TESTING INDUCTORS

An inductor is a device consisting of one or more windings of wire with or without a magnetic core. Frequent causes of inductor coil failures are shorted turns, open turns, and changes in inductor value. Small power supply transformers are similar in construction to inductors and can be tested with the capacitor-inductor analyzer shown in figure 4-30.

Inductors may be tested in the circuit, but the circuit impedance will have some effect on the readings. It is recommended that you remove the inductor or transformer from the circuit before you perform any tests.

Inductor Value

Inductor coils can change in value. This may be caused by overstressing the wire in the winding process. The wire may then relax after a period of time. This changes its position and shape. You may also find coils that have been altered in inductance by a previous technician who spread or compressed the windings in an attempt to correct a faulty circuit.

Inductance value can be checked by performing the following steps:

1. Balance out the inductance of the test leads according to the operator's manual instructions.

2. Connect the test leads to the coil or transformer to be tested.

3. Depress the INDUCTORS VALUE button.

4. Read the value of inductance of the coil or transformer on the digital display. The LED will light in front of the μH if the value is in microhenrys or in front of the mH if the value is in millihenrys.

NOTE: A reading of flashing 888 with a steady 0 indicates an open circuit. Recheck your lead connections to ensure proper terminations.

Inductor Opens

Coils frequently open up. Occasionally, too much stress is put on the coil when it is wound. This may cause the wire to break. At other times, the coil is stressed when it is placed in the circuit or removed for testing. Coils can also open if a screwdriver or some other object accidently falls or makes contact with the windings. Finally, the coil may open if excessive current burns the wire open.

Open windings in coils are easily spotted with the inductor analyzer. Just hook up the instrument to check the inductance value. If the display shows flashing 888 with a stationary 0, then the coil is open. Check the lead connections to the coil to be sure. If the coil is a small wire type, be sure to check the fine wires that go to the solder lugs on the coil form. The fine wire can be broken easily from tension or extreme heat and cold variations.

On large transformers that have several taps or windings in series, simply check them from top to bottom for an open. The actual open can be isolated by moving one lead down the series of taps until the instrument gives an inductance reading. The tap above this point has the open winding.

Inductor Ringing

The most common coil defect is a shorted turn or group of shorted turns. Shorted turns are generally caused by weak insulation that breaks down under voltage. A circuit malfunction that causes excessive voltage across the coil can also lead to shorted turns.

The ringing test allows you to determine if a coil (without an iron core) is good or bad with an accurate but easy-to-perform test of the quality of Q factor. A good coil should show a reading of 10 rings or more on the digital display. A bad coil will show less than 10 ringing cycles.

The ringing test measures the Q factor by applying a reference pulse to the coil and then digitally counting the number of ringing cycles produced until the signal is damped to a preset level. A shorted turn in a coil will lower its Q and cause the ringing to dampen faster than in a good coil. An open coil will show no ringing.

The ringing test should not be used on coils and transformers having laminated iron cores, such as power transformers, audio output transformers, and filter chokes. The iron core in these types of transformers and coils absorbs the ringing energy of the coil and results in low readings that are unreliable.

Good coils below 10 μH in value may not ring 10 cycles. The low inductance of these coils generally allows only about two to four cycles. A comparison test should be made on a known good coil to see if the Q factor results are correct.

Some coils above 10 μH may not show 10 or more rings because of the nature of the construction or core material used in the coil. These may show eight or nine rings and still be good. The quality of these coils may be confirmed by adding a "shorted turn" and rechecking the ringing of the coil. If the coil is bad, the number of rings will not change or will change little, indicating the coil already has a shorted turn. If the number of rings drops off drastically, then the coil is good. A good shorted turn can be made from a piece of solder wrapped around the coil tightly and twisted together at the ends. Small diameter wire or stranded wire does not give the same effect and could give misleading results. Be sure to use solder or a heavy-gauge solid wire for the shorted turn.

To test the quality of a coil with the ringing test:

1. Connect the test leads to the inductor to be tested.

2. Depress the RINGING TEST button. Hold the button down and rotate the IMPEDANCE MATCH switch through all six positions for regular inductors or through the last four positions for TV yokes and flybacks.

3. If a reading of 10 or more appears on the display in one or more positions of the IMPEDANCE MATCH switch, the inductor is good. If a reading of less than 10 is displayed on all positions of the switch, the inductor is defective.

If a continuously changing reading occurs, move the coil being tested to a location away from the source of ac radiation, and check the connections to the coil. If you suspect that the coil may be open or the leads may not be connected properly, merely recheck the inductance value. If the readout shows a flashing 888 with a stationary 0, the coil is open or the leads are not connected properly.

Coils and transformers that are shielded with a metal shield may not give a reading of good on the ringing test. The metal shield may absorb the ringing energy, depending on how close the shield is to the coil. You should consider a shielded coil good if it shows 10 or more rings. If the coil shows less than 10 rings in all positions of the IMPEDANCE MATCH switch, you should either remove the shield and repeat the test or make a comparison test on a known good shielded coil. For accurate results, be sure the coil is identical to the one in the circuit being tested.