Explanation of resistors, types, working principles

Resistor is a passive electronic component used in almost all circuits. This article explains What is a Resistor , How Resistor works and How to use Resistors in practical circuits.

Quick Answer

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.

what is a resistor

The resistor is a passive electrical component that creates resistance in the flow of electric current. In almost all electrical networks and electronic circuits they can be found. The resistance is measured in ohms (Ω). An ohm is the resistance that occurs when a current of one ampere (A) passes through a resistor with a one volt (V) drop across its terminals. The current is proportional to the voltage across the terminal ends. This ratio is represented by Ohm’s law:

R=VIR=VI

Resistors are used for many purposes. A few examples include limiting electric current, voltage division, heat generation, matching and loading circuits, gain control, and setting time constants. They are commercially available with resistance values over a range of more than nine orders of magnitude. They can be used as electric brakes to dissipate kinetic energy from trains, or be smaller than a square millimeter for

what does a resistor do

Resistors have many uses in circuits. Below are some of the more common functions; most applications require multiple resistors in serial or parallel configurations. Some of the more common uses of resistors include:

Potential dividers. Two or more resistors in series will give a voltage at their junction point proportional to the ratio of their values. This functionality is widely used in circuits for generating intermediate voltages

Biasing resistors. Transistors and many other devices need to have their AC and DC operating characteristics and gain values set up for correct operation. This is done with multiple resistors and is often called biasing

Op-amp gain and feedback. Most op-amp circuits need to have their gain and feedback functionality set by resistors external to the amplifier chip; resistors are the primary means of doing this

Current limiting. Resistors can be used to limit the amount of current that flows in a circuit element. This is a useful safety function in many circuits e.g. limiting the current that can flow into an LED to manage its brightness

Impedance matching. To maximise power transmission at high frequencies the impedance of the receive and transmit ends of a circuit need to be the same. Resistors can perform at least part of this requirement

Current measuring. Many circuits need to know how much current is flowing, however, it is much easier to measure voltage, so inserting a resistor into the circuit to ‘develop’ a voltage – remember Ohm's law- is a common technique for measuring current

Data and address bus pullups. This functionality helps to reduce noise issues on high-speed computer busses. When a data bus tri-states or is driven high, it is often necessary to pull it into a known state or to make sure its output high is well above the switching point of other logic elements on the bus, and a pull up helps to do this

how to read resistors

Resistor Colour Code

The system is a little simpler for through-hole resistors. A colour code has been in place for many years and is still current. This diagram shows the system in use. Each resistor has three or four bands specifying the value, plus a tolerance band. For a 4-band resistor, the first two bands are numerical values and the third band is a multiplier.

If, for instance, we had a case where the first two bands were red, and the third band was orange, then the resistor value would be 22KΩ. If the fourth band was also red, then the resistor would have a tolerance of 2%. If we take the case of a 5-band resistor with the first three colours as red, and the fourth as brown, we would have a value of 2.22KΩ.

how does a resistor work

People who make electric or electronic circuits to do particular jobs often need to introduce precise amounts of resistance. They can do that by adding tiny components called resistors. A resistor is a little package of resistance: wire it into a circuit and you reduce the current by a precise amount. From the outside, all resistors look more or less the same. As you can see in the top photo on this page, and the one below, a resistor is a short, worm-like component with colored stripes on the side. It has two connections, one on either side, so you can hook it into a circuit.

What's going on inside a resistor? If you break one open, and scratch off the outer coating of insulating paint, you might see an insulating ceramic rod running through the middle with copper wire wrapped around the outside. A resistor like this is described as wire-wound. The number of copper turns controls the resistance very precisely: the more copper turns, and the thinner the copper, the higher the resistance. In smaller-value resistors, designed for lower-power circuits, the copper winding is replaced by a spiral pattern of carbon. Resistors like this are much cheaper to make and are called carbon-film. Generally, wire-wound resistors are more precise and more stable at higher operating temperatures.

Types of Resistors

There are different types of resistors available in the market with diverse ratings and sizes. Some of these are described below.

Wire wound resistors

Metal film resistors

Thick film and Thin film resistors

Network and Surface Mount Resistors

Variable Resistors

Special resistors

Wire-wound Resistors

These resistors vary in physical appearance and size. These wire-wound resistors are commonly a length of wires usually made of an alloy such as nickel-chromium or copper-nickel manganese alloy. These resistors are the oldest type of resistors having excellent properties like high power ratings and low resistive values. During their use, these resistors can become very hot, and for this reason, these are housed in a finned metal case.

