One of the passive components - basic knowledge of resistors

Resistance, capacitance and inductance are common and important passive components. This series of articles will be divided into three parts to introduce the detailed basic knowledge of these three components. The resistance in this paper will be introduced from the type designation, classification, main characteristic parameters, marking method of resistance value, and common resistors.



The resistance of the conductor to the current is called resistance, which is expressed by the symbol R, and the unit is ohm, kiloohm and megaohm, which are expressed by Ω, K Ω and M Ω respectively.




1、 Type designation of resistance:



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



Part I: The main name, expressed in letters, represents the name of the product. For example, R represents resistance and W represents potentiometer.



The second part: materials, expressed in 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-nitriding film, C-deposition film, I-glass glaze film, X-wire winding.



The third part: classification, which is generally represented by numbers, and individual types are represented by letters, indicating the type of products. 1-common, 2-common, 3-UHF, 4-high resistance, 5-high temperature, 6-precision, 7-precision, 8-high voltage, 9-special, G-high power, T-adjustable.



Part IV: Serial number, expressed in numbers, indicates different varieties of the same product to distinguish the appearance size and performance index of the product, for example: R T 11 common carbon film resistor



2、 Classification of resistors



1. Wirewound resistor: general wirewound resistor, precision wirewound resistor, high-power wirewound resistor, high-frequency wirewound resistor.



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



3. Solid resistor: inorganic synthetic solid carbon resistor, organic synthetic solid carbon resistor.



4. Sensitive resistor: varistor, thermistor, photosensitive resistor, force sensitive resistor, gas sensitive resistor, humidity sensitive resistor.



3、 Main characteristic parameters



1. Nominal resistance value: the resistance value indicated on the resistor.



2. Allowable error: the percentage of the difference between the nominal resistance value and the actual resistance value and the ratio of the nominal resistance value is called the resistance deviation, which represents the accuracy of the resistor. The corresponding relationship between allowable error and accuracy grade is as follows: ± 0.5% - 0.05, ± 1% - 0.1 (or 00), ± 2% - 0.2 (or 0), ± 5% - Grade I, ± 10% - Grade II, ± 20% - Grade III



3. Rated power: the maximum power dissipated by the resistor during long-term operation under the normal atmospheric pressure of 90-106.6KPa and ambient temperature of - 55 ℃~+70 ℃.



Rated power series of wire wound resistor 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: voltage converted from resistance and rated power.



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



6. Temperature coefficient: the relative change of resistance value caused by temperature change of 1 ℃. The smaller the temperature coefficient, the better the stability of the resistance. The resistance value that increases with temperature is positive temperature coefficient, and vice versa.



7. Aging factor: the percentage of relative change of resistance value of resistor under rated power and long-term load. It is a parameter indicating the life of resistor.



8. Voltage coefficient: within the specified voltage range, the relative change of the resistor for each voltage change of 1 volt.



9. Noise: An irregular voltage fluctuation in the 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 at any two points of the conductor to change irregularly.



4、 Marking method of resistance value of resistor



1. Direct marking method: mark the resistance value on the surface of the resistor with numbers and unit symbols, and the allowable error is directly expressed as a percentage. If no deviation is noted on the resistance, it is ± 20%.



2. Text symbol method: the nominal resistance value is expressed by the regular combination of Arabic numerals and text symbols, and the allowable deviation is also expressed by text symbols. The number in front of the symbol represents the integer resistance value, and the number after the symbol represents the first decimal resistance value and the second decimal resistance value in turn.



Text symbol indicating allowable error



Text symbol D F G J K M



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



3. Digital method: a method of marking the nominal value with three digits on the resistor. The number is from left to right. The first and second digits are valid values, and the third digit is the index, that is, the number of zeros. The unit is Europe. Deviation is usually indicated by text symbols.



4. Color coding method: mark the nominal resistance value and allowable deviation on the surface of the resistor with strips or dots of different colors. Most of foreign resistors use 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 resistance is four rings, the last ring must be gold or silver, the first two digits are significant figures, the third digit is the multiplier, and the fourth digit is the deviation.



