Resistor

It is very likely that the first electronic component you have come across is the resistor. Resistors are one the simplest electronic components around and it is almost impossible to build a circuit that does not have one. So, in this article, we’ll look at what resistors are, resistance, color code, and what they’re used for. We’ll even go into the different kinds of resistors you can use in your circuits.

Different types of resistors

A Resistor is a two-terminal device that is used to resist the flow of electricity or current. A resistor has no polarity. Resistance (also known as ohmic resistance or electrical resistance) is a measure of the opposition to current flow in an electrical circuit. The basic unit of electrical resistance is the Ohm. This is often denoted by the Greek symbol omega (Ω). The value of resistance for conductors is less whereas for insulators it is more. Resistance plays a major role in Ohm’s Law. 

Resistance of a material depends on the type of material of which the resistor is made, the length of the resistor, the thickness of the material, and temperature. Each resistor has a different value of resistance which tells us how strongly it can resist the flow of current. More the value of resistance more is the capability of resisting the current.

Representation of resistors(fixed, variable)

How does a Resistor work?

The resistors are designed with 2 flexible ends called leads for bending and inserting in the breadboard. The core is made up of a spiral or helical conducting material wrapped around an insulating core. The material is extremely thin which forces the current to slow down to pass through it, which in turn provides resistance. To increase the amount of resistance, the number of loops in the spiral can be increased.

Representation and colour strip of resistor to find resistance

Resistor Color Codes

Each resistor has two leads and between these two leads is a ceramic part that resists the flow of current. Resistor consists of three colored strips that indicate the value of resistance. Each color strip on the resistor corresponds to a digit.

Some resistors come with four colored strips. In such a case, the fourth strip indicates the value of tolerance. Tolerance is the value of the deviation of resistance from its given value on the resistor.

The table below contains the value corresponding to each color:

ColorDigitMultiplierTolerance(%)
Black0100
Brown11011
Red21022
Orange3103
Yellow4104
Green51050.5
Blue61060.25
Violet71070.1
Grey8108
White9109
Gold10-15
Silver10-210
No Strip20

For example, the resistance for a resistor having color code Yellow-Violet-Orange-Gold can be calculated by

        First digit (Yellow) = 4

        Second digit (Violet) = 7

        Multiplier (Orange) = 103

          Tolerance (Gold) = 5%     

        Color Code = 47 x 103  Ω with a tolerance of +/- 5%.

                         = 47 kΩ with a tolerance of +/- 5%.        

The resistance for a resistor having color code Green-Red-Gold-Silver can be calculated by

        First digit (Green) = 5

        Second digit (Red) = 2

        Multiplier (Gold) = 10-1

          Tolerance (Silver) = 10%     

        Color Code = 52 x 10-1  Ω with a tolerance of +/- 10%.

                         = 5.2 Ω with a tolerance of +/- 10%. 

The resistance for a resistor having color code White-Violet-Black can be calculated by

        First digit (White) = 9

        Second digit (Violet) = 7

        Multiplier (Black) = 100 = 1

         Tolerance (No strip) = 20%     

        Color Code = 97 x 100  Ω with a tolerance of +/- 20%.

                         = 97 Ω with a tolerance of +/- 20%. 

You can also use a Multimeter to calculate the resistance of a resistor.

Ohm’s Law

Ohm’s Law states that the current flowing in a circuit is directly proportional to the applied voltage and inversely proportional to the resistance of the circuit, provided the temperature remains constant. Therefore, resistance can be derived from Ohm’s law.

I ∝ V

I ∝ 1/R

I = V/R

 V = I x R 

    R  =   V/I

                                                                

            where I = current, , V = voltage,  R = resistance. 

How to use Resistor to limit current?

Suppose, you want to light a LED using a DC power source or simply a battery. If you connect the LED directly to the battery, your LED will burn out right away because the battery will pass a large amount of current through the LED which will burn it out.

To protect your LED from such damages you can connect a resistor in between your LED and battery. Resistor controls the amount of current flowing from the battery to the LED. 

The value of resistance can be calculated using Ohm’s Law. Suppose, you have a LED with voltage rating  12 V and current rating of 0.1 A. 

V  = 12 Volts

I  =  0.1 Ampere

From Ohm’s Law: R  =    V/I

   R = 12/ 0.1

    = 120 Ω

So, in order to protect our LED, we need a resistor with a resistance of 120 Ω.

Calculating resistance of resistor to connect in series with LED

Combination Of Resistors

Resistors can be used in combination. Resistors can be combined in parallel or serial manner.

Resistors in Parallel

If the resistors are connected in parallel, then the total resistance will be equal to the sum of reciprocals of all resistance.