Metal Film Resistors

These resistors are made from metal oxide or small rods of ceramic-coated metal. These are similar to carbon-film resistors and their resistivity is controlled by the thickness of the coating layer. The properties like reliability, accuracy, and stability are considerably better for these resistors. These resistors can be obtained in a wide range of resistance values (from a few ohms to millions of ohms).

Thick Film and Thin Film Types of Resistors

Thin film resistors are made by sputtering some resistive material onto an insulating substrate (a method of vacuum deposition) and are therefore more expensive than thick film resistors. The resistive element for these resistors is approximately 1000 angstroms. Thin-film resistors have better temperature coefficients, lower capacitance, low parasitic inductance, and low noise.

Thick film and Thin film Resistors — Thick film and Thin-film Resistors

These resistors are preferred for microwave active and passive power components such as microwave power terminations, microwave power resistors, and microwave power attenuators. These are mostly used for applications that require high accuracy and high stability.

Usually, thick film resistors are made by mixing ceramics with powered glass, and these films have tolerances ranging from 1 to 2%, and a temperature coefficient between + 200 or +250 and -200 or -250. These are widely available as low-cost resistors and compared with the thin film, thick film resistive element is thousands of times thicker.

Surface Mount Resistors

Surface-mount resistors come in a variety of packages sizes and shapes agreed by the EIA (Electronics Industry Alliance). These are made by depositing a film of resistive material and don’t have enough space for color-code bands owing to their small size.

The tolerance may be as low as 0.02% and consists of 3 or 4 letters as an indication. The smallest size of the 0201 packages is a tiny 0.60mm x 0.30mm resistor and this three number code works in a similar way to the color code bands on wire-ended resistors.

Network Resistors

Network resistors are a combination of resistances that give identical value to all pins. These resistors are available in dual inline and single inline packages. Network resistors are commonly used in applications such as ADC (Analog to digital converters) and DAC, pull up or pull down.

Variable Resistors

The most commonly used types of variable resistors are potentiometers and presets. These resistors consist of a fixed value of resistance between two terminals and are mostly used for setting the sensitivity of sensors and voltage division. A wiper (moving part of the potentiometer) changes the resistance that can be rotated with the help of a screwdriver.

These resistors have three tabs, in which the wiper is the middle tab that acts as a voltage divider when all the tabs are used. When the middle tab is used along with the other tab, it becomes a rheostat or variable resistor. When only the side tabs are used, then it behaves as a fixed resistor. Different Types of Variable Resistors are potentiometers, rheostats, and digital resistors.

Special Types of Resistors

These are classified into two types:

Thermistors

Light-Dependent Resistors

Light-dependent Resistors (LDR)

Light-dependent resistors are very useful in different electronic circuits, especially in clocks, alarms, and street lights. When the resistor is in darkness, its resistance is very high (1 Mega Ohm) while in flight, the resistance falls down to a few kilo Ohms.

These resistors come in different shapes and colors. Depending on the ambient light, these resistors are used to turn ‘on’ or turn ‘off’ devices.

Fixed Resistors

The fixed resistor can be defined as the resistance of a resistor that does not vary through the change in temperature/voltage. These resistors are available in different sizes as well as shapes. The main function of an ideal resistor gives a stable resistance in all situations whereas the practical resistor’s resistance will be changed somewhat by an increase in temperature. The fixed resistors resistance values which are used in most of the applications are 10Ω, 100Ω, 10kΩ & 100KΩ.

These resistors are expensive as compared with other resistors because if we want to change the resistance of any resistor, we need to buy a new resistor. In this case, it is different because a fixed resistor can be used with different resistance values. The resistance of the fixed resistor can be measured through the ammeter. This resistor includes two terminals which are mainly used for connecting through other kinds of components within the circuit.

Types of fixed resistors are surface mount, thick film, thin film, wire wound, metal oxide resistor & metal film chip resistor.

Varistors

When the resistance of a resistor can be changed based on the applied voltage is known as a varistor. As the name suggests, its name has been coined through the linguistic blend of words like varying & resistor. These resistors are also recognized through the name VDR (voltage-dependent resistor) with non-ohmic characteristics. Therefore, they come under the nonlinear type of resistors.

