What is resistance? The resistance of a conductor to the current is called the resistance of the conductor. Resistance (usually represented by "R") is a physical quantity that represents the magnitude of the resistance of a conductor to the current in physics. The greater the resistance of a conductor, the greater the resistance of the conductor to the current. Different conductors generally have different resistances, and resistance is a property of the conductor itself. The resistance of a conductor is usually represented by the letter R, and the unit of resistance is ohm, abbreviated as ohm, with the symbol Ω.
Resistors
So is the resistor perfect? Of course not. It is because of the existence of resistors that we can control the magnitude of the current. In order to make the current do work according to people's wishes, people created resistors.A resistor is a component that limits current, usually referred to as a resistor. It is one of the most basic and commonly used electronic components and is widely used in various electronic circuits.Due to different manufacturing materials and structures, there are many types of resistors. The common ones include carbon film resistors, metal film resistors, organic solid core resistors, wirewound resistors, rated tap resistors, variable resistors, sliding wire varistors and chip resistors.Carbon film or metal film resistors are commonly used in electronic manufacturing. Carbon film resistors have the characteristics of high stability, good high-frequency characteristics, small negative temperature coefficient, stable pulse load and low cost, and are widely used. Metal film resistors have the characteristics of high stability, small temperature coefficient, good heat resistance, low noise, wide operating frequency range and small size, and are also widely used.
Classification and structure of resistors
Axial Leaded ResistorAxial lead resistors are usually cylindrical, and the two outer electrodes are axial wires at both ends of the cylinder. They can be further divided into many types according to different materials and processes.Wire Wound Resistor
Wirewound resistors are made by winding nickel-chromium alloy wires on an alumina ceramic substrate, with the resistance controlled by turns. Wirewound resistors can be made into precision resistors with a tolerance of 0.005% and a very low temperature coefficient. The disadvantage is that the parasitic inductance of wirewound resistors is relatively large and cannot be used for high frequencies. The volume of wirewound resistors can be made very large, and then with an external heat sink, they can be used as high-power resistors.Carbon Composition Resistor
Carbon composite resistors are mainly made of carbon powder and adhesive sintered together into a cylindrical resistor body. The concentration of carbon powder determines the resistance value. Tinned copper leads are added at both ends and finally packaged. Carbon composite resistors have simple processes and easy to obtain raw materials, so they are the cheapest. However, the performance of carbon composite resistors is not very good, the tolerance is relatively large (that is, it cannot be made into precision resistors), the temperature characteristics are not good, and the noise is usually relatively large. Carbon composite resistors have good voltage resistance performance. Since the inside can be regarded as a carbon rod, it will basically not be punctured and burned.Carbon Film Resistor
Carbon film resistors are mainly made by forming a layer of carbon mixture film on a ceramic rod, such as directly coating a layer. The thickness of the carbon film and the carbon concentration in it can control the size of the resistor. In order to more accurately control the resistance, spiral grooves can be processed on the carbon film. The more spirals, the greater the resistance. Finally, metal leads are added and the resin is encapsulated. The process of carbon film resistors is a little more complicated and can be used to make precision resistors, but due to the carbon quality, the temperature characteristics are still not very good.Carbon film resistors are a type of film resistors. They are made by using high-temperature vacuum coating technology to tightly attach carbon to the surface of a porcelain rod to form a carbon film, then adding appropriate joints and cutting, and coating the surface with epoxy resin for sealing and protection. The surface is often coated with green protective paint. The thickness of the carbon film determines the size of the resistance value, and the resistor is usually controlled by controlling the thickness of the film and carving grooves.Metal Film Resistor
Similar to the structure of carbon film resistors, metal film resistors mainly use vacuum deposition technology to form a layer of nickel-chromium alloy coating on ceramic rods, and then process spiral grooves on the coating to accurately control the resistance. Metal film resistors can be said to be resistors with better performance, high precision, and can be made into E192 series, and then have good temperature characteristics, low noise, and more stability.Metal film resistors are made by vacuum plating nickel-chromium or similar alloys on the surface of white porcelain rods, and then cut and adjust the resistance value to achieve the final precision resistance required. Metal film resistors provide a wide range of resistance values, with precise resistance values and small tolerance range. They can also be used in metal film fuse resistors. Carbon film resistors are currently the most widely used in electronic, electrical, and information products, with the cheapest price and high reliability in quality stability. They are made by separating the carbon of organic compounds from high-temperature vacuum, tightly adhering to the carbon film body on the surface of the porcelain rod, and cutting and adjusting after appropriate joints, and coating the surface with epoxy resin to seal for protection. From the appearance, the metal film resistor has five rings (1%) and the carbon film resistor has four rings (5%). The metal film resistor is blue, and the carbon film resistor is khaki or other colors.Metal Oxide Film Resistor
Similar to the structure of metal film resistors, metal oxide film mainly forms a layer of tin oxide film on the ceramic rod. In order to increase the resistance, a layer of antimony oxide film can be added on the tin oxide film, and then spiral grooves are processed on the oxide film to accurately control the resistance. The biggest advantage of metal oxide film resistors is high temperature resistance.
