1. What is Resistor?
2. Resistor types.
3. Resistor construction.
4. Resistor formulas.
5. Resistance measurement
6. Use of resistors.

# What is Resistor?

A resistor is a two-terminal passive electronic component that can reduce the flow of electrons, can be used as a voltage divider and can adjusting the signal levels in electronic circuits. And also, resistors are used to complement active components like the transistor, op-amp, microcontroller, and other integrated circuits.

The resistance of a resistor

The resistance is a property of a resistor to oppose the electric current through it. This resistance can be calculated by Ohm's law

According to Ohm's law, the flow of electric current in the circuit is directly proportional to the applied voltage i.e

Where R is called the resistance of the wire.

The hydraulic analogy of resistance

The concept of resistance, current, and voltage can be explained by a hydraulic analogy. Take two containers one filled with full water and another filled with half water.

If we connect these two containers with a pipe then the flow of water always flows from full-filled containers to half-filled containers because of the pressure difference.

When the aperture of the pipe decreases then the flow of water through the pipe is restricted by a constriction.

Now if we compare this hydraulic analogy with voltage, current and resistance then we can say the flow of water is equivalent to the current, the pressure difference is equivalent to the voltage and the constriction is equivalent to the resistance.

The resistance property depends on the cross-sectional area and length of the conductor. The resistance is directly proportional to the length 𝑙 and inversely proportional to the cross-sectional area A of the conductor i.e

$R=\rho&space;.\frac{l}{A}$

Where 𝜌 is called the resistivity of the material.

Unites of resistance

The electrical resistance of the resistor measured in ohms(Ω). The unit ohm is the S.I unit of the resistor. there are other units of resistance like kΩ, MΩ.

1kΩ = 10^3Ω
1MΩ = 10^6Ω

Ohm's law gives that the resistance of the resistor is equal to voltage/current which means the unit of resistance is volt(V)/ampere(A) and it is called Ohm(Ω).

In the unit of the resistor, 1Ω means the resistance between two points where 1volt(V) of the applied potential difference gives 1ampere(A) of current.

Symbols of resistors

All the resistors have two terminal, one connection on each end of the resistor. But in the symbol of the variable resistor, there are three terminals that mean two terminals are connected on each end of the resistor, and the other shows the variation of the value of resistance.

There is another symbol used to show the light-sensitive resistor (to show it arrow sign is used).

## Types of Resistor

There are different kinds of resistors used in the electronic circuits for different types of applications. The resistors are available in different shapes, sizes, and materials. Normally resistors can be classified into two types called linear and nonlinear resistors. This classification is given below

A. Linear resistors

a. Fixed resistors:

1. Carbon composition resistors.
2. Wire wound resistors.
3. Thin-film resistors.
• Carbon film resistors
• Metal film resistors.
4. Thick film resistors.
• Metal oxide resistors.
• Cermet oxide resistors (Network resistors).
• Fusible resistors.

b. Variable resistors:

1. Potentiometer.
2. Rheostats.
3. Trimmers.

B. Non-Linear resistors

1. Thermisters.
2. Varistors.
3. Photoresistors or Light dependent resistors (LDR).

## Resistor construction

If you break a resistor or open it and scratch off the outer coating of insulating paint then you can see an insulating ceramic rod running through the middle with copper wire wrapped around the outside.

The number of copper turns controls the resistance very precisely, the more copper turns, and the thinner the copper, the higher is the resistance.

For the smaller value of resistors and designed for lower power circuits, the copper winding is replaced by a spiral pattern of carbon. Generally, wire wound resistors are more precise and more stable at higher operating temperatures.

## Resistor formulas

Resistance

The electrical resistance of a circuit or device is equal to the ratio of the voltage(V) applied to the electric current(I) which flows through it i.e

$R=\frac{V}{I}$

Resistivity and conductivity

The resistance property depends on the cross-sectional area and length of the conductor. The resistance of the resistor is directly proportional to the length 𝑙 and inversely proportional to the cross-sectional area A of the conductor and would be expected to depend upon the material out of which the wire is made.

Experimentally, the dependence upon these properties is a straightforward one for a wide range of conditions, and the resistance of a wire can be expressed as

$R=\rho&space;.\frac{l}{A}$

Where 𝜌 is called the resistivity and it depends on the nature of the material.

The inverse of resistivity is called conductivity i.e

$\sigma&space;=\frac{1}{\rho&space;}$

Resistor combination

Resistor circuits that combine series and parallel resistor networks together are generally known as resistor combinations.

1. The series combination of resistor:

If N number of resistors are connected in series then the equivalent resistance is given as

2. The parallel combination of resistor:

If N number of resistors are connected in parallel then the equivalent resistance is given as

$\frac{1}{R_{equivalent}}=\frac{1}{R1}+\frac{1}{R_{2}}+\frac{1}{R_{3}}+...+\frac{1}{R_{n}}$

## Resistance measurement

Steps for measuring resistance with a digital multimeter:

1. Turn off the ability to run the circuit. If a circuit includes a capacitor then discharge the capacitor before taking any resistance reading.

2. Rotate shaft in the resistance mode, which often shares a dial spot with one or more test modes( like continuity, capacitance, or diode modes).

Notes:

If the display shows OL, it means in resistance mode even before test leads are connected to the component, a digital multimeter(DMM) automatically begins taking a resistance measurement.

If the symbol M may appear, then it means the resistance of open test leads is very high. Pressing the RANGE button to manually set the range.

3. At first, insert the black test lead into the COM jack.

4. And then insert the red lead into the V.

5. Connect the test leads across the component being tested. Make sure that contact between the test leads and the circuit is good.

6. Read the measurement on the display.

7. Turn off the multimeter to prevent the battery drain when the measurement is finished.

8. Press the RANGE button to select the specific fixed measurement.

Note:

Be sure to note the annunciator (such as K or M) after the measurement on the display.

9. Press the HOLD button to capture a stable measurement. When the multimeter beeps, it recorded a new reading at each time.

10. Press the MIN/MAX button to capture the lowest and the highest measurement. When the multimeter beeps, it recorded a new reading at each time.

11. You can measure the resistance by using resistor color code.

## Use of resistors

Resistors are present everywhere in our life. They are used in a wide variety of applications. All electronics boards and devices utilize them. Our laptops, mobile, home accessories contain SMD resistors.

Different types of resistors are used for different types of application like -

1. Fan speed controller

All of us are familiar with the fan regulator which controls the fan speed. Actually, in this regulator, a potentiometer is used whose rotation changes the amount of resistance.

2. Street lighting

In the automatic street light systems, the light-dependent resistors(LDR) are used. During daylight, the resistance of lamps is adjusted to turn off the light.

And in the night absence of light, the resistance also changes, this variation in resistance is used to turn the lights on.

3. Power supply

In our daily life, we used some power supply like a laptop charger, mobile charger. These chargers contain resistors that are responsible for controlling the required amount of current.

4. Heater resistor

Because the resistor converts electrical energy into heat they make good heating elements for toasters, heater, electric stoves, and similar devices.