Types of Resistors
Like all electronic components, resistors are also available in different sizes, shapes and types. These variations in resistors bring advantages in some types and limitations in other types. This in turn makes some resistors suitable for some applications rather than others.
Resistors Types Based on Composition
Carbon Composition Resistors
Carbon Composition Resistors are commonly used resistors which are manufactured at low cost. This is because of the simpler construction process. They are generally called carbon resistors. The main composition is finely ground carbon along with ceramic clay acting as a binding agent
Carbon resistors can be manufactured in wide range of values ranging from 1Ω to a high value as 22 MΩ. Due to its low cost, they are used in circuits where cost is a criterion rather than the performance.
The advantages of carbon resistors are its ability to remain undamaged from high energy pulses, available at very low cost and at all local vendors and good durability. The disadvantages are high sensitivity to temperature, unstable noise properties and stability issues when hot.
Carbon composition resistors are suitable for high frequency applications as they have low inductance. They are easily affected by humidity and hence the tolerance is only 5%. They also have a low-medium range power rating i.e. < 5W.
Metal Film type resistors have much higher tolerance and better temperature stability when compared to carbon resistors. Hence they are used in applications like active filters where low temperature coefficient and tight tolerance are required.
Metal oxide resistors have much better temperature stability and better surge current capacity.
Thin Film Resistors
Thin film resistors are manufactured by depositing a resistive layer on an insulating base like ceramic. The thickness of the resistive film is equal to or smaller than 0.1 micro meters.
Vacuum deposition is the technique used to deposit the resistive film on the ceramic. The resistive material which is often an alloy of nickel and chromium called Nichrome is sputtered on an insulator base which is ceramic. This process will create a uniform film of 0.1 micrometer thick.
The thickness of the metallic film can be controlled by controlling the time of sputtering. Patterns are created by laser trimming process on the dense and uniform layer to create and calibrate the resistive path and resistance value.
Thin film resistors can be produced as SMD resistors or axial leaded resistors. Because of their high tolerance and low temperature coefficient, thin film resistors are used in precision applications.
Thick Film Resistors
In thick film resistors, the thickness of resistive film is nearly 1000 times thicker than that in thin film resistors. The main difference between thick film and thin film resistors is the procedure for applying the resistive film. The resistive film in thick film resistors is made from a mixture of a binder, carrier and metal oxide.
Glass frit bonding is used to bind the mixture. Carrier is the extract of organic solvent and oxides of iridium or ruthenium are used. This mixture is made as a paste and the resistive film is produced by applying this paste on to a ceramic base using stencil and screen printing process.
Wire Wound Type Resistors
Wire wound resistors are the most precise and high power rated resistors. The construction of wire wound resistors involves a winding of thin metal or metal alloy wire around an insulating substrate.
Generally the metals used are manganin or constantan and a nickel chromium alloy which is also called as nichrome is used in case of metal alloy. The resistive value can be varied by varying wrap pattern, diameter, length and type of alloy.
The resistance tolerance of wire wound resistors is as tight as .005% and the power ratings are in the range of 50W-300W. These are precision wire wound resistors. In case of power resistors, the tolerance is 5% and the power rating is in the range of kilo watts.
They are limited to low frequency applications because of the nature of their construction. Since there is a metal wire wound as a coil around an insulator, they act as inductors. This results in reactance and inductance and when used in A.C circuits there is a chance of phase shift when operated at higher frequencies.
There is a possibility to overcome this limitation by winding each half of wire in different directions. This will cancel each other’s inductive effect. These resistors are called as Non-Inductive Wire Wound Resistors. Normally the cost of wire wound resistors is higher when compared to carbon composition resistors. In high frequency applications Non-Inductive Wire Wound Resistors can be used but their cost is more than normal wire wound resistors.
Generally the metals used are manganin or constantan and a nickel chromium alloy which is also called as nichrome is used in case of metal alloy. The resistive value can be varied by varying wrap pattern, diameter, length and type of alloy.
The resistance tolerance of wire wound resistors is as tight as .005% and the power ratings are in the range of 50W-300W. These are precision wire wound resistors. In case of power resistors, the tolerance is 5% and the power rating is in the range of kilo watts.