Metal foil resistors are made by vacuum melting nickel-chromium alloy, rolling it into metal foil, bonding it to an alumina ceramic substrate, and then using photolithography to control the shape of the metal foil, thereby controlling the resistance. Metal foil resistors are currently the best resistors with controllable performance.Thick Film Resistor
Thick film resistors use a screen printing method, which is to stick a layer of palladium silver electrode on the ceramic substrate, and then print a layer of ruthenium dioxide between the electrodes as the resistor body. The resistor film of thick film resistors is usually thicker, about 100 microns. The specific process flow is shown in the figure below.
Thick film resistors are the most widely used resistors at present. They are cheap and have tolerances of 5% and 1%. Most products use 5% and 1% thick film chip resistors.
Thin film resistors are nickel-chromium films formed on an alumina ceramic substrate by vacuum deposition. They are usually only 0.1um thick, only one thousandth of thick film resistors, and then the film is etched into a certain shape through photolithography. The photolithography process is very precise and can form complex shapes. Therefore, the performance of thin film capacitors can be well controlled.
Thin film resistors and thick film resistors are widely used resistors. Their appearance is also very similar. Many users confuse them or directly regard them as the same components. So are they the same? What is the difference?
Thin film resistors and thick film resistors, patterns shown under translucent shells (Source: digikey)
The biggest difference between the two is: first of all, of course, the difference in film thickness. The film thickness of thick film resistors is usually greater than 10μm, while that of thin film resistors is usually less than 10μm, and most of them are less than 1μm; secondly, the difference in manufacturing process. Thick film resistors are usually made by screen printing, while thin film resistors use vacuum evaporation, magnetron sputtering and other process methods to evaporate materials with a certain resistivity on the surface of insulating materials to make resistors. In addition, the accuracy of thick film resistors is generally not as high as that of thin film resistors. The common accuracy of thick film resistors is 10%, 5%, 1%, etc., while the accuracy of thin film resistors can reach 0.1%, 0.01%, etc.; at the same time, in terms of temperature coefficient, thick film resistors are usually larger, while the temperature coefficient of thin film resistors can be very low, such as 5PPM/℃, 10 PPM/℃, etc., so the resistance value of thin film resistors changes less with temperature and is more stable and reliable.Thin film resistors have more advantages and are relatively expensive. They are often used in various instruments, medical equipment, power supplies, power equipment, electronic digital products, etc. When selecting resistors, of course, you cannot blindly choose the most expensive ones, but choose the appropriate ones according to actual needs. When the temperature coefficient and accuracy requirements are high, use thin film resistors, and if it is general requirements, use thick film resistors.
A variable resistor is a resistor whose resistance value can change. There are two types: one is a resistor whose resistance value can be adjusted manually; the other is a resistor whose resistance value can change according to other physical conditions.