Not like rheostats & potentiometers, where the resistance varies from the least value to the highest value. In Varistor, the resistance will change automatically when applied voltage changes. This varistor includes two semiconductor elements to provide over-voltage safety within a circuit like a Zener diode.

Magneto-resistors

When the electrical resistance of a resistor is changed once an external magnetic field is applied is known as a magneto resistor. This resistor includes a variable resistance which depends on the strength of the magnetic field. The main purpose of a magneto resistor is to measure the presence, direction, and strength of a magnetic field. An alternate name of this resistor is MDR (magnetic dependent resistor and it is a subfamily of magnetometers or magnetic field sensors.

Film Type Resistor

Under film type, three types of resistors will come like carbon, metal, and metal oxide. These resistors are normally designed with the deposition of pure metals like nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate. This resistor’s resistance value can be controlled by increasing the width of the deposited film so it is known as a thick-film or thin-film resistor.

Whenever it is deposited, then a laser is employed for cutting a high accuracy spiral helix groove type model into this film. So the film cutting will influence the resistive path or the conductive path similar to taking a long length wire to form it into a loop. This kind of design will allow the resistors which have much closer tolerance like 1% or below as evaluated with the simpler carbon composition type resistors.

Carbon Film Resistor

This kind of resistor comes under the type of fixed resistor that utilizes carbon film to control the flow current to a certain range. The applications of carbon film resistors mainly include in the circuits. The designing of this resistor can be done by arranging the carbon layer or carbon film on a ceramic substrate. Here, carbon film works like the resistive material toward the electric current.

Hence, the carbon film will block some amount of current whereas the ceramic substrate works like the insulating material toward the electricity. So, the ceramic substrate does not permit heat throughout them. Thus, these types of resistors can endure at high temperatures without any harm.

Carbon Composition Resistor

An alternative name for this resistor is carbon resistor and it is very commonly used in different applications. These are easy to design, less costly, and are mainly designed with carbon clay composition covered through a plastic container. The resistor lead can be made with a tinned copper material.
The main benefits of these resistors are less cost & extremely durable.

These are also available in different values which range from 1 Ω to 22 Mega Ω. So these are suitable for Arduino starter kits.
The main drawback of this resistor is extremely sensitive to temperature. The range of tolerance for this resistor ranges from ± 5 to ± 20 %.

This resistor generates some electric noise because of the electrical current flow from one particle of carbon to another particle of carbon. These resistors are applicable where the low-cost circuit is designed. These resistors are available in a different color band which is used to find out the resistance value of the resistor with tolerance.

What are Ohmic Resistors?

The Ohmic resistors can be defined as the conductors which follow ohm’s law is known as ohmic resistors otherwise linear resistances. The characteristic of this resistor when a graph designed for the V (potential difference) & I (current) is a straight line.

We know that ohms law defines that the potential disparity among two points can be directly proportional toward the electric current supplied through physical conditions as well as the conductor’s temperature.

The resistance of these resistors is constant or they obey the ohms law. When the voltage is applied across this resistor, while measuring voltage and current, plot a graph in between voltage and current. The graph would be a straight line. This resistor is used wherever a linear relation among V & I is expected like filters, oscillators, amplifiers, clippers, rectifiers, clampers, etc. Most of the simple electronic circuits use ohmic resistors or linear resistors. These are normal components used to limit the flow of current, select frequency, divide voltage, bypass current, etc.

Carbon Resistor

Carbon resistor is one of the most common types of electronics used. They are made from a solid cylindrical resistive element with embedded wire leads or metal end caps. Carbon resistors come in different physical sizes with power dissipation limits commonly from 1 watt down to 1/8 watt.

Different materials are used for generating resistance mainly alloys & metals such as brass, nichrome, tungsten alloys & platinum. But, the electrical resistivities of most of them have less, not like a carbon resistor, which creates it complex to generate high resistances without turning into huge. So, resistance is directly proportional to length × resistivity.

But, they generate highly precise resistance values & usually used to calibrate as well as compare resistances. The different materials used to make these resistors are ceramic core, lead, nickel cap, carbon film & protective lacquer.

In most of the practical applications, these are mostly preferred because of some benefits like these are very cheap to create, solid & they can be printed directly onto circuit boards. They also regenerate resistance quite well in practical applications. As compared with metal wires, which are costly to generate, carbon is plentifully obtainable making it inexpensive.

Explanation of resistors, types, working principles

what is a resistor

what does a resistor do