1. Introduction to resistors

2. Physical drawings and symbols of resistors

3. Classification of resistors

4. Naming of resistors

5. Main parameters of resistors

6. Identification of resistors

7. Capacitor selection specifications

8. How to judge the quality of resistors

A potentiometer is an adjustable resistor and one of the most versatile components in electronic circuits. It has three lead-out ends, two of which are fixed ends and the other is a central shaft head. Rotate or adjust the rotary shaft of the potentiometer, and the resistance between the central shaft head and the fixed end will change.

1. Classification of potentiometers Usually potentiometers can be divided into different types according to different conditions. (1) According to the adjustment method: rotation (single turn, multiple turns), straight sliding. (2) According to the number of joints: single joint, double joint. (3) Press whether there is a switch: with switch (rotation, push-pull), without switch. (4) According to the output function characteristics, that is, the relationship between resistance value and operating quantity: linear (X/B) (D/C) - tone control; exponential (Z/A)

2. Structure and working principle of potentiometer The resistor body of the potentiometer has two fixed ends. By adjusting the rotating shaft or slide handle and changing the position of the movable contact on the resistor body, the resistance value between the movable contact and either fixed end is changed, thereby changing the voltage. with the size of the current. Taking the rotary potentiometer as an example, the potentiometer is mainly composed of a resistor, a sliding piece, a rotating shaft, a welding piece and a shell.

3. Main parameters of potentiometer (1) Nominal resistance: Nominal resistance, the same as resistance. (2) Rated power: In a DC or AC circuit, when the atmospheric pressure is 87~107kPa, at the specified rated temperature The maximum power allowed to be consumed by the long-term continuous load on the two fixed ends under the temperature. (3) Degree of compliance: The degree of compliance between the actual output function characteristics of the potentiometer and the required theoretical function characteristics. It is expressed as the maximum deviation between the actual characteristics and the theoretical characteristics as a percentage of the total applied voltage. (4) Resistance change characteristics: linear, logarithmic, exponential. (5) Zero resistance: the resistance at both ends when the movable contact slides to the fixed end. (6) Resolution (resolution): The theoretical accuracy of the potentiometer. Wirewound potentiometers and linear potentiometers: The change in resistance caused by each turn of the moving contact on the winding is related to the total resistance. It is expressed as a percentage of resistance, that is, the reciprocal of the total number of turns N of the winding. Potentiometer with functional characteristics: Since the resistance of each turn of the winding is different, the resolution is a variable. With function properties The section with the largest slope on the curve is used as the average resolution. (7) Sliding noise: Improper distribution of potentiometer resistance, improper coordination of the rotating system, and contact resistance of the potentiometer, etc. Causes noise superimposed on the signal. (8) Wear resistance: The total number of reliable movements of the movable contact of the potentiometer under the specified test conditions, commonly used "weeks" express.

4. Identification of potentiometer Potentiometers generally use the direct marking method, and letters and numbers are used to mark their models and ratings on the potentiometer shell. Power, nominal resistance, relationship between resistance and angle, etc.

5. Selection specifications for potentiometers The selection of potentiometers must also comply with the specifications for resistor selection, such as power, surface temperature, working voltage, strong current circuit usage environment, and high-frequency characteristics of the resistor. At the same time, note: Choose the appropriate resistance ratio according to the application; if high resolution is required, non-wirewound potentiometers and multi-turn potentiometers can be used; if no further adjustment is required after adjustment, use trimmer potentiometers. The main requirements for the potentiometer are that the resistance value meets the requirements, the contact between the center sliding end and the resistor body is good, the rotation is smooth, and for the switch potentiometer, the switch part should act accurately, reliably, and flexibly

6. Identification method and detection of potentiometer The following points need to be noted when detecting potentiometers: (1) Whether the mechanical parts are intact, making noise on and off, whether the rotation is smooth, etc.; (2) Measure whether the resistance between the fixed ends is consistent with the nominal resistance, and rotate the sliding contact at the same time, and it remains unchanged; Its value should be fixed (3) During the measurement process, slowly rotate the shaft. Under normal circumstances, the reading should change smoothly in one direction; Is the insulation resistance between each terminal, shell and rotating shaft large enough?

