Electronic Design Basics: Resistors

　　Resistance, because the material has an obstructive effect on the current, it is called a resistive material under this effect. Resistance will cause a change in the amount of electron flow. The smaller the resistance, the greater the amount of electron flow, and vice versa. Materials with no resistance or very low resistance are called conductors, or conductors for short. Materials that cannot form current transmission are called electrical insulators, or insulators for short.

　　In physics, resistance is used to indicate the magnitude of a conductor's resistance to electric current. The greater the resistance of a conductor, the greater the resistance of the conductor to electric current. Different conductors generally have different resistances, and resistance is a characteristic of the conductor itself. A resistor is an energy-consuming element that resists electric current.

　　The resistance value of a resistor element is generally related to temperature. The physical quantity that measures the effect of temperature on resistance is the temperature coefficient, which is defined as the percentage change in resistance when the temperature rises by 1°C.

　　Resistors are the most used components in all electronic circuits.

　　The resistance of a conductor to electric current is called resistance, represented by the symbol R, and its units are ohm, kiloohm, and megaohm, represented by Ω, kΩ, and MΩ respectively.

　　1. Resistor model naming method:

　　The model of domestic resistors consists of four parts (not applicable to sensitive resistors)

　　The first part: Main name, expressed in letters, represents the name of the product, such as R for resistor and W for potentiometer.

　　The second part: materials, represented by letters, indicate what materials the resistor is made of, T-carbon film, H-synthetic carbon film, S-organic solid, N-inorganic solid, J-metal film, Y-nitride film, C-deposited film, I-glass glaze film, X-wire wound.

　　Part 3: Classification, generally represented by numbers, and some types are represented by letters, indicating what type the product belongs to. 1-General, 2-General, 3-UHF, 4-High Resistance, 5-High Temperature, 6-Precision, 7-Precision, 8-High Voltage, 9-Special, G-High Power , T-Adjustable.

　　The fourth part: serial number, expressed in numbers, indicates different varieties of the same product to distinguish the product's dimensions and performance indicators, etc. For example: RT 1 1 type ordinary carbon film resistor a1.

　　2. Classification of resistors

　　1. Wirewound resistors: general purpose wirewound resistors, precision wirewound resistors, high power wirewound resistors, high frequency wirewound resistors.

　　2. Thin film resistors: carbon film resistors, synthetic carbon film resistors, metal film resistors, metal oxide film resistors, chemical deposition film resistors, glass glaze film resistors, metal nitride film resistors.

　　3. Solid resistors: inorganic synthetic solid carbon resistors and organic synthetic solid carbon resistors.

　　4. Sensitive resistors: varistors, thermistors, photoresistors, force-sensitive resistors, gas-sensitive resistors, and humidity-sensitive resistors.

　　3. Main characteristic parameters

　　1. Nominal resistance: the resistance marked on the resistor.

　　2. Allowable error: The percentage of the difference between the nominal resistance and the actual resistance to the nominal resistance is called resistance deviation, which indicates the accuracy of the resistor.

　　The corresponding relationship between the allowable error and the accuracy level is as follows: ±0.5%-0.05, ±1%-0.1 (or 00), ±2%-0.2 (or 0), ±5%-level I, ±10%-level II, ±20%-level III

　　3. Rated power: The maximum power that a resistor is allowed to dissipate during long-term operation under normal atmospheric pressure of 90-106.6KPa and ambient temperature of -55℃～+70℃.

　　The rated power series of wirewound resistors are (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 4, 8, 10, 16, 25, 40, 50, 75, 100, 150, 250, 500

　　The rated power series of non-wirewound resistors are (W): 1/20, 1/8, 1/4, 1/2, 1, 2, 5, 10, 25, 50, 100

　　4. Rated voltage: The voltage converted from resistance and rated power.

　　5. Maximum operating voltage: The maximum continuous operating voltage allowed. When working at low pressure, the maximum operating voltage is lower.