When the resistance is five rings, the distance between the last ring and the front four rings is large. The first three digits are significant digits, the fourth digit is the multiplier, and the fifth digit is the deviation.




5、 Common resistor



1. Potentiometer



The potentiometer is a kind of electromechanical component. It obtains the output voltage which is in a certain relationship with the brush displacement by sliding the brush on the resistance body.



1.1 Synthetic carbon film potentiometer



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



1.2 Organic solid potentiometer



The organic solid potentiometer is a new type of potentiometer. It uses the method of heating and 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, the temperature coefficient is large, the dynamic noise is large, the moisture resistance is poor, the manufacturing process is complex, and the resistance accuracy is poor. It is used to regulate voltage and current in miniaturized, highly reliable and wear-resistant electronic equipment and AC and DC circuits.



1.3 Metal glass uranium potentiometer



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



1.4 Winding potentiometer



The winding potentiometer is made of copper wire or nickel-chromium alloy wire as the resistance body and wound on the insulating framework. The winding potentiometer is characterized by low contact resistance, high accuracy and low temperature coefficient. Its disadvantages are poor resolution, low resistance and poor high-frequency characteristics. It is mainly used as voltage divider, rheostat, instrument middle note zero and working point, etc.



1.5 Metal film potentiometer



The resistance 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, low temperature coefficient, low dynamic noise and good smoothness.



1.6 Conductive plastic potentiometer



Use a special process to cover the DAP (diallyl phthalate) resistance slurry on the insulating body, heat and polymerize it to form a resistance film, or press the DAP resistance powder into the groove of the insulating substrate to form a solid body as the resistance body. Features: good smoothness, excellent resolution, good wear resistance, long service life, low dynamic noise, high reliability and chemical corrosion resistance. It is used for servo systems of space devices, missiles, aircraft radar antennas, etc.



1.7 Potentiometer with switch



There are rotary switch potentiometer, push-pull switch potentiometer and push-pull switch potentiometer



1.8 Preset potentiometer



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



1.9 Direct slide potentiometer



Change the resistance value by direct sliding.



1.10 Double-connected potentiometer



There are two types of potentiometers, namely, the heteroaxial double-connected potentiometers and the coaxial double-connected potentiometers



1.11 Contactless potentiometer



The contactless potentiometer eliminates mechanical contact, has long service life and high reliability, and is divided into photoelectric potentiometer and magnetic sensitive potentiometer.



2. Solid carbon resistor



A solid resistor is made by mixing carbonaceous particles with strong conductive substances, fillers and adhesives. Features: low price, but its resistance error, noise and voltage are large, and its stability is poor, so it is rarely used at present.



3. Wound resistor



It is made by winding high resistance alloy wire on the insulating framework, and coated with heat-resistant glaze insulating layer or insulating paint. Winding resistance has low temperature coefficient, high resistance accuracy, good stability, heat resistance and corrosion resistance. It is mainly used for precision high-power resistance. Its disadvantages are poor high-frequency performance and large time constant.



4. Film resistor



The material with certain resistivity is evaporated on the surface of insulating material by evaporation. The main contents are as follows:



4.1 Carbon film resistor



Crystalline carbon is deposited on the ceramic rod skeleton. Carbon film resistor is the most widely used resistor with low cost, stable performance, wide resistance range, low temperature coefficient and voltage coefficient.



4.2 Metal film resistor.



The alloy material is evaporated on the surface of the ceramic rod skeleton by vacuum evaporation. Metal film resistance has higher accuracy, better stability, lower noise and temperature coefficient than carbon film resistance. It is widely used in instruments and communication equipment.



4.3 Metal oxide film resistor



A layer of metal oxide is deposited on the insulating rod. Because it is an oxide, it is stable at high temperature, resistant to heat shock and has strong load capacity.



4.4 Synthetic film resistance



It is obtained by coating the conductive composite suspension on the substrate, so it is also called paint film resistance. Due to the granular structure of its conductive layer, it has high noise and low accuracy. It is mainly used to manufacture high-voltage, high-resistance and small resistors.



5. Metallic glass uranium resistor



The metal powder and glass uranium powder are mixed and printed on the substrate by screen printing. It is resistant to humidity, high temperature and low temperature coefficient, and is mainly used in thick film circuits.



