1. Adjustable inductor
Commonly used adjustable inductors include oscillating coils for semiconductor radios, horizontal oscillating coils for TV sets, horizontal linear coils, intermediate frequency trap coils, frequency compensation coils for acoustics, and choke coils.
(1) Oscillating Coil for Semiconductor Radios
In a semiconductor radio, the oscillating coil is connected with a variable capacitor to generate a local oscillation signal higher than 465 kHz for the input radio signal received by the tuning circuit. The outer part is a metal shield, and the inner part is composed of a nylon lining, an H-shaped magnetic core, a magnetic cap, and a pin outlet. There are windings with high-strength enameled wire on the H-shaped core. The magnetic cap is mounted on the nylon frame inside the shield, which can be rotated up and down. By changing the distance between the cap and the coil, we can also change the inductance.
(2) Horizontal Oscillating Coils for TV
Horizontal oscillating coils were used in early black and white TV sets, which can form self-excited oscillation circuits (three-point oscillator, intermittent oscillator, or multivibrator) with peripheral resistor-capacitor units and horizontal oscillation transistors to generate a rectangular pulse voltage signal of 15625HZ.
There is a square hole in the center of the magnetic core, and the line synchronization adjustment knob is directly inserted into it. If rotating the adjustment knob, we can change the relative distance between the core and the coil, thereby changing the inductance of the coil and keeping the line oscillation frequency at 15625 Hz. In this way, this oscillation frequency, with the line synchronization pulse sent by the automatic frequency control circuit (AFC), will generate asynchronous oscillation.
(3) Horizontal Linear Coil
The horizontal linear coil is a nonlinear magnetic saturation inductance coil, whose inductance decreases with the increase of current. It is generally connected in series in the line deflection coil circuit to compensate for linear distortion of the image with its magnetic saturation characteristics.
The horizontal linear coil is wound with enameled wires on an H-shape high-frequency ferrite core or ferrite bar, and an adjustable permanent magnet is installed beside the coil. By adjusting the relative position of the permanent magnet and the coil, we can change the size of the coil inductance, so as to achieve linear compensation.
2. Choke Inductors
Choke inductors refer to the inductive coils used to block the AC path in the circuit,
It is divided into a high-frequency choke coil and a low-frequency choke coil.
(1) High-frequency choke coil
A high-frequency choke coil is used to prevent high-frequency alternating currents. It works in high-frequency circuits and is mostly with hollow or ferrite cores. The skeleton is made of ceramic materials or plastics, and the coils are wound with honeycomb segment winding or multilayer flat segment winding.
(2) Low-frequency Choke Coil
A low-frequency choke coil is used in the current circuit, audio circuit, or field output circuit. Its function is to prevent low-frequency AC from passing.
Generally, the low-frequency choke coil used in the audio circuit is called the audio choke coil, the low-frequency choke coil used in the field output circuit is called the field choke coil, and that used in the current filter circuit is named the smoothing choke coil.
Low-frequency choke coils mostly use "E" shaped silicon-steel sheet iron core, permalloy iron core, or ferrimagnetic core. In order to prevent magnetic saturation caused by a large DC, an appropriate gap should be left in the core during the installation.
