Resistors are versatile electric components that are commonly used in almost every electrical device around you. The main function of the resistors is that they offer resistance to the flow of electric current in the electrical devices. They are widely used in transistors and integrated circuits to protect them from the overflow of electric current as they limit the electric current. Resistors are called passive electrical elements as they themselves do not release any energy, instead, they absorb the energy and release it in the form of heat, when the electric current is flowing through them. Resistors can be connected in the electrical circuits in two ways, i.e., either in series or in parallel. When resistors are connected in such a way, that the same amount of current flows through each resistor in the circuit, they are said to be connected in series, and the net resistance of the circuit will be the sum of the resistance of each individual resistor in the circuit, i.e., Rnet=R1+R2+R3+…..+Rn. However, if the resistors are connected in such a way that each resistor has the same voltage at its terminal as their terminals are connected to the same two points in the circuit, they are said to be connected in parallel. The net inverse resistance of the resistors connected in the parallel is the sum of the reciprocal of each resistance in the circuit, i.e., 1/Rnet=1/R1+1/R2+1/R3+….1/Rn.
The resistance offered by the resistors is measured in ohm (Ω). According to Ohm’s law, ‘The current passing through the circuit is directly proportional to the voltage across its terminals and inversely proportional to the resistance,’ i.e., V=IR, where V is the voltage across the terminal, I is the current passing through the circuit, and R is the resistance offered to the flow of the current. By using Ohm’s law, we can say that the circuit offers one-ohm resistance when one ampere of current is passed through the circuit at the one-unit voltage across the terminal.
Types of Resistors
Various types of resistors are available these days based on their characteristics and applicability. Let us discuss some major categories into which resistors are divided.
1. Fixed Resistors
These are the most commonly used resistors. They have a fixed value of resistance, and they are broadly categorized into the following categories.
Carbon Composition
These resistors are made up of carbon particles, for example, graphite and ceramic dust are bound together with a binder under high temperature and high-pressure conditions. This mixture is then molded in a case and fixed either with metal wires or leads. These resistors can withstand high energy and are mostly used in high voltage power supplies.
Wirewound Resistors
In these types of resistors, a nichrome or a manganin wire is wounded around a core. Here, the wire conducts electricity with some resistance, while the core is non-conductive. The core is usually made up of ceramic, plastic, or glass. These resistors work very accurately for high power ratings and low resistance values.
Thick Film
These are the widely used resistors in the electronic industry. In these types of resistors, a ceramic base is layered with a thick resistive film, which is a paste of glass and metal oxide mixture. They are usually available in SMD packaging and are comparatively cheaper than other types of resistors. Thick film resistors have three main types, i.e., Fusible resistors, metal oxide, and cermet film resistors.
Thin Film
In these resistors, a thin resistive layer is deposited over the insulating device; the thickness of the layer is almost 0.1μm or less. These resistors have a better temperature coefficient than thick film resistors. The two important types of thin-film resistors are carbon film and metal film resistors.
Other Types
Some other types of Fixed resistors are printed carbon resistors, ammeter shunt, foil resistor, grid resistor, and Surface-mount resistors.
2. Variable Resistors
In these types of resistors, we can adjust the value of resistance, which is why they are used in many electrical devices for controlling the flow of current. They find their applications in the tuning of the circuits, dimming the lights, and controlling the audio of the devices. The main categories of variable resistors are potentiometer, rheostat, digital resistor, and preset.
3. Thermistors
The resistance of the thermistors changes with the change in the temperature. They are so sensitive that even a small change (say, one degree) in the temperature can cause a resistance change of even more than 100 ohms. They are broadly divided into two types, negative temperature coefficient (NTC) and positive temperature coefficient (PTC). The resistance of NTC thermistors decreases with the increase in temperature because the conductivity of the electrons increases in the semiconductor due to the increase in the heat, whereas the resistance of the PTC thermistors increases with the increase in the temperature as they are made up of silicon or polycrystalline ceramic material, and they become more resistant as temperature increases.
Thermistors
4. Varistors
The electrical resistance of the varistors varies with the applied voltage. So, they are also known as voltage-dependent resistors (VDR). They offer high electrical resistance at low voltages and low resistance at high voltages, and their resistance drops very rapidly above a certain value of threshold voltage. The most commonly used varistor is Metal oxide varistor (MOV), which is widely used in protecting telecommunication lines, and building surge protector power strips.
5. Photoresistors
Photoresistors are also known as light-dependent resistors (LDR) as their resistance decreases with the increase in the intensity of light due to the photoconductivity process. Photoresistors are used in the light-sensitive detectors, and light & dark activated switching circuits. They have a very high resistance in terms of mega-ohms in the dark, while a very low resistance of few ohms in the light.
6. Magneto Resistors
The resistance of the magneto resistors or magnetic dependent resistors (MDR) varies with the strength and the direction of the magnetic field. MDR works on the principle of magnetoresistance effect (It is the property of the material to change its resistance value on the application of magnetic field). It is widely used in electronic compass, position sensors, and ferrous metal detection.
Examples of Resistors
Resistors find their applications in almost every electrical component that we encounter in our everyday life. They allow only limited or the desired amount of current to pass through the electrical devices. Hence, they ensure the proper functioning of the devices. Let’s discuss some examples of resistors in real life.
1. Street Lighting
The street lights get automatically switched on in the evening and switched off in the sunlight. This is because they sense the brightness and the darkness due to the presence of photoresistors in them. The resistance of the photoresistors changes with the change in the intensity of the light, by using this phenomenon the circuits of the street lights are designed, and they automatically switched on and off during the night and day respectively. The position of the photoresistors is adjusted in such a way that factors other than sunlight, say, car headlights or bird’s shadow do not affect the working of the streetlights. Photoresistors also find their applications in burglar alarms and photographic devices.