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4 +  ………     + 1/Rn

Resistors in Series

If the resistors are connected in series, then the total resistance will be equal to the sum of all resistance.

R = R1 + R2 + R3 + R4 +  ………     + Rn

How to choose the perfect resistor?

Listed below are some characteristics you need to consider while choosing the perfect resistor for your circuit.

1. Power Rating/ Watt Rating

It is the maximum amount of power that can be passed through a resistor without it being burned out  or how much current you can run through the resistor before it overheats and burns up.

     Most of the resistors that we use in our circuits come with a rating of 1/2W or 1/4W.

     You can calculate the appropriate power rating for your circuit using the formula

  P = I2 R

      or

  P = IV

2. Tolerance

Tolerance is the value of the deviation of resistance from its given value on the resistor. High tolerance means a higher percentage of error in the resistance value and low tolerance means a low percentage of error. So low tolerance is better.

The last color band in the resistor gives the tolerance of the resistor.

3. Voltage rating

It is the maximum voltage that can be applied through a resistor without it being burned out. Power rating and voltage rating are related.

4. Temperature coefficient

Temperature coefficient (TC) is the relative change of resistance per degree of temperature change. It is measured in ppm/°C (1 ppm = 0.0001%). 

Ohm’s Law tells us that the more current we pass through a component, the more power that component dissipates; this results in a rise in the temperature of the component itself. In the case of resistors, this can change the value of the resistor.

A positive TC means that an increase in temperature gives rise to an increase in resistance.

A negative TC means that an increase in temperature gives rise to a decrease in resistance.

5.  Noise

In resistors, as temperature increases, vibration of electrons inside the material increases which produces an electric AC signal across the terminal of the resistor. As the vibrations are completely random, they are unwanted and are called noise.  

Noise in a resistor is dependent on the material that composes it, the applied voltage, and physical dimensions of the resistor.

6. Frequency response

The frequency response of a resistor tells us at what frequencies the resistor still acts as a pure resistor.

At high frequencies, some resistors also have characteristics of capacitance and/or inductance.

Types Of Resistors

Fixed resistors

Fixed value resistors have a fixed-resistance and are not adjustable. Fixed resistors are the most commonly used resistors and in general one of the most used electronic components. When people talk about resistors they are most probably referring to fixed resistors.

Fixed resistor

Variable resistors

The main property of variable resistors is that their resistance value is adjustable. Most of them use mechanical movements(linear or rotary) to adjust the resistance value. If they are used as a variable voltage divider, then they are called potentiometers. If they are used as a variable resistance to control the current in a circuit, then they are called rheostats. Digital potentiometers are controlled electronically instead of by mechanical action. Variable resistors have a higher tolerance of 20% compared to fixed resistors where 5% is the most common.

Variable Resistor

Thermistors

In Thermistors, the resistance changes significantly when temperature changes. The two most common types are the NTC and PTC thermistor. NTC thermistors decrease their resistance when the temperature rises, while PTC thermistors increase their resistance when the temperature rises. Thermistors are often used as temperature sensors or thermal protection devices.

Varistors

A varistor is a voltage dependent resistor (VDR). The resistance of a varistor is variable and depends on the voltage applied. When voltage increases resistance decreases and if the voltage increases excessively, their resistance drops. This property makes them suitable to protect the circuits during voltage surges. Lightning strikes and electrostatic discharges can be the causes of a surge.

Varistor

Light-dependent resistors

Light dependent resistors or photoresistors are also variable resistors and depend on light intensity. When light intensity increases resistance decreases. They are usually used to identify light or dark situations. For example, these types of resistors are used in the street lamps to switch the lights on in the evening.

Light-dependent resistors

Magneto-resistors

Magneto resistors, also known as magnetic dependent resistors (MDR), have a variable resistance which is dependent on the magnetic field strength. Magneto resistors can be used to measure magnetic field presence, strength and direction. A magneto resistor is a subfamily of magnetic field sensors or magnetometers.

Applications of Resistors

Current Limiting

As we have discussed above, resistors play a major role in making sure LEDs do not get damaged when the current is passed. Not only for LEDs, but resistors also protect other electronic components also. By connecting a resistor in series with an LED, the current flowing through the two components can be limited to a safe value

Current Limiting application of resistor

Voltage Dividers

A voltage divider is a resistor circuit which converts a large voltage into a smaller one. Using just two resistors in series, an output voltage can be created that’s a fraction of the input voltage.

Voltage Divider

Conclusion

Resistor is a small and simple device, but its applications are endless. It is way more powerful than what it actually looks like. In this tutorial, you have learnt about resistors, resistance and different types of resistors.

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