They are limited to low frequency applications because of the nature of their construction. Since there is a metal wire wound as a coil around an insulator, they act as inductors. This results in reactance and inductance and when used in A.C circuits there is a chance of phase shift when operated at higher frequencies.
There is a possibility to overcome this limitation by winding each half of wire in different directions. This will cancel each other’s inductive effect. These resistors are called as Non-Inductive Wire Wound Resistors. Normally the cost of wire wound resistors is higher when compared to carbon composition resistors. In high frequency applications Non-Inductive Wire Wound Resistors can be used but their cost is more than normal wire wound resistors.
Wire wound resistors are used in many applications. Some of them are circuit breakers, transducers, temperature sensors and current sensors.
Resistors Types Based on Termination and Mounting
SMD Resistors
Surface Mount Devices (SMD) are produced as a result of a technique called Surface-mount Technology (SMT). In this technique the components are placed directly on the printed circuit board.
SMD Resistors are also developed similarly. The development of Surface-Mount Technology and Surface Mount Devices is a result of requirement of smaller, faster, cheaper and more efficient components by PCB manufacturers.
SMD resistors consist of an insulator substrate which is generally ceramic and a layer of metal oxide film is deposited on this substrate. The value of resistance is determined by the thickness of the film.
Because of their small size they are suitable for circuit boards. They have very little inductance and capacitance and can perform well at radio frequencies.
These SIP and DIP resistors are generally used in resistor ladder networks, pull-up and pull-down networks, bus terminators etc.
Through-hole Resistors
Through-hole is a mounting technique where the components are inserted into holes that are drilled on a PCB. For this purpose, the electronic component consists of small metallic leads. All the resistors with leads coming out of them for contact purpose come under Through-hole resistors.
Through-hole resistors are available in carbon composition resistors, carbon film resistors, metal film resistors, metal oxide resistors, wire wound resistors and many others.
Apart from discrete components, through-hole resistors can be found as pack of resistors with the usage of Dual in-line package and Single in-line package techniques.
Resistors Types Based on Shape
Resistors are classified based on their physical shape. They are Square chip, leadless, open and sealed type in the category of Surface–mount technology. Radial and axial type resistors come under leaded type category.
Square chip type includes SMD resistors
Leadless type resistors are round chip resistors and metal electrode lead less face or MELF resistors.
Open type are general wire wound resistors
Radial lead type resistors are vertical taping compatible. Axial lead type resistors are those in which the leads come out of the body axially.
Resistors Types Based on Power Ratings
Resistors can be classified based on their power ratings. The power rating of a resistor is a benchmark used to indicate the maximum permissible power through a resistor for uninterrupted operation at a specified temperature. Beyond this power, the resistor gets hot and may burn up. If a resistor is rated as 0.25W, then a maximum power of 0.25W can be fed to it.Hence it is important to find the power in a circuit.
Power is rate of doing work. In electrical terminology, power is the rate at which energy is transferred by a circuit. Here the energy is electrical energy.
Power P = VI = V2/R = I2R watts.
When a resistor with known resistance and a fixed supply voltage is present then the power is calculated with this resistance along with supply voltage.
For example a resistor of resistance 400Ω and voltage of 12V is used to power up an LED. The power is calculated as
P = V2/R = (12)2 / 400 = 0.36W
Then a resistor of a power rating of 0.50W should be used.
he standard available power ratings are 0.25W, 0.5W, 1W, 2W, 5W, and 25W.Generally resistorsare available with power ratings up to 500W.
Other Types of Resistors
Fixed Resistors
As the name indicates fixed resistors are those which have a predefined or fixed value of resistance. When the term resistor is used, it generally refers to fixed resistor. Ideally fixed resistors should work independent to changes in temperature, voltage and frequency.
However, this is not possible practically as all resistor materials have temperature coefficient which leads to temperature dependency. The stray capacitance which is present in all resistors will result in impedance and hence the actual resistance will be different from expected.
Fixed resistors are available in different sizes, shapes, leaded, leadless, etc. They can be manufactured based on carbon composition type, carbon film type, metal film type, metal oxide film type, wire wound type, SMD, etc.
Variable Resistors
When there are fixed resistors, there is a scope for resistors with resistance value that is not fixed. Variable resistors are those in which the value of resistance can be varied or adjusted.