2. Laptop and Mobile Chargers
Laptops and mobile chargers contain many resistors in them as they control the flow of current and dissipate heat. Various current readings like 1 A, 2A, 500mA, 700mA, etc., are inscribed on every charger; these readings represent the amount of current that a particular charger can allow to pass through the mobile or laptop and the speed of the charger in charging the devices.
3. Temperature Control
The temperature or heat in the circuit can be varied by changing the resistance in the circuit. This can be understood by Joule’s law of heating; Joule’s law states that the heat in the circuit is directly proportional to the square of the current (I), Resistance (R), and time (t), i.e., H={I}_{2}RT. So, it is clear from this expression that the temperature of the circuit can be varied by varying the values of current, time, and resistance.
4. Fan Speed Controller
We can change the speed of the ceiling fans by rotating the knob present on the circuit board. This knob is attached to a variable resistor, called a potentiometer. When we rotate that knob, the resistance values change that results in a change in the electric current. Hence, the speed of the fan can be controlled by using the potentiometer.
5. Measuring Electrical Current
If we connect the resistors of known resistance in series, then the electric current in the circuit can be calculated by measuring the voltage drop across the resistor; this resistor is known as a shunt resistor, and they usually have high power ratings and low resistance value. The electric current in the circuit is calculated with the help of Ohm’s law (V=IR) by using the known values of current and voltage across the terminal.
6. Temperature Sensor
The temperature sensors are used to measure the degree of hotness or the coolness of both living and non-living things. Thermistors are used in temperature sensors because they can easily detect small changes in the temperature as the change in the temperature of the body is directly proportional to the change in the resistance of the diode. If the temperature of the body is low, then the resistance will also become low, but if the temperature of the body is high, then the resistance will also be high. The resistance is thus detected and measured by the temperature sensor and is converted into electrical signals, which give us readable units of temperature like Fahrenheit, Celsius, etc.
7. In-Circuit Functioning
Many electrical devices in which current controlling is required such as changing the musical pitch of the tone, loudness of an amplifier, speed of the electric motors, uses variable resistors in them. Variable resistors allow us to change the amount of current flow in these devices by changing the resistance by simply sliding or rotating the knob.
8. Dividing Voltage
Resistors are used in electrical circuits as voltage dividers. A voltage divider divides the source voltage into different parts of the electric circuits and gives the desired operating voltage at the output or the load terminal. In accordance with Ohm’s law, the voltage drop is high for the high resistance and low for the low resistance. The simplest voltage dividing circuit consists of two resistors (R1 and R2) connected in series that give lesser output voltage (Vo) than the source voltage (Vs) as per the requirement at the output terminal.
The output voltage can be calculated by the given formula,
Vo=Vs(R2/R1+R2)
Where Vo is the output voltage, Vs is the source voltage, R1 and R2 are the resistance of the resistors connected in series.
Voltage Dividing Circuit
9. Heating Appliances and Lighting
Heating appliances like heaters, kettles, toasters, and electric ovens, use resistors in them. Resistors turn the electric current into heat and slowly dissipate this heat to make the appliances warm. The filament in the light bulb used in these heat appliances also behaves as a resistor as it reduces the current and heats up the filament wire until it glows. The heat dissipated by the resistors is measured in terms of heating power (Watts), which is given by,
P={I}^{2}R
Where P is the heating power (watts), I is the electric current in the circuit (amps), and R is the resistance offered by the resistor (ohms).
10. LEDs & Transistors Protection
Transistors and LEDs are very sensitive to the electric current. The overflow of electric current can disrupt the sensitive components of these devices, and very little current in the circuit can affect their proper functioning. A resistor of the fixed values is connected in series with the led as they allow only the defined value of the current range to pass through these devices. Resistors that are used in LEDs are often known as ballast resistors; ballast resistors minimize the current flow in LEDs and protect them from getting burnt. The resistance of the ballast resistor is calculated by an expression derived by using Kirchhoff’s law and Ohm’s law, which is given by,
R=(V-{V}_{LED})/I
Where V is the voltage of the source,{V}_{LED} is the voltage across the led, and I is the current at which the led is operating.
11. Timing Circuits
Resistors are used in various devices like electronic sirens, light flashers, and various other similar devices that consist of timing circuits. The timing circuit consists of either resistors and capacitors combination (RC) or resistors and inductor combination (RL) that are connected in series or in parallel with each other. Capacitors and inductors are used to store the energy supplied by the voltage source; capacitors retard the change in the voltage, while inductors retard the change in the electric current. The ability of the capacitor and inductor to charge themselves depends upon the amount of resistance used in the circuit, they take a long time to charge up if the resistance is high or vice-versa. The time value of the circuit can be obtained by multiplying the value of resistance (in terms of ohms) by the value of the capacitor (in terms of farads) or inductor (in terms of henry). The time period of the circuit can be increased by increasing the resistance in the circuit as the electric current in the circuit gets slow down.
Time Delay Circuit with Resistors and Capacitors.
12. Lighting Circuits in Houses
Parallel circuits of resistors are preferred over the series circuits in the lighting systems in homes because if we connect resistors in the series, then every bulb of the home gets switched off if we turn off only one light bulb. Moreover, the voltage is not the same across all the loads in the series, a larger number of loads means less voltage per load, and the farther away load from the source will get the least voltage. Hence, we use parallel resistor circuits as in this case, the voltage across each load becomes equal, and all the light bulbs won’t get switched off just by switching off any one of the bulbs in the circuit as each bulb in the parallel circuit has its own voltage source.
13. Blower Motor Resistor
A Blower motor is used to run a fan that maintains the air ventilation system of the cars. The blower resistor is connected in series to the blower fan so that the current passing through the blower motor can be controlled by changing