Knowledge of inductors and component testing
Electrical measurement and Ohm's law

Knowledge of inductors and component testing

(1) Inductor
The inductor (see Figure 1 and Figure 2) is an electronic component composed of a wire coil. When current passes through the coil, a magnetic field is generated, and the inductor uses the magnetic field to store energy. When the current in the inductor disappears, the magnetic field returns to zero. The ability of an inductor to build a magnetic field is called inductance, and its unit of measurement is Henry (H). The symbol for inductance is L.

Knowledge of inductors and component testing
Figure 1 The more turns of the coil, the greater the inductance
Knowledge of inductors and component testing
Figure 2 Circuit symbol of the inductor

①The structure of the inductor
Inductors are usually made by winding a conductor coil on a core material. The core body has two functions: ①It is used to stabilize the wire and provide support for the wire winding; ②Some core body materials can enhance the inductance.

One type of core body is the air core (see Figure 3), which is made of cylindrical cardboard, wood or ceramics, and can be hollow. This type of core is only used to provide coil winding support without affecting the inductance value. Another type of core is made of ferromagnetic materials (such as iron, nickel or ferrite) (see Figure 4). This core material can concentrate the magnetic field and increase the inductance of the inductor.

Knowledge of inductors and component testing
Figure 3 Air Core
Knowledge of inductors and component testing
Figure 4 Ferromagnetic material core

②Factors affecting inductance
Inductance is a function of the number of turns of the wire, the length of the coil, the cross-sectional area of ​​the coil, and the permeability of the core material. The inductance of a cylindrical inductor can be calculated by the following formula
In the formula, L is the inductance (Henry); μ is the magnetic permeability of the core (H/m); A is the cross-sectional area (㎡) is the length of the core (m), and N is the number of turns of the wire. Magnetic permeability is a measure of how easy it is to generate a magnetic field in a certain material. The greater the permeability of the material, the easier it is to build a magnetic field. Please note that the inductance is proportional to the square of the number of turns of the wire, so the number of turns has a significant effect on the inductance of the inductor.

example 1
Figure 5 shows an inductor with a solid nickel core. The area (A) is 0.25㎡, the length (l) is 0.05m, the number of turns (N) is 5, and the core permeability (μ) is 0.000125 (H/m). Calculate the inductance.

Knowledge of inductors and component testing
Figure 5 Example 1

(2) Testing of components
The manufacturer’s tests on resistors, capacitors, and inductors include static tests and dynamic tests where AC and DC voltage and current are applied.

However, it is difficult to carry out such a “high-tech” test at the use site. But you can use an ohmmeter to perform some “low-tech” tests to determine whether the components have short-circuit and open-circuit faults. First, isolate the component under test from other parts of the circuit and disconnect the power supply.

When testing resistors or inductors, first calibrate the ohmmeter and then connect it to both ends of the resistor or coil. You may need to switch gears (for example, switch between R×1 or R×10 gears) ). When testing the resistance value, the resistor and coil should not be short-circuited or open-circuited, but the precise resistance value (if necessary) should be determined by referring to the specifications. Certain types of inductors are not made of wire coils, so they cannot be tested with an ohmmeter.

When testing capacitors, the power supply must be disconnected first. Generally, an ohmmeter can be used for short-circuit testing of small electrolytic capacitors and large oil-immersed capacitors. As shown in Figure 6, connect the ohmmeter pen across the two leads of the capacitor. The pointer of the meter head will swing quickly and then begin to stabilize. Reverse the ohmmeter pen and perform repeated measurements. The capacitor should not be short-circuited.

Knowledge of inductors and component testing
Figure 6 Test capacitor with ohmmeter

The test of the ohmmeter does not show that the capacitor can work normally under high voltage. In fact, it is difficult to perform high-voltage testing at the use site. If the capacitor is suspected of high voltage breakdown, it should be replaced.