　　6. Temperature coefficient: The relative change in resistance caused by a 1°C change in temperature. The smaller the temperature coefficient, the better the stability of the resistor. The resistance increases with increasing temperature, which is a positive temperature coefficient, and vice versa, which is a negative temperature coefficient.

　　7. Aging coefficient: The percentage of relative change in resistance of a resistor under long-term load at rated power. It is a parameter that indicates the life of the resistor.

　　8. Voltage coefficient: The relative change in resistance for every 1 volt change in voltage within the specified voltage range.

　　9. Noise: An irregular voltage fluctuation generated in a resistor, including thermal noise and current noise. Thermal noise is caused by the irregular free movement of electrons inside the conductor, which causes the voltage between any two points of the conductor to change irregularly.

　　4. Resistor resistance marking method

　　1. Direct marking method: Use numbers and unit symbols to mark the resistance value on the surface of the resistor. The allowable error is directly expressed in percentage. If the deviation is not noted on the resistor, it is ±20%.

　　2. Text symbol method: Use a regular combination of Arabic numerals and text symbols to indicate the nominal resistance value, and the allowable deviation is also indicated by text symbols. The number before the symbol indicates the integer resistance value, and the number after it indicates the first decimal resistance value and the second decimal resistance value respectively.

　　Text symbols indicating allowable errors:

　　Text symbolDFGJKM

　　Allowable deviation ±0.5% ±1% ±2% ±5% ±10% ±20%

　　3. Digital method: A marking method that uses three digits to indicate the nominal value on a resistor. From left to right, the first and second digits are effective values, and the third digit is the exponent, i.e. the number of zeros, in ohms. Deviations are usually indicated by text symbols.

　　For example: 101 means 10*10^1Ω, i.e., a 100-ohm resistor; 102 means 10*10^2Ω, 10^3 means 10KΩ, 10^4 means 100KΩ. If a resistor is marked with 22*10^3, then the resistor is 22KΩ.

　　4. Color code method: Use different colored strips or dots to mark the nominal resistance and allowable deviation on the surface of the resistor. Most foreign resistors use the color code method.

　　Black-0, Brown-1, Red-2, Orange-3, Yellow-4, Green-5, Blue-6, Purple-7, Gray-8, White-9, Gold-±5%, Silver-±10%, Colorless-±20%

　　When the resistor has four rings, the last ring must be gold or silver, the first two digits are valid digits, the third digit is the exponent, and the fourth digit is the deviation. When the resistor has five rings, the last ring is farther away from the first four rings. The first three digits are valid digits, the fourth digit is the exponent, and the fifth digit is the deviation.

　　For example, the colors of a 4-color resistor from left to right are red (2), purple (7), yellow (x10000), and silver (plus or minus 10%). The resistance of this resistor is 27Ωx10000=270000Ω, which is 270KΩ, and the tolerance is plus or minus 10%.

　　For example, the colors of a five-color resistor from left to right are red (2), green (5), blue (6), red (x100), and brown (plus or minus 1%). The resistance of this resistor is 256Ωx100=25600Ω, which is 25.0KΩ, and the tolerance is plus or minus 1%.

　　5. Function

　　The main function is to prevent the current from flowing through, and it is used for current limiting, current shunting, voltage reduction, voltage division, load and capacitor matching as filters and impedance matching, etc. The functions of the digital circuit include pull-up resistors and pull-down resistors.

　　6. Commonly used resistors

　　1. Potentiometer

　　A potentiometer is an electromechanical component that relies on the sliding of a brush on a resistor to obtain an output voltage that is related to the displacement of the brush.

　　1.1 Synthetic carbon film potentiometer

　　The resistor body is made of ground carbon black, graphite, quartz and other materials coated on the surface of the substrate. The process is simple and it is the most widely used potentiometer. The characteristics are high resolution, good wear resistance and long life. The disadvantages are current noise, large nonlinearity , poor moisture resistance and resistance stability.

