Coil winding, on the surface, the coil is like winding a metal wire into a spring coil to form a spiral. The center of the coil can be hollow, or a magnetic material with high magnetic flux density can be inserted.

The role is to collect the magnetic force in a relatively close range to the coil, so that the coil can be seen in the transformer, common mode inductor, and differential mode inductor.

When current is passed through the coil, the coil generates a magnetic field. The strength of the magnetic field is proportional to the strength of the current. Whenever the coil is switched from a state of no current to the moment when the circuit is turned on, the current does not enter a stable state immediately. The reason is that when the current flows through the coil and generates a magnetic field, the generated magnetic field will create a resistance that prevents the coil current from continuing to increase, We called it is inductance.
 

Coils, also called inductors, have a similar electrical principle to capacitors :
That is, the inductor can also store energy and is stored in the form of a magnetic field, and when the power supply is interrupted, a high voltage will be generated to release energy, so that the circuit will not return to the zero voltage state in real time; Capacitors, on the other hand, store energy in the form of an electric field and release current when power supply is interrupted.

The direction of the self-inductive electromotive force of the coil will hinder the change of the original magnetic field. This is because the original magnetic field is generated by the current in the coil, and the self-induction hinders the change of the current passing through the coil. This hindering effect is called inductive reactance. The size of the inductive reactance is related to the current inductance of the coil and the frequency passing through the inductive coil. The greater the inductance, the greater the inductive reactance he forms. Under the same inductance, the higher the frequency of the AC current, the greater the inductive reactance.
 

In the interlinked magnetic field passing through the current coil, place another coil, the magnetic field lines in the interlinked magnetic field will pass through this coil, and an induced electromotive force will be generated in this coil, we call this phenomenon mutual inductance. The interaction between the primary coil at the input and the secondary coil at the output is called coupling. The coupling coefficient is related to factors such as the position, method, and presence of magnetic cores of the two coils. The mutual inductance principle of the inductor coil is also the common transformer principle.

The coil is defined on the winding. The point where the winding starts is called the starting point, and the point where the winding ends is called the ending point. On the characteristic diagram, the black dot will be the starting point.

According to the picture, PIN1 is wound to PIN2, P4 is wound to PIN3, there are two windings.

The windings also have parallel winding and reverse winding, dense winding, sparse winding, and series-parallel winding, which are used in different designs.

The inductance has polarity, because the coil also has the beginning and end of the winding, and the directionality of the winding and the reverse winding, but because the inductor is usually a coil, in actual application, the positive and negative connections are not significant. The difference does not affect the normal execution of the circuit, so it is often ignored directly. However, in circuits with high precision requirements, the direction of polarity cannot be ignored. At this time, it is critical to accurately distinguish the polarity of the inductor.