil meter such as the Dick Smith Recoil Tester. A small sized coil I would usually just test it with an analog meter and might as well check it on board. Set your analog meter to X1 ohm and place the probes across the small coil. The meter should show some reading (or continuity) and this showed that the coil winding is fine. Small coils seldom go bad because they have less winding compared to large coils where they could have many turns of winding and the chances of shorting are very high.

When testing larger coils or inductors, such as the computer monitor’s B+ coil, you need an inductance meter to find out the exact value of the inductance, which is in the unit of henry (h). Experienced use of an inductance meter to check coils for good or bad is not recommended because a shorted coil (shorted between windings) could have a good inductance value and you would miss the opportunity to check a bad coil. Unless you want to use the inductance meter to calculate the reading and do rewind, loop, etc. on that coil. I would only test a large coil with a Dick Smith return gauge. This meter could easily detect any shorted winding.

Now it’s time to share my real case example: a computer dealer sent me a monitor for repair with a power flickering symptom. I usually don’t fix the monitor right away, but I would first use the flyback tester to scan all the main coils (smps, flyback, b+ coil and horizontal deflection coil) before using my digital or analog meter to test. When measuring the B+ coil, the LED lights went out and it is supposed to show at least 5 LED bars or more. When soldering the coil, I didn’t see any burn marks on the winding or loop and it actually looks shiny. Because I trust the meter, I opened up the winding and to my surprise the inner winding had burned to a crisp, but the outer winding looks really good! A new B+ coil brought the monitor back to life.

By using a flyback tester to test the coil, it has helped me locate many shorted coils in the switched mode power transformer primary winding, B+ coil, flyback transformer primary winding and horizontal deflection coil. The flyback meter can also be used to check the condition of the ballast in a fluorescent lamp!