What is Resistor
A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.
Advantages of Resistor
Voltage and current control
Resistors are commonly used to control the flow of current and voltage in a circuit. By adjusting the resistance value, you can regulate the amount of current flowing through a circuit or the voltage across specific components.
Dividing voltage
Resistors are often used in voltage divider circuits to divide the input voltage into smaller voltages. This is useful in various applications where you need to obtain a fraction of the input voltage.
Limiting current
Resistors can be used to limit the current flowing through a circuit to protect components from damage due to excessive current. They act as current-limiting devices in such situations.
Temperature stability
Some types of resistors exhibit good temperature stability, which means their resistance values remain relatively constant over a wide range of temperatures. This property is important in applications where temperature variations can affect circuit performance.
Matching impedances
In some cases, resistors are used to match the impedance between different components in a circuit. This helps in maximizing power transfer and minimizing signal reflections.
Protection
Resistors can also be used for protection purposes, such as in current-limiting circuits or voltage clamping circuits, to protect sensitive components from overvoltage or overcurrent conditions.
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Linear resistors
The resistors whose value changes when temperature and voltage values are changed are known as linear resistors. These are further of two types- fixed and variable resistors.
Fixed resistors- These resistors have a fixed value that cannot be changed. Some examples of fixed resistors are- thin-film resistors, wire-wound resistors, carbon composition resistors, etc.
Variable resistors–These resistors do not possess a fixed value but values that can be changed using a knob, dial etc. Some examples of variable resistors are- rheostats, potentiometers, etc.
Non-Linear resistors
The resistors whose value changes when temperature and voltage values are changed but do not follow ohm's law are known as non-Linear resistors. For e.g. Thermistor, Varistor, Photo resistors etc.
Thermistor-A thermistor is a type of resistor whose resistance is strongly dependent on temperature in standard resistors.
Varistor- A varistor is a resistor whose resistance varies with the applied voltage.
Photoresistors- A photoresistor is a resistor, also a sensor that changes its resistance when light shines on it.
Wirewound (WW) Resistors
The process of creating Wire Wound Resistors involves spiraling resistance wire around a non-conductive core. Usually, they are made for applications requiring a high degree of precision and power. Typically, the resistance wire is composed of an alloy of nickel and chromium, while the core is constructed of ceramic or fiberglass. Applications involving frequencies more than 50 kHz are not appropriate for them.
Metal Film Resistors
The resistive substance is usually composed of a mixture of metal and ceramic. Although they are better at handling higher frequencies, metal film resistors are typically less stable with temperature than wire wound resistors.
Metal Oxide Film Resistors
Compared to metal film resistors, these resistors function at greater temperatures and are dependable and stable. For this reason, applications requiring great durability employ metal oxide film resistors.
Carbon Film Resistors
Carbon film resistors consist of an insulating cylindrical core covered in a thin layer of carbon with a spiral cut in it to improve the resistive path. This raises the resistance value and makes it possible for the resistance value to be more precise. Resistors made of carbon composition are not nearly as accurate as carbon film resistors. Applications requiring strong pulse stability employ special carbon film resistors.
The process of creating Wire Wound Resistors involves spiraling resistance wire around a non-conductive core. Usually, they are made for applications requiring a high degree of precision and power. Typically, the resistance wire is composed of an alloy of nickel and chromium, while the core is constructed of ceramic or fiberglass. Applications involving frequencies more than 50 kHz are not appropriate for them.
Metal Film Resistors
The resistive substance is usually composed of a mixture of metal and ceramic. Although they are better at handling higher frequencies, metal film resistors are typically less stable with temperature than wire wound resistors.
Metal Oxide Film Resistors
Compared to metal film resistors, these resistors function at greater temperatures and are dependable and stable. For this reason, applications requiring great durability employ metal oxide film resistors.
Carbon Film Resistors
Carbon film resistors consist of an insulating cylindrical core covered in a thin layer of carbon with a spiral cut in it to improve the resistive path. This raises the resistance value and makes it possible for the resistance value to be more precise. Resistors made of carbon composition are not nearly as accurate as carbon film resistors. Applications requiring strong pulse stability employ special carbon film resistors.
Working Principle of Resistor
Atomic structure
The behavior of resistors is rooted in the atomic structure of the materials they are made of. Most common resistors are made from materials like carbon, metal films, or metal wire. These materials have electrons that are somewhat loosely bound, allowing them to move relatively freely through the material.
Electric field and electrons
When a voltage (potential difference) is applied across the ends of a resistor, an electric field is established within the material. This electric field exerts a force on the loosely bound electrons, causing them to move through the material in response to the voltage.
Resistance
As the electrons pass through the conductor, they encounter resistance due to collisions with atoms and other electrons within the material. These collisions slow down the electron flow, transforming some of the electrical energy into heat energy. This heat dissipation is why resistors get warm when current passes through them.
Ohm's law
Ohm's law is a fundamental principle in electronics that describes the relationship between voltage, current, and resistance in a conductor, such as a resistor. It was named after the german physicist georg simon ohm, who first formulated this law in the 1820s.