6. SMT



The sheet resistance is a form of metallic glass uranium resistance. Its resistance body is made of highly reliable ruthenium series glass uranium material sintered at high temperature. The electrode is made of silver palladium alloy paste. Small size, high precision and good stability. Because it is a chip element, it has good high-frequency performance.



7. Sensitive resistor



Sensitive resistor refers to the resistor whose characteristics are sensitive to temperature, voltage, humidity, light, gas, magnetic field, pressure, etc. The symbol of sensitive resistance is to add a slash to the symbol of ordinary resistance and mark the type of sensitive resistance beside it, such as t V, etc.



7.1 Varistor



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



7.2 Humidity sensitive resistor



It is composed of humidity sensing layer, electrode and insulator. The humidity sensitive resistor mainly includes lithium chloride humidity sensitive resistor, carbon humidity sensitive resistor and oxide humidity sensitive resistor. The resistance of lithium chloride humidity sensor decreases with the increase of humidity. The disadvantages are small test range, poor repeatability of characteristics and great influence of temperature. The disadvantage of carbon humidity sensitive resistance is low sensitivity at low temperature, the resistance value is greatly affected by temperature, and is less used due to aging characteristics. The oxide humidity sensitive resistor has superior performance, can be used for a long time, and has little influence on temperature. The resistance value is linear with the change of humidity. There are tin oxide, nickel ferrite, and other materials.



7.3 Photosensitive resistor



Photosensitive resistor is an electronic element whose conductivity changes with the change of light force. When a material is illuminated, the carrier concentration increases, which increases the conductivity, which is called photoconductivity effect.



7.4 Gas sensitive resistor



It is made by using some semiconductors to absorb certain gases and undergo oxidation-reduction reaction. The main components are metal oxides. The main varieties are: metal oxide gas sensors, composite oxide gas sensors, ceramic gas sensors, etc.



7.5 Force sensitive resistor



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



7.6 Thermistor



Thermistor is a kind of sensitive element, and its resistance value will show a step change with the change of the temperature of the thermistor body, which has the characteristics of semiconductor.



Thermistors are divided into positive temperature coefficient thermistor (PTC thermistor for short) and negative temperature coefficient thermistor (NTC thermistor for short) according to temperature coefficient.



PTC Thermistor



PTC is the abbreviation of PosiTIve Temperature Coefficient, which means positive temperature coefficient, and generally refers to semiconductor materials or components with high positive temperature coefficient. Usually, PTC refers to positive temperature coefficient thermistor, or PTC thermistor for short.



PTC thermistor is a typical temperature-sensitive semiconductor resistance. When the temperature exceeds a certain temperature (Curie temperature), its resistance increases step by step with the increase of temperature.



PTC thermistors are divided into ceramic PTC thermistors and organic polymer PTC thermistors according to their different materials.



Currently, PTC thermistors are widely used:



PTC thermistor for constant temperature heating



PTC thermistor for overcurrent protection



PTC thermistor for air heating



PTC thermistor for delayed start



PTC thermistor for sensor



PTC thermistor for automatic degaussing



In general, organic polymer PTC thermistors are suitable for overcurrent protection purposes, and ceramic PTC thermistors can be used for various purposes listed above.



Negative temperature thermistor (NTC Thermostor)



NTC is the abbreviation of Negative Temperature Coefficient, which means negative temperature coefficient, and generally refers to semiconductor materials or components with large negative temperature coefficient. In general, we refer to NTC as negative temperature coefficient thermistor, or NTC thermistor for short.



NTC thermistor is a typical temperature-sensitive semiconductor resistor. Its resistance value decreases step by step with the increase of temperature.



NTC thermistor is made of manganese, cobalt, nickel, copper and other metal oxides by ceramic process. These metal oxide materials have semiconductor properties, because they are completely similar to semiconductor materials such as germanium and silicon in terms of conducting mode. At low temperature, the number of carriers (electrons and holes) of these oxide materials is small, so their resistance values are high; As the temperature increases, the number of carriers increases, so the resistance value decreases.



NTC thermistors are divided into:



Power NTC thermistor



Compensated NTC thermistor