　　1.2 Organic solid potentiometer

　　Organic solid potentiometer is a new type of potentiometer. It uses the method of heating and plastic pressing to press the organic resistance powder into the groove of the insulator. Compared with carbon film potentiometer, organic solid potentiometer has the advantages of good heat resistance, high power, high reliability and good wear resistance. However, it has large temperature coefficient, large dynamic noise, poor moisture resistance, complex manufacturing process and poor resistance accuracy. It is used to adjust voltage and current in miniaturized, highly reliable and highly wear-resistant electronic equipment and AC and DC circuits.

　　1.3 Metallic Glass Uranium Potentiometer

　　The metal glass uranium resistor paste is coated on the ceramic substrate according to a certain pattern by screen printing, and then sintered at high temperature. The characteristics are: wide resistance range, good heat resistance, strong overload capacity, moisture resistance, wear resistance, etc. It is a very promising potentiometer variety, but the disadvantages are large contact resistance and current noise.

　　1.4 Wirewound Potentiometer

　　The wire-wound potentiometer is made by winding constantan wire or nickel-chromium alloy wire as the resistor body on an insulating frame. The characteristics of the wire-wound potentiometer are small contact resistance, high precision, and small temperature coefficient. Its disadvantages are poor resolution, low resistance value, and poor high-frequency characteristics. It is mainly used as a voltage divider, variable resistor, zero adjustment and working point in instruments , etc.

　　1.5 Metal Film Potentiometer

　　The resistor body of the metal film potentiometer can be composed of alloy film, metal oxide film, metal foil, etc. It is characterized by high resolution, high temperature resistance, small temperature coefficient, small dynamic noise and good smoothness.

　　1.6 Conductive plastic potentiometer

　　The DAP (diallyl phthalate) resistor paste is coated on the insulating body by a special process, and then heated and polymerized into a resistor film, or the DAP resistor powder is thermoplastically pressed into the groove of the insulating substrate to form a solid body as the resistor body. The characteristics are: good smoothness, excellent resolution, good wear resistance, long life, low dynamic noise, extremely high reliability, and chemical corrosion resistance. It is used in servo systems of space devices, missiles, and aircraft radar antennas .

　　1.7 Potentiometer with switch

　　There are rotary switch potentiometers, push-pull switch potentiometers, and push-push switch potentiometers.

　　1.8 Pre-adjustable potentiometer

　　Once the pre-adjusted potentiometer is debugged in the circuit, the adjustment position is sealed with wax and it will not be adjusted under normal circumstances.

　　1.9 Slide Potentiometer

　　Use direct sliding method to change the resistance value.

　　1.10 Dual Potentiometer

　　There are heteroaxial double-connected potentiometers and coaxial double-connected potentiometers

　　1.11 Non-contact potentiometer

　　Contactless potentiometers eliminate mechanical contact, have long life and high reliability, and are divided into photoelectric potentiometers, magnetic sensitive potentiometers, etc.

　　2. Solid core carbon resistor

　　A solid resistor is made by mixing carbonaceous particles, strong conductive materials, fillers and binders.

　　Features: Low price, but large resistance error and noise voltage, poor stability, and currently less used.

　　3. Wirewound resistor

　　It is made of high-resistance alloy wire wound on an insulating frame and coated with a heat-resistant glaze insulation layer or insulating paint on the outside.

　　Wirewound resistors have a low temperature coefficient, high resistance accuracy, good stability, heat resistance and corrosion resistance. They are mainly used as precision high-power resistors. Their disadvantages are poor high-frequency performance and large time constant.

　　4. Thin film resistors

　　It is made by evaporating a certain resistivity material onto the surface of the insulating material. The main points are as follows:

　　4.1 Carbon Film Resistors

　　Made by depositing crystalline carbon on a ceramic rod skeleton. Carbon film resistors are the most widely used resistors at present due to their low cost, stable performance, wide resistance range, low temperature coefficient and voltage coefficient.