The resistance path is provided by the track and the terminals of the device are connected to the track. The wiper is used to increase or decrease resistance through its motion.
Potentiometer
A potentiometer or pot is an electro mechanical resistor with three terminals and is the most commonly used variable resistor.
The two terminals on the either end will deliver a constant resistance which is the formal resistance. The terminal in the center is movable and is called the wiper. This movable wiper maintains contact with the resistive surface.
The resistance between first terminal and the wiper plus the resistance between the wiper and the second terminal is equal to the formal resistance of the device. The name potentiometer is given to this device as it adjusts the voltage using voltage divider principle.
The best application is their use in tuning circuits in radio receivers.Preset
Preset is a variable resistor which is used in occasional adjustment conditions.
Generally presets are mounted on printed circuit board and are adjusted using the rotary control present on top of it with the help of a screw driver. In contrast to potentiometers where the resistance varies linearly, the resistance in preset varies exponentially.
Presets are made available in single turn and multi turn operations. Presets are used in designs where the value of the resistance is set in the circuit during the time of production. Due to their sensitivity, presets are often used in sensing circuits like temperature or light sensing.
Presets are made available in single turn and multi turn operations. Presets are used in designs where the value of the resistance is set in the circuit during the time of production. Due to their sensitivity, presets are often used in sensing circuits like temperature or light sensing.
Rheostat
A rheostat is a two terminal variable resistor. In the rheostat, one end of the resistive track of a variable resistor and its wiper terminal are connected to the circuit. This connection will limit the current in the circuit according to the position of the wiper.
Rheostats are used to control the resistance without interrupting the flow of current. Because of this significant flow of current, rheostats are made as wire wound resistors.
Rheostats are used in applications where current is more important than power rating. They are generally used in tuning circuits and power control applications.
Light Dependent Resistor (LDR)
Light Dependent Resistors or Photo resistors are light sensitive resistors whose resistance varies according to the intensity of the light incident on them.
Light dependent resistors are made of semiconductors with high resistance. In the absence of light or in the dark, the resistance of light dependent resistors is very high usually in the range of Mega Ohms (MΩ).
When light is incident on the surface of light dependent resistors, photons fall on the semiconductor material and the valence electrons of the semiconductor are excited to conduction band.
For the valence electrons to jump to conduction band there should be enough energy in the photons. Therefore the light incident should exceed a certain frequency and the number of free electrons depends on frequency of light. The free electrons will conduct current and hence lowers the resistance.
Based on the semiconductor material used,light dependent resistors are divided into intrinsic and extrinsic. Intrinsic Light dependent resistors use undoped or pure semiconductors like silicon.
There should be enough energy in the photons to excite the entire band gap. Therefore intrinsic light dependent resistors are used for shorter wavelength or higher frequency photons.
On the other hand, extrinsic light dependent resistors use semiconductor materials with impurities in them. These impurities are called dopants and generally boron or phosphorous are used. These impurities create an intermediate energy band which is closer to the conduction band.
Hence the energy required to excite these electrons is less. Lower energy photons i.e. longer wavelength or lesser frequency like Infra-red are suitable for extrinsic light dependent resistors.
Network Resistors
Network resistors are single package resistors with two or more resistors. They generally come in Single in-line package or Dual in-line package.
Resistor networks are used to reduce the board space, improve reliability, reduce solder connections and improve tolerance matching. Generally resistor networks are used in resistor ladders, bus terminators and small computer system interface terminators.
They are available as both surface mount devices and through-hole devices.
Varistor
Varistor is the portmanteau of the variable resistor. It is an electronic component with non-linear current voltage characteristics like the diode. The resistance in the varistor is changed according to the change in voltage across it. This makes it a voltage sensitive device hence it is also called Voltage Dependent Resistor. Generally varistors are made from semiconductor materials.
The resistance of the varistor is very high under normal operating conditions. But the resistance decreases dramatically when the voltage increases beyond the rated value of the varistor.
Metal oxide Varistors are the most common type of varistors. Grains of Zinc oxide are used because it provides P-N diode characteristics. Hence it is used to protect electronic and electrical circuits from over voltage surges.
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