The inductance coil is made of a wire, and the wire is wound around the insulating tube, the wire is insulated from each other, and the insulating tube can be hollow, and can also contain iron core or magnetic powder core. With L said that the unit has Henry (H), Henry (mH), Wahenry (uH), 1H=10^3mH=10^6uH.

Classification of inductance
According to the inductive form of classification: fixed inductance, variable inductance.
According to the nature of the magnetic guide: air core coil, ferrite coil, iron core coil, copper core coil.
According to the classification of the nature of the work: antenna coil, coil, choke coil, notch coil, deflection coil.
According to the classification of single coil winding structure, multilayer coil, honeycomb coil.

Two, the main characteristics of the inductance coil parameters
1, inductance L
Inductance L indicates that the inherent characteristics of the coil is not related to the current size. In addition to the special induction coil (inductance, inductance codes) is generally not specially marked on the coil, and a specific name label.
2, inductance XL
Hinder the inductance coil of AC current size inductance XL ohms. The relationship between it and the inductance L and the frequency of F XL=2 PI fL
3, quality factors Q
The quality factor Q is a physical quantity of the coil quality, the ratio of Q to XL and equivalent inductance resistance is Q=XL/R
The higher the Q value of the coil, the smaller the loss of the circuit. The Q value of the coil is related to the DC resistance of the wire, the dielectric loss of the skeleton, the loss caused by the shielding cover or the iron core, and the effect of the high frequency skin effect. The Q value of the coil is usually tens to hundreds.
4, distributed capacitance
Turn and turn coil coil and shield between the cover and the bottom plate, and the coil between the capacitance is called distributed capacitance. The existence of distributed capacitance reduces the Q value of the coil, and the stability becomes worse, so the distribution capacitance of the coil is as small as possible. Three, commonly used coil
1, single coil
Single coil is a circle around the enclosure to get a paper or bakelite skeleton insulated wire. Such as the transistor radio wave antenna coil.
2, duolateral
If the coil winding, the plane is not parallel with the rotating surface, but at a certain point of view, this is called honeycomb coil coil. And its rotation a week, the wire back and forth bending times, often referred to as the number of points. The advantages of the honeycomb type winding is of small volume, small distributed capacitance, and large inductance. Duolateral coil winding machine is the use of the hive to wound break point more, the distributed capacitance is small
3, ferrite core and iron powder core coil
The size of the inductance of the coil is related to the size of the core. The ferrite core is inserted into the air core coil, which can increase the inductance and improve the quality of the coil.
4, copper core coil
Copper core coil in ultrashort wave range is widely applied to change the inductance using the copper core coil position, this adjustment is convenient and durable.
5, color code inductor
Is the color code inductor inductor with fixed inductance, the inductance of the same sign as to the color ring resistance method to mark.
6, choke (choke)
Limiting the current through the coil of the choke coil, divided into high-frequency choke coil and low-frequency choke coil.
7, deflection coil
Deflection coil is the output stage of the output stage of the TV scanning circuit, the deflection coil requirement: high deflection sensitivity, uniform magnetic field, high Q value, small size, low price.
Transformer
The transformer is the device transform AC voltage, current and impedance, when there is communication through the primary coil current, the core (or core) will be generated in the exchange flux, the secondary coil induces a voltage (or current). The transformer is composed of an iron core (or magnetic core) and a coil, and the coil has two or more than two windings, wherein the primary coil is connected with the power source, and the rest is called the secondary coil.
(a), classification
By cooling category: dry (from cold) transformer, oil immersed transformer (from cold), fluoride (evaporative cooling) transformer.
According to the classification of moistureproof way: open type transformers, potting transformers, sealed transformers.
According to the classification of the core or coil structure: core type transformer (core inserts, C cores, ferrite core, shell type transformer (core), insert the C cores, ferrite core), toroidal transformer, transformer metal foil.
According to the power of the number of categories: single-phase transformers, three-phase transformers, multi-phase transformer.
Classification: power transformers, voltage transformers, audio transformers, intermediate frequency transformers, high frequency transformers, pulse transformers.
(two) the characteristic parameters of power transformer
1 working frequency
Transformer core loss and frequency of a great relationship, it should be based on the frequency of use to design and use, this frequency is called the frequency of work.
2 rated power
Under the specified frequency and voltage, the transformer can work for a long time, but not more than the output power of the specified temperature rise.
3 rated voltage
The voltage applied to the coil of a transformer shall not be greater than the specified value at work.
4 voltage ratio
Refers to the transformer primary voltage and secondary voltage ratio, there is no load voltage ratio and the difference between the load voltage ratio.
5 no load current
Transformer secondary open circuit, the primary is still a certain current, this part of the current is called the no-load current. No load current from the magnetizing current (flux generated) and the iron loss currents (caused by core loss). For a 50Hz power transformer, the no-load current is essentially equal to the magnetizing current.
6 no-load loss: refers to the transformer secondary open circuit, measured in the primary power loss. The main loss is the core loss, followed by the no-load current in the primary coil of copper resistance loss (copper loss), this part of the loss is very small.
7 efficiency
Refers to the secondary power P2 and the primary power P1 ratio of the percentage of. Usually the greater the power rating of the transformer, the higher the efficiency.
8 insulation resistance
To represent the insulation performance between the coil of the transformer and the coil and the iron core. The insulation resistance is related to the performance of the insulating material, the temperature and the humidity.
(three) the characteristic parameters of the audio transformer and the high frequency transformer
1 frequency response
The characteristics of the secondary output voltage of the transformer with the change of working frequency.
2 pass band
If the transformer in the middle frequency of the output voltage of U0, when the output voltage (input voltage is kept constant) down to the frequency range of 0.707U0, called the transformer pass band B.
3 primary and secondary impedance ratio
The transformer primary and secondary access to the appropriate impedance Ro and Ri, so that the transformer primary and secondary impedance matching, the ratio of Ro and Ri is known as the primary and secondary impedance ratio. In the case of impedance matching, the transformer works in the best condition, and the transmission efficiency is the highest.