Basic knowledge of inductor coils
Inductor coils are made of wires wound around an insulating tube in circles. The wires are insulated from each other, and the insulating tube can be hollow or contain an iron core or a magnetic powder core, referred to as an inductor.
1. Classification of inductors
(1) Classification by inductor form: fixed inductor, variable inductor.
(2) Classification by magnetic conductor properties: air core coil, ferrite coil, iron core coil, copper core coil.
(3) Classification by working properties: antenna coil, oscillation coil, choke coil, trap coil, deflection coil.
(4) Classification by winding structure: single-layer coil, multi-layer coil, honeycomb coil.
2. Main characteristic parameters of inductor coils
(1) Inductance L
Inductance L represents the inherent characteristics of the coil itself and has nothing to do with the current size. Except for special inductor coils (color-coded inductors), inductance is generally not specifically marked on the coil, but marked with a specific name.
(2) Inductive reactance XL
The magnitude of the inductor coil’s resistance to AC current is called inductive reactance XL, and the unit is ohm. Its relationship with inductance L and AC frequency f is XL=2πfL
(3) Quality factor Q
Quality factor Q is a physical quantity that represents the quality of the coil. Q is the ratio of inductive reactance XL to its equivalent resistance, that is: O=XL/R. The higher the Q value of the coil, the smaller the loss of the loop. The Q value of the coil is related to factors such as the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shielding cover or the core, and the influence of the high-frequency skin effect. The Q value of the coil is usually tens to hundreds.
(4) Distributed capacitance
The capacitance between turns of the coil, between the coil and the shield, and between the coil and the base plate is called distributed capacitance. The existence of distributed capacitance reduces the Q value of the coil and deteriorates its stability. Therefore, the smaller the distributed capacitance of the coil, the better.
3. Commonly used coils
(1) Single-layer coils
Single-layer coils are made by winding insulated wires one by one on a paper tube or bakelite frame. Such as the medium wave antenna coil of a transistor radio.
(2) Honeycomb coils
If the plane of the coil is not parallel to the rotating plane, but intersects at a certain angle, this coil is called a honeycomb coil. The number of times the wire bends back and forth during one rotation is often called the number of inflection points. The advantages of honeycomb winding are small size, small distributed capacitance, and large inductance. Honeycomb coils are all wound using honeycomb winding machines. The more inflection points, the smaller the distributed capacitance.
(3) Ferrite core and iron powder core coils
The inductance of the coil is related to whether there is a magnetic core. Inserting a ferrite core into an air-core coil can increase the inductance and improve the quality factor of the coil.
(4) Copper core coil
Copper core coils are commonly applied in the ultra-short wave range. The inductance is adjusted by rotating the copper core within the coil, providing a convenient and long-lasting method of tuning.
(5) Color-coded inductor
Color-coded inductor is an inductor with a fixed inductance. Its inductance is marked with a color ring in the same way as resistors.
(6) Choke
The coil that limits the passage of alternating current is called a choke. It is divided into high-frequency choke and low-frequency choke.
(7) Deflection coil
The deflection coil serves as the load for the output stage in the TV scanning circuit. It is characterized by its high deflection sensitivity, a uniform magnetic field, a high Q factor, compact dimensions, and cost-effectiveness.