Adjustable resistor . When you were in middle school, you should have used a sliding rheostat to do experiments. By moving the sliding rheostat, the small light bulb can become brighter or darker. A sliding rheostat is an adjustable resistor, and the principle is the same.Adjustable resistors are usually divided into three types:
Potentiometer
Potentiometer or voltage divider, which is a three-port device. The potentiometer is divided into two resistors by the middle tap. By changing the resistance of the two resistors through the middle tap, the divided voltage can be changed.
A variable resistor is actually a potentiometer. The only difference is that a variable resistor only needs two ports. It is a pure resistor that can accurately adjust the resistance value.
Trimmer
A trimmer is actually a potentiometer, but it does not need to be adjusted frequently. For example, it can be adjusted when the equipment leaves the factory. It usually requires a special tool such as a screwdriver to adjust it.Sensitive resistors are a type of sensitive components. Most of these resistors are particularly sensitive to certain physical conditions. Once the physical conditions change, the resistance value will change accordingly. They can usually be used as sensors, such as photoresistors, humidity-sensitive resistors, magnetoresistors, etc. Thermistors and varistors are more commonly used in circuit design and are often used as protection devices.Thermistor , PTC thermistor, the full English is Positive Temperature CoeffiCient, translated as positive temperature coefficient thermistor, the product characteristics are that as the temperature rises, the resistance is a positive coefficient that increases. NTC thermistor, the full English is Negative Temperature CoeffiCient, translated as negative temperature coefficient thermistor, the product characteristics are that as the temperature rises, the resistance is a positive coefficient that decreases.Varistors are usually metal oxide variable resistors, or Metal Oxide Varistors (MOVs), whose resistance material is a mixture of zinc oxide particles and ceramic particles that are sintered together. The characteristic of MOV is that when the voltage exceeds a certain threshold, the resistance drops rapidly and large current can pass through, so it can be used for surge protection and overvoltage protection.
Zinc oxide ceramics are made into multilayer varistors, or MLVs, using a process similar to that of MLCCs. MLVs are smaller in size and are usually sheet-shaped. Their rated voltage and current capacity are much smaller than those of MOVs, and they are suitable for low-voltage DC applications.
Classification according to the scope of use and purpose of resistors
Ordinary type: refers to resistors that can meet general technical requirements, with a rated power range of 0.05~2W, a resistance of 1Ω~22MΩ, and an allowable deviation of ±5%, ±10%, ±20%, etc.Precision type: It has higher precision and stability, the power is generally not more than 2 watts, the nominal value is between 0.01Ω~20MΩ, and the accuracy is divided into grades between ±2%~±0.001%.High-frequency type: The inductance of the resistor itself is very small, often called non-inductive resistor. Used in high-frequency circuits, the resistance is less than 1kΩ, and the power range is wide, up to 100W.High voltage type: used in high voltage devices, with a power between 0.5 and 15W, a rated voltage of more than 35kV, and a nominal resistance of up to 1 (1000MΩ). High resistance type: resistance is above 10MΩ, up to 1014Ω. Integrated resistor (resistor array): This is a resistor network with the characteristics of small size, regularity, and high precision. It is particularly suitable for electronic instruments and computer products.Sensitive resistors . Various types of sensitive resistors can be divided into two types: slow-change type and sudden-change type according to their information transmission relationship. They are widely used in technical fields such as detection and automatic control.Varistor . Mainly include zinc oxide, silicon carbide and zinc oxide varistors.Humidity resistor . Humidity resistor consists of humidity sensing layer, electrode and insulator. Lithium chloride hygroscopic resistor has a small test range, poor characteristic repeatability and great temperature influence. Carbon hygroscopic resistor has the disadvantages of low sensitivity at low temperature and great temperature influence on resistance, so it is rarely used. Oxide hygroscopic resistor has superior performance, can be used for a long time, is less affected by temperature, and has a linear relationship between resistance and humidity change.Photoresistors . Photoresistors are mostly made of semiconductor materials. They use the photoconductivity of semiconductors to change the resistance of the resistor with the intensity of the incident light. When the incident light increases, the resistance value decreases significantly; when the incident light decreases, the resistance value increases significantly.Gas resistors . Gas resistors are made by using certain semiconductors to absorb certain gases and undergo redox reactions. The main components are metal oxides. The main varieties include metal oxide gas resistors, composite oxide gas resistors, ceramic gas resistors, etc.Force sensitive resistor . Force sensitive resistor is a resistor whose resistance changes with pressure. 