A capacitor consists of a layer of non-conductive insulating material (dielectric) sandwiched between two conductors close to each other. Electric current passes between capacitors in the form of an electric field. Capacitance is usually represented by the letter C. The basic unit of capacitance is Farad (F). The conversion is as follows: lF=10°mF=10°uF=10'nF=10'pF (3-4) For the determining formula of plate capacitance: C=eS/4πkd, where the electrostatic constant k=8.988×10N·m2/C2, and ε is the dielectric constant. The capacitance C of the capacitor reflects the energy storage capacity of the capacitor Q=CU. It can be seen that the capacitance size is related to the dielectric and the facing area and diameter.

Based on analytical statistics, capacitors can be classified in the following ways. (1) Classified according to structure: fixed capacitors, variable capacitors and trimmer capacitors. (2) Classification by electrolyte: organic dielectric capacitors, inorganic dielectric capacitors, electrolytic capacitors, electrothermal capacitors and air dielectric capacitors, etc. (3) Classified by use: high-frequency bypass, low-frequency bypass, filtering, tuning, low-frequency coupling, small capacitors. High frequency bypass: ceramic capacitors, mica capacitors, glass film capacitors, polyester capacitors, glass glaze capacitors. Low-frequency bypass: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors. Filtering: aluminum electrolytic capacitors, paper capacitors, composite paper capacitors, liquid tantalum capacitors. Tuning: Ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors. Low frequency coupling: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors, solid tantalum capacitors. Small capacitors: metalized paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polystyrene capacitors, solid tantalum capacitors, glass glaze capacitors, metalized polyester capacitors, polypropylene capacitors, mica capacitors. (4) According to different manufacturing materials, they can be divided into: porcelain capacitors, polyester capacitors, electrolytic capacitors, tantalum capacitors, and advanced polypropylene capacitors, etc.

The main parameters of the capacitor are as follows. (1) Nominal capacitance: The capacitance marked on the capacitor is also serialized in a similar manner to the resistor. (2) Allowable deviation: deviation between actual capacitance and nominal capacity. Usually marked with an accuracy grade. Accuracy levels include 01 (1%), 02 (2%), I (5%), II (10%), III (20%), IV (-30%~ 20%), V (50%~-20 %), VI (-10%~100%), general capacitors are commonly used in grades 1, Ⅱ, and Ⅲ, and electrolytic capacitors are in grades I, V, and VI, which are selected according to the use. (3) Rated voltage: The highest effective DC voltage that can be continuously applied to the capacitor at the lowest ambient temperature and rated ambient temperature. It is generally marked directly on the capacitor shell. When the voltage across the capacitor reaches a certain level, the intermediate medium can also conduct electricity. This voltage is called breakdown voltage. Capacitor breakdown will cause irreparable and permanent damage. (4) Insulation resistance Rm: DC voltage is applied to the capacitor and leakage current is generated. The ratio between the two is called insulation resistance. The larger the value, the better. When the capacitance is small, it mainly depends on the surface state of the capacitor; when the capacity is >0.1μF, it mainly depends on the performance of the medium. (5) Time constant of capacitor: The time constant is introduced to properly evaluate the insulation condition of large-capacity capacitors. It is equal to the product RmC of the insulation resistance and capacity of the capacitor. (6) Frequency characteristics: As the frequency increases, the capacitance of general capacitors decreases. (7) Loss: The energy consumed by a capacitor due to heating per unit time under the action of an electric field is called loss. All types of capacitors have specified allowable losses within a certain frequency range. The loss of a capacitor is mainly caused by dielectric loss, conductance loss and the resistance of all metal parts of the capacitor. Under the action of DC electric field, the loss of the capacitor exists in the form of leakage conduction loss, which is generally small. Under the action of an alternating electric field, the loss of capacitance is not only related to leakage conduction, but also to the periodic polarization establishment process.