　　4.2 Metal Film Resistors

　　The alloy material is deposited on the surface of the ceramic rod skeleton by vacuum evaporation.

　　Metal film resistors have higher precision, better stability, lower noise and lower temperature coefficient than carbon film resistors. They are widely used in instruments and communication equipment.

　　4.3 Metal Oxide Film Resistors

　　A layer of metal oxide is deposited on the insulating rod. Since it is an oxide itself, it is stable at high temperatures, resistant to thermal shock, and has a strong load capacity.

　　4.4 Synthetic membrane resistance

　　It is obtained by coating a conductive composite suspension on a substrate, so it is also called a paint film resistor.

　　Because its conductive layer has a granular structure, it has high noise and low precision. It is mainly used to make high-voltage, high-resistance, and small resistors.

　　5. Metal glass uranium resistor

　　The metal powder and glass uranium powder are mixed and printed on the substrate using screen printing.

　　Resistant to moisture, high temperature, small temperature coefficient, mainly used in thick film circuits.

　　6. Chip resistor SMT

　　Chip resistor is a form of metal glass uranium resistor. Its resistor body is made of high-reliability ruthenium series glass uranium material sintered at high temperature, and the electrode is made of silver-palladium alloy paste. It has small size, high precision, good stability, and good high-frequency performance because it is a sheet component.

　　7. Sensitive resistor

　　Sensitive resistors are resistors whose device characteristics are sensitive to temperature, voltage, humidity, light, gas, magnetic field, pressure, etc.

　　The symbol of a sensitive resistor is the symbol of an ordinary resistor with a slash added to it, and the type of sensitive resistor is marked next to it, such as: t. v, etc.

　　7.1 Varistor

　　There are mainly silicon carbide and zinc oxide varistors, and zinc oxide has more excellent properties.

　　7.2 Humidity-sensitive resistor

　　Composed of a humidity sensing layer, electrodes, and insulators, hygroscopic resistors mainly include lithium chloride hygroscopic resistors, carbon hygroscopic resistors, and oxide hygroscopic resistors. Lithium chloride hygroscopic resistors decrease in resistance as humidity increases, but have the disadvantages of a small test range, poor repeatability, and great temperature influence. Carbon hygroscopic resistors have the disadvantages of low sensitivity at low temperatures, great temperature influence on resistance, and aging characteristics, so they are rarely used.

　　Oxide hygroscopic resistors have superior performance, can be used for a long time, are less affected by temperature, and have a linear relationship between resistance and humidity. They are made of tin oxide, nickel ferrite, and other materials.

　　7.3 Photoresistor

　　A photoresistor is an electronic component whose conductivity changes with the amount of light. When a substance is exposed to light, the concentration of carriers increases, thereby increasing the conductivity. This is the photoconductivity effect.

　　7.4 Gas Sensor

　　It is made by using certain semiconductors to absorb certain gases and then undergo redox reactions. The main components are metal oxides. The main varieties are: metal oxide gas resistors, composite oxide gas resistors, ceramic gas resistors, etc.

　　7.5 Force Sensitive Resistor

　　Force sensitive resistor is a resistor whose resistance changes with pressure, which is called piezoresistors abroad. The so-called piezoresistance effect is the effect that the resistivity of semiconductor materials changes with mechanical stress. It can be made into various torque meters, semiconductor microphones, pressure sensors , etc. The main varieties are silicon force sensitive resistors and selenium tellurium alloy force sensitive resistors. Relatively speaking, alloy resistors have higher sensitivity.

        7.6 Thermistor

　　Thermistors are a type of sensitive components, which are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC) according to their temperature coefficients. The typical characteristic of thermistors is that they are sensitive to temperature and show different resistance values ​​at different temperatures. The higher the temperature, the greater the resistance value of the positive temperature coefficient thermistor (PTC), while the higher the temperature, the lower the resistance value of the negative temperature coefficient thermistor (NTC). They are both semiconductor devices.