The relationship between voltage, current, and resistance in a resistor is described by ohm's law, which states that the current passing through a resistor (i) is directly proportional to the voltage across the resistor (v) and inversely proportional to its resistance (r).
Use of Resistor
In every life, the gazettes use the resistors to operate easily without damaging itself. Today's life depends upon lots of electrical and electronic applications. These applications use resistors in several ways. To heat the water, you need geysers, to watch a movie, the requirement of TVs/mobiles are a must. To do any kind of work in today's life, electronic gazettes are need of the hour. All these equipment being used are having resistors in some way or other.
In electronic components, sometimes a single resistor does not give the desired result. To get the desirable results, resistors are in use in series or parallel pattern.
To enhance the value of resistance, resistors are in use in the series pattern. When the resistors are arranged in the series pattern, the total resistance of the connected resistors is the summation of individual resistances.
For this arrangement of resistors, the total equivalent resistance RTotal is
RTotal=R1+R2+R3
To reduce the value of resistance, the use of resistors in a parallel pattern is recommended. When the resistors are used in the parallel pattern, the reciprocal total resistance of the connected resistors is the reciprocal summation of individual resistances. For this arrangement of resistors, the total equivalent resistance RTotal is
1RTotal =1R1+1R2+1R3
Resistor Specifications
Temperature Coefficient
This is a measure of the variation of the nominal value as a result of temperature changes. Generally quoted as a single value in parts per million per degree centigrade (or Kelvin), it can be positive or negative. The equation for calculating the resistance at a given temperature is:
Rt=Ro[1+α(T-To)]
Where Ro is nominal value for room temperature resistance, To is the temperature at which the nominal resistance is given, T is operating temperature and α is the TCR.
Put simply, a 1 MΩ resistor with a TCR of 50ppm/K will change by 50Ω per 1 degree of temperature rise or fall. This may not sound like much but consider if you were using this resistor as the gain resistor in a x10 non-inverting amplifier circuit with 0.3v on the + input. The worst-case change in output could be as much as 7.5mv which is equivalent to about 5LSBs in a 5v 12-bit ADC circuit. This kind of change can be quite noticeable in precision design. Remember also that the TCR is quoted as ±x ppm/C so it is feasible, although unlikely, that the second resistor in the circuit could change in the opposite direction hence double the possible error. Finally, it's worth noting that some precision resistors quote variable TCRs over the temperature range the circuit is operating in, and this can complicate the design process significantly.
Resistor Ageing or Stability
Ageing and stability are a complex amalgam of multiple changes to the value of a resistance value over time and are the result of temperature cycling, high-temperature operation, humidity ingress and so on. Typically, the value will lead to an increase in resistance over time as conduction atoms migrate within the device.
Thermal Resistance
The thermal resistance is a measure of how well the resistor can dissipate power into the environment. In practice, engineers use thermal resistance to model the heat dissipation for a system – it is thought of as a set of series ‘thermal resistors', each representing one element of the heat dissipation of the system.
This is mainly important if the design means the resistor is running at or near its maximum value and can significantly affect the long-term reliability of the system. An example of where this parameter could be used is to calculate the size of a PCB pad or ground plane requirement that would be used to keep the resistor's value and operating temperature within acceptable limits.
Thermal and Power Rating
All resistors come with a maximum power rating, specified in watts. This can be anything from 1/8th watt right up to 10s of watts for power resistors. In a first pass analysis, the engineer would check that the resistor is operating within its rated value. The equation for calculating this is P=I² R, where p is the power dissipated in the resistor, i is the current flowing and R is the resistance. Sadly, things can be more complicated than this; for exact work, the engineer needs to take account of the thermal derating curve for the resistor. This specifies the amount by which the designer needs to de-rate the maximum power dissipation above a given temperature.
This might seem theoretical as often the de-rating kicks in at quite high temperatures, but a power circuit in an enclosed housing in a hot region can often exceed the cut in point and the maximum power dissipation will need to be reduced appropriately. It's also worth noting that the maximum operating voltage of a resistor is de-rated with power dissipation.
Resistor Noise
Any electronic component that has flowing electrons is going to be a source of noise, and resistors are no different in this respect. In high gain amplifier systems or when dealing with very low voltage signals, it needs to be considered.
The major contributor to noise in a resistor is thermal noise caused by the random fluctuation of electrons in the resistive material. It is generally modelled as white noise (i.e. a constant RMS voltage over the frequency range) and is given by the equation E=√4RkT∆F where E is the RMS noise voltage, R is the resistance value, k is Boltzmann's constant, T is the temperature and Δf is the bandwidth of the system.
It is possible to lessen system noise by reducing the resistance, the operating temperature or the system's bandwidth. Additionally, there is another type of resistor noise called current noise which is a result of the electron flow in devices.
High-Frequency Behaviour
The final challenge to consider is the high-frequency performance of the particular resistor. In simple terms, you can model a resistor as a series inductor, feeding the resistor which has a parasitic capacitor in parallel with it.
At frequencies as low as 100Mhz (even for surface mount resistors which have lower parasitic values than through-hole parts) the parallel capacitance can start to dominate, and the impedance will drop below nominal. At a higher frequency still, the inductance may predominate, and the impedance will start to increase from its minima and may well end up above the nominal value.
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