1. Direct marking method

2. Digital representation

3. Text symbol method

4. Color marking method

coupling

filter

decoupling

energy storage

Tuning

Basic ideas for selecting capacitors: (1) Meet the requirements of electronic equipment for the main parameters of capacitors; (2) Choose a type that meets the circuit requirements; (3) Consider the outer surface and shape of the capacitor; (4) Choose the appropriate model according to different circuits and the frequency of signals in the circuit, reasonably determine the accuracy of the capacitor and the rated working voltage and capacity of the capacitor, try to choose a capacitor with a large insulation resistance, and consider the temperature coefficient and frequency characteristics and use. environment. The following is some common sense for capacitor selection: (1) Large-capacity capacitors are usually suitable for filtering out low-frequency interference noise; (2) Small-capacity capacitors are usually suitable for filtering out high-frequency interference noise; (3) Mica capacitors and high-frequency mica capacitors can be selected for harmonic circuits. Ceramic capacitors; (4) When blocking DC, mica capacitors, polyester capacitors, ceramic capacitors and electrolytic capacitors can be selected; (5) When making a filter, electrolytic capacitors should be selected.

An inductor is a component that can convert electrical energy into magnetic energy and store it. The structure of an inductor is similar to that of a transformer, but it has only one winding. An inductor has a certain inductance, which only blocks changes in current. If the inductor is in a state where no current is flowing through it, it will try to block the flow of current through it when the circuit is turned on; if the inductor is in a state where current is flowing through it, it will try to maintain the current flow when the circuit is off. Inductors are also called chokes, reactors, and dynamic reactors. Inductance is usually represented by the letter L, and the units of inductance are represented by Henry (H), millihenry (mH) and microhenry (μH). The unit conversion relationship is 1H=10² mH=10°μH

Inductors are classified as follows. (1) According to working characteristics: fixed and variable. (2) According to whether there is a magnetic core: hollow, magnetic core. (3) According to the installation form: vertical, horizontal, small fixed type. (4) Divided according to working frequency: high frequency and low frequency. (5) According to the application: antenna coil, oscillation coil, choke coil, filter coil, notch coil, deflection coil.

The magnetic bead is rated according to the impedance it produces at a certain frequency (100MHz), so its unit is 2. The higher the frequency, the greater the resistance, so it is usually used to absorb high frequencies. Ferrite is its main material. Magnetic beads have three parameters: initial magnetic flux, Curie temperature, and operating frequency (core material).

The connection and difference between inductors and magnetic beads: (1) The inductor is an energy storage component, while the magnetic beads are energy conversion (consumption) devices; (2) Inductors are mostly used in power supply filter circuits, and magnetic beads are mostly used in signal circuits for electromagnetic compatibility (electromagnetic) (3) Magnetic beads are mainly used to suppress electromagnetic radiation interference, while inductors are used in this area to focus on suppressing conductive interference. Both can be used to deal with EMC and electromagnetic interference (EMI) problems. There are two ways of EMI, namely radiation and conduction. Different ways use different suppression methods. The former uses magnetic beads and the latter uses inductors; (4) Magnetic beads are used to absorb ultra-high frequency signals, such as radio frequency (RF) circuits, phase locked loop (PLL) oscillation circuits, and ultra-high frequency memory circuits, all need to be in the power input part Add magnetic beads, and the inductor is an energy storage component used in LC resonant circuits, LC oscillation circuits and medium and low-frequency filter circuits, etc. Its application frequency range rarely exceeds 50 MHz; (5) Inductors are used for circuit matching and signal quality control. Generally, inductors are used for ground connections and power connections. Magnetic beads are used where analog ground and digital ground are combined, and magnetic beads are also used for signal lines. The size of the magnetic bead (specifically the characteristic curve of the magnetic bead) depends on the frequency of the interference wave that the magnetic bead needs to absorb. The data sheet of magnetic beads usually comes with frequency and impedance characteristic curves.