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 force sensitive resistors have higher sensitivity.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.Characteristics of thermistors: ① High sensitivity, its resistance temperature coefficient is 10 to 100 times greater than that of metal; ② Wide operating temperature range, normal temperature devices are suitable for -55℃ to 315℃, high temperature devices are suitable for temperatures above 315℃ (currently up to 2000℃), and low temperature devices are suitable for -273℃ to 55℃; ③ Small size, able to measure the temperature of gaps, cavities and blood vessels in organisms that other thermometers cannot measure; ④ Easy to use, the resistance value can be selected arbitrarily between 0.1 and 100kΩ; ⑤ Easy to process into complex shapes and can be mass-produced; ⑥ Good stability and strong overload capacity.Types of thermistors: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). The resistance of a PTC thermistor increases as the temperature rises, while the resistance of an NTC thermistor decreases as the temperature rises. Currently, the most widely used is the NTC thermistor.
Fuse resistor . Fuse resistors are commonly known as fuse resistors. They are dual-function components that have the functions of fuse and resistor. Under normal circumstances, they have the functions of ordinary resistors. Once a circuit fails, the resistor will melt and open the circuit within a specified time due to overload, thereby protecting other circuits. Fuse resistors are mostly gray, and the resistance value is indicated by color rings or numbers.
Compared with traditional fuses and other protective devices, fuse resistors have the advantages of simple structure, easy use, low fusing power and short fusing time, and are widely used in electronic equipment.Magnetoresistor . Magnetoresistor is made by using the principle that magnetoelectric effect can change the resistance value of a resistor. Its resistance value will change with the change of the magnetic flux density passing through it. Its notable feature is that in a weak magnetic field, the relationship between the resistance value and the magnetic field strength is a square relationship, and it has a high sensitivity.
Resistor selection - based on resistor material
Resistors have six important application areas: 1) high voltage; 2) circuit protection; 3) high power and dissipation; 4) current sensing; 5) pulse/surge conditions; 6) signal conditioning and instrumentation.
Generally speaking, professional resistor manufacturers will meet the requirements of these various fields through various advanced physical configurations and high-purity material construction technologies.
According to the manufacturing and materials of resistors, there are mainly the following types:
1. Composition resistance
The resistor element material is distributed throughout the substrate.
2. Wirewound resistor
Wirewound resistors are made by winding metal wire around an insulating rod and then soldering it to metal end caps.
3. Thin film resistor
Thin film resistors are made by coating a very thin layer of conductive material on a substrate.
4. Bulk metal resistor
Bulk metal resistors have thicker metal wires or strips forming a substrate-free support structure.
5. Thick film resistor
Thick Film Resistors A paste containing glass and metal particles is fired to form the resistor layer.
Resistor selection - according to the resistor type
This article mainly introduces the following different types of circuits: chip resistors, plug-in resistors, wirewound resistors, current sensing resistors, thermistors, and potentiometers. It mainly focuses on characteristics, applications, packaging, and component selection.
1. Chip resistor
Chip resistors have a size advantage over plug-in resistors and are well suited for printed circuit boards (PCBs). Some common applications are pull-up/pull-down, voltage division, current limiting, and filtering signals at certain frequencies in high-pass/low-pass/band-pass filters. It is also possible to use 0Ω resistors as jumpers. There are two types of chip resistors: thin film resistors and thick film resistors.
1) Thin film resistor
Thin film resistors are used in applications such as high-precision audio, medical or test equipment. Compared to thick film resistors, they have lower resistance variation (accuracy 0.1%-2%), lower temperature coefficient (5 ppm/K) and less noise, but are more expensive.
