Inductors and transformers

An inductor coil is a coil of wire wound around an insulating tube. The wires are insulated from each other, and the insulating tube can be hollow or contain an iron core or a magnetic powder core. It is referred to as an inductor. It is represented by L, and the units are Henry (H), millihenry (mH), and microhenry (uH). 1H=10^3mH=10^6uH. 1. Classification of inductors Classification by inductance form: fixed inductance, variable inductance. Classification by magnetic conductor properties: air core coil, ferrite coil, iron core coil, copper core coil. Classification by working properties: antenna coil, oscillation coil, choke coil, trap coil, deflection coil. 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), the inductance is generally not specifically marked on the coil, but marked with a specific name. 2. Inductive reactance XL The magnitude of the inductive coil's resistance to AC current is called inductive reactance XL, and its unit is ohm. Its relationship with inductance L and AC frequency f is XL=2πfL 3. Quality factor Q The quality factor Q is a physical quantity that represents the quality of the coil. Q is the ratio of the inductive reactance XL to its equivalent resistance, that is, Q=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 iron 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 shielding cover, and between the coil and the bottom plate is called distributed capacitance. The existence of distributed capacitance reduces the Q value of the coil and deteriorates its stability, so the smaller the distributed capacitance of the coil, the better. III. Commonly used coils 1. Single-layer coils Single-layer coils are insulated wires wound around a paper tube or bakelite skeleton in circles. Such as the medium wave antenna coil of a transistor radio. 2. Honeycomb coil If the plane of the wound 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 widely used in the ultra-short wave range. The position of the rotating copper core in the coil is used to change the inductance. This adjustment is more convenient and durable. 5. Color-coded inductors Color-coded inductors are inductors with fixed inductance. The inductance marking method is the same as that of resistors, marked with color rings. 6. Choke coil: The coil that limits the passage of alternating current is called a choke coil, which is divided into high-frequency choke coil and low-frequency choke coil. 7. Deflection coil: The deflection coil is the load of the output stage of the TV scanning circuit. The deflection coil has the following requirements: high deflection sensitivity, uniform magnetic field, high Q value, small size and low price.

Transformer Transformer is a device that transforms AC voltage, current and impedance. When AC current flows through the primary coil, AC magnetic flux is generated in the iron core (or magnetic core), which induces voltage (or current) in the secondary coil. The transformer consists of an iron core (or magnetic core) and a coil. The coil has two or more windings, of which the winding connected to the power supply is called the primary coil, and the remaining windings are called the secondary coil. 1. Classification Classification by cooling method: dry-type (self-cooling) transformer, oil-immersed (self-cooling) transformer, fluoride (evaporative cooling) transformer. Classification by moisture-proof method: open transformer, potting transformer, sealed transformer. Classification by core or coil structure: core transformer (insert iron core, C-type iron core, ferrite iron core), shell transformer (insert iron core, C-type iron core, ferrite iron core), toroidal transformer, metal foil transformer. Classification by the number of power phases: single-phase transformer, three-phase transformer, multi-phase transformer. Classification by use: power transformer, voltage regulating transformer, audio transformer, medium frequency transformer, high frequency transformer, pulse transformer. 2. Characteristic parameters of power transformer 1. Working frequency The core loss of the transformer is closely related to the frequency, so it should be designed and used according to the frequency of use, which is called the working frequency. 2. Rated power Under the specified frequency and voltage, the transformer can work for a long time without exceeding the output power of the specified temperature rise. 3. Rated voltage Refers to the voltage allowed to be applied to the coil of the transformer, which shall not be greater than the specified value during operation. 4. Voltage ratio Refers to the ratio of the primary voltage to the secondary voltage of the transformer, which is distinguished by the no-load voltage ratio and the load voltage ratio. 5. No-load current When the secondary of the transformer is open, there is still a certain current in the primary, which is called the no-load current. The no-load current consists of the magnetizing current (generating magnetic flux) and the iron loss current (caused by the core loss). For a 50Hz power transformer, the no-load current is basically equal to the magnetizing current. 6. No-load loss: refers to the power loss measured at the primary when the secondary of the transformer is open. The main loss is the core loss, followed by the loss (copper loss) caused by the no-load current on the copper resistance of the primary coil, which is very small. 7 Efficiency Refers to the percentage of the ratio of secondary power P2 to primary power P1. Generally, the larger the rated power of the transformer, the higher the efficiency. 8 Insulation resistance Indicates the insulation performance between the coils of the transformer and between the coils and the iron core. The insulation resistance is related to the performance of the insulating material used, the temperature and the humidity. III. Characteristic parameters of audio transformers and high-frequency transformers 1 Frequency response Refers to the characteristics of the secondary output voltage of the transformer changing with the operating frequency. 2 Passband If the output voltage of the transformer at the intermediate frequency is U0, the frequency range when the output voltage (input voltage remains unchanged) drops to 0.707U0 is called the passband B of the transformer. 3 Primary-secondary impedance ratio The primary and secondary of the transformer are connected to appropriate impedances Ro and Ri to match the primary and secondary impedances of the transformer. The ratio of Ro and Ri is called the primary-secondary impedance ratio. Under the condition of impedance matching, the transformer works in the best state and the transmission efficiency is the highest.