Education

# What is a Resistor and How it Works?

A resistor is a two-terminal passive electrical component that restricts the passage of current in electrical or electronic circuits. Resistors come in a variety of sizes and shapes. Its size is related to the amount of power it has. The maximum amount of power that resistors can dissipate without being destroyed by excessive heat build-up is the power rating. The greater surface area resistors cover, the more power they can dissipate.

Resistors are found in almost all electronic and electrical circuits. Resistors resist the flow of electricity, as their name implies, and this function is critical to the operation of most circuits. Some resistors are used for specific purposes, such as variable resistors, while others are utilized for giving varying resistance with temperature. All of these qualities can be utilized. The operation of the circuit in which the resistor is employed may be influenced by power dissipation, noise, inductance, thermal stability, and a variety of other variables.

Resistors are utilized in a variety of applications. Delimit electric current, voltage division, heat generation, matching and loading circuits, control gain, and establish time constants are only a few examples. They have resistance levels spanning more than nine orders of magnitude and are commercially available. They can be used to disperse kinetic energy from trains as electric brakes, or they can be smaller than a square millimeter for electronics.

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## What is a resistor?

The primary function of a resistor is to reduce current flow and lower the voltage in a specific area of the circuit. It’s comprised of copper wires that are wrapped around a ceramic rod and have an insulating paint coating outside its surface. They are found in all electrical networks and electronic circuits. Resistors ability to resist current flow is known as resistance and resistance of resistor is measured in ohms. When current of one ampere travels through it with one volt drop across its terminals, the resistance is measured as one ohm. Resistance depends on applied voltage and temperature.

According to Ohm’s law, the voltage across the terminal ends is directly proportional to the value of current.

V ∝ I

V = I R

R = V / I

Here R is resistance, I is current and V is voltage. Symbol of resistor is shown below:

## How resistor works?

When a resistor is connected to a circuit, it reduces the current by a specific amount. When the electrons (current) enters the resistor, they collide with the atoms of the resistor. As the result, energy is wasted as heat and power dissipation occurs. More collisions will cause more restriction for electrons thus blocking their path. In this way current is minimized, according to required value. If certain resistor has more value, it can reduce current more.

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From the outside, resistors almost always appear to be the same. If you crack it open, you’ll find an insulating ceramic rod running through the middle and copper wire wrapping around the exterior. The resistance is determined by the copper turns. The higher the resistance, the more difficult it is for electrons to travel through thin copper. Some conductor materials allow electrons to travel more easily than insulators.

George Ohm investigated the relationship between resistance and the size of the resistor’s material. He demonstrated that a material’s resistance (R) increases as its length grows. Longer and thinner wires provide additional resistance in this case. Resistance reduces as wire thickness increases. Thus he formulated an equation that is given as

R ∝ L / A

R = ρ L / A

Here R is resistance, ρ is resistivity of material, L is the length of wire and A is its cross sectional area. Sometimes the terms resistor and resistance are mixed. Both are separate actually. Resistor is a device that resists the flow of electrons while resistance is the property of a material to stop the current flow. Resistance of each material is different.

## Types of resistor

Linear and non-linear resistors are the two types of resistors.

Linear resistors: The resistors whose value doesn’t change with temperature and applied voltage. They are sub-divided into two types: fixed and variable.

Fixed resistors:   The most common type of resistor is the fixed resistor. They are used in electronic circuits to set the proper operating conditions. Their values are set during the circuit’s design phase, and they should never need to be updated to make adjustments. The value of fixed resistors cannot be changed. Following are the different types of fixed resistors:

• Carbon composition resistors
• Wire wound resistors
• Thin film resistors
• Thick film resistors

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## What are variable resistors?

A fixed resistor element plus a slider that taps into the main resistor element make up these resistors. The component has three connections: two connections to the fixed element and the third to the slider. If all three connections are used, the component serves as a variable potential divider. It is possible to connect the slider and one end to create a variable resistance. Their vale can be changed with the help of a knob, screw or dial. Types of variable resistors are given as:

• Potentiometers
• Rheostats
• Trimmers

Variable resistors and potentiometers are commonly used for a variety of control applications, including volume controls on radios and sliders in audio mixers, as well as a variety of other applications. A potentiometer is a component with a fixed resistor and a slider to produce a potential division from the voltage at the top. A variable resistor is essentially the same as a fixed resistor, except with the slider connected to one end of the resistor, allowing actual variable resistance.

### Non-linear resistors

The resistors whose value change with respect to applied voltage and temperature. Types of non-linear resistors are given as:

• Thermisters
• Varisters
• Photo resistors

## Color coding of resistor

Although resistors do not show their values outside, the color pattern created by their resistance can be computed. Color-coding is used on PTH (plated-through-hole) resistors (which adds a lot of flare to circuits), and SMD (surface-mount-device) resistors have their own value-marking method.

Several colored bands around the component body represent the resistance value and tolerance. The significant digits of the resistance value are determined by the first two bands of the resistor, the multiplying factor is determined by the third band, and the tolerance is determined by the fourth band. A resistor color coding chart can be used to look up which color corresponds to which number.

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The table below shows the color codes for resistors:

The table below indicates the tolerance of resistors.

However, if there is no fourth band than tolerance of ± 20 is taken.

## Applications of resistor

Resistors are used everywhere in the electronic circuits to control the value of current, here are some of the applications listed.

• Wire wound resistors are used in shunt with ampere meter applications where balanced current regulation, high sensitivity, and precise measurement are required.
• Photo resistors are used in flame detectors, burglar alarms, and photographic gadgets, among other things.
• Temperature and voltmeter control are achieved by the use of resistors.
• Resistors are employed in modulators, demodulators and transmitters, as well as digital multimeters, amplifiers, telephony and oscillators.