2) Thick film resistor
Thick film resistors are the most common type of resistor and are required in most applications. Thick film resistors have a larger variation (accuracy 1%-5%), a higher temperature coefficient (50 ppm/K), and are noisier than thin film resistors. If there are no specific performance requirements, thick film resistors are usually the first choice.
Package: 0201, 0402, 0603, 0805 and 1206 packages are the most common.
The numbers represent the imperial size, 0402 is 0.04 X 0.02 inches, 0603 is 0.06 X 0.03 inches, and so on.
2. Plug-in resistor
Plug-in resistors are popular and widely used, especially in circuit prototyping, easy to replace and can be used with breadboards. The main functions are pull-up/pull-down, voltage division, current limiting and filtering. There are many types of plug-in resistors.
Among plug-in resistors, carbon film resistors and metal film resistors are the most popular.
1) Carbon film resistor
Carbon film resistors have a wide range of resistance values (accuracy 2%-10%). They are most commonly found in E12 (± 10%), E24 (± 5%), and E48 (±2%) packages. Metal film resistors have replaced carbon film resistors in most applications. The temperature coefficient (TC) of carbon film resistors is typically negative—approximately -500ppm/K—but the exact value depends on the resistor value and size.
2) Metal film resistor
The resistance value of metal film resistors varies little (accuracy 0.1%-2%) and has high stability.
Metal film resistors are most commonly found in E48 (±2%), E96 (±1%), and E192 (±0.5%, ±0.25%, and ±0.1%) packages. Because they offer better performance than carbon film resistors and are less expensive, metal film resistors have a temperature coefficient (TC) of approximately ±100 ppm/K - some parts have a positive TC, while others have a negative TC.
Carbon component resistors have large variations and poor stability, and have been replaced by carbon or metal film resistors. However, they have good high-frequency characteristics and are good at withstanding high-energy pulses, and are used in welding equipment and high-voltage power supplies.
Metal oxides were the first replacement for carbon composition resistors but are being replaced by metal film resistors in most applications. They are still used in high durability applications because they can handle higher temperatures and have higher power ratings (>1W).
Generally speaking, the resistance value is determined by the color ring.
3. Wirewound resistor
Wirewound resistors are made by winding thin wire around a ceramic rod. They are used in high-precision equipment such as multimeters, oscilloscopes, and other measuring equipment. Wirewound resistors that can pass large currents without overheating are used in power supplies and other high-current circuits.
Wirewound resistors are available in very high power ratings (up to 1000W) and can operate at very high temperatures (up to 300 degrees C). They also have good long-term stability - 15-50 ppm/year variation compared to metal film resistors which vary 200-600 ppm/year, and are the best choice for noise performance.
Disadvantages: Only suitable for low ohm range (0.1Ω to 100kΩ). Because the wire winding creates resistance, they have their own inductance, so they have the worst high frequency characteristics of all resistor types. They are also more expensive than other common types of resistors.
Applications: Commonly used in circuit breakers and fuses due to their high power capabilities.
4. Thermistor
A thermistor is a resistor whose resistance value changes significantly with changes in temperature.
If you need to change resistance over a wide temperature range, use an NTC thermistor. NTC thermistors have a resistance that decreases with temperature and are a good choice for temperature sensors between -55C and 200C.
PTC thermistors are used when you need a sudden change in resistance at a specific temperature. PTC thermistors are popular in overcurrent protection applications. The holding current is the current when the PTC thermistor is absolutely "short-circuited", while the tripping current is the current when the PTC thermistor is absolutely "open-circuited".
5. Potentiometer
Potentiometers provide variable resistance and can be used in a variety of applications such as amplifier gain control, circuit tuning, etc. Trimmers (or trimmers) are small potentiometers that can be mounted on a PCB and adjusted using a screwdriver. They can be SMD or plug-in and can have top or side adjustment orientation. They can also be single turn or multi-turn. Single turn potentiometers are used in applications such as amplifiers where only a single turn of control is required. Multi-turn potentiometers are used for more precise control and can have up to 25 turns.
Resistor selection - based on resistor parameters
1. Resistance value
Resistance (R) is the primary specification of a resistor component and represents the single resistance value or resistance range provided by the component.
Using Ohm's law, the voltage (V) and current (I) of the system can be used to find the required resistance value of the resistor element.
R = V / R (resistance selection calculation formula)
The value of a resistor depends on the length, cross-sectional area, and resistivity of the resistive material from which it is made.
2. Temperature coefficient
The change in resistance due to temperature change is usually very small within a specific temperature range. This is because the manufacturer selects materials whose resistivity is not greatly affected by temperature. That is, the material (and thus the resistor) has a low temperature coefficient.
In other words, there is only a small change in value per degree Celsius. This change in value is usually measured in parts per million (ppm), so it is part of its specification. You can check this on the datasheet.
The change in resistance value with temperature is less dependent on the change in the size of the component, as it will expand or contract due to temperature changes. It is mainly the change in the resistivity of the material caused by the movement of the atoms that make up the material.
3. Frequency response
Ideally, a resistor should act as a pure resistor, without any properties of other types of components, and is usually considered a pure resistor when used in DC circuits.
In AC circuits, some resistors may have characteristics that make them unsuitable for certain purposes.
At high frequencies, some resistors also have the properties of capacitance and/or inductance. As a result, they will have a property called reactance, similar to resistance, but dependent on the frequency of the AC signal passing through the component.
The frequency response of a resistor tells us at what frequency the resistor still acts as a pure resistor without any significant influence from these other types of frequency dependent components.
Frequency response: Carbon component resistors > Film resistors > Wirewound resistors (worst)
4. Power consumption
Power dissipation is a measure of the amount of power a resistor can dissipate without causing overheating.
Resistors are manufactured to standard power ratings, most are fractions of 1W, with some of the larger carbon and metal resistors rated from 1W to 5W, and wirewound resistors are often rated up to about 25W.
5. Power derating
The power rating is the maximum power that the resistor can support. This rating is measured in watts (W) and describes how much heat energy the resistor can dissipate without overheating and sustaining damage. A resistor can operate at any voltage and current combination as long as the power rating is not exceeded, and the current rating and voltage rating of the component are not exceeded.
Using Joule's law, the voltage (V) and current (I) of a system can also be used to find the power in the system.
P = IV (resistance selection formula)
The resistance and system current can be used to determine the power by substituting Ohm's law into the equation.
P = I (IR) = I 2 R (Resistor selection power rating formula)
This curve shows how the specified power rating of a resistor must be reduced (derated) at various temperatures above the normal operating range.
6. Maximum temperature
Resistors are designed to operate within a specified temperature range, and parameters such as tolerance and temperature coefficient cannot exceed this specified temperature range.
The most likely limit to the temperature range to be achieved in most uses will be the maximum value due to heat generated by the operating circuitry in addition to any ambient temperature.
7. Maximum voltage
When current flows through a resistor, the voltage developed across the resistor causes electrical stress to the material the resistor is made of. If this voltage exceeds the maximum allowed, the resistor may suddenly break down and experience a voltage flashover. The maximum voltage for different types of resistors varies greatly, from just a few volts for some surface mount types to several thousand volts for some specialized high voltage resistors.
When repairing equipment, it is recommended that replacement parts provided by the original manufacturer be used whenever possible. When repairing any electronic device, it is important to pay close attention to the manufacturer's repair manual for the specific device being worked on.
8. Tolerance (precision)
Tolerance is a measure of the resistor's accuracy. A lower tolerance means the resistor will deviate less from the indicated resistance value. For coded individual resistors, a fourth color usually indicates accuracy. No tolerance band usually indicates an accuracy of ±20%. A 1kΩ resistor with gold accuracy (±5) can actually have any value between 950Ω and 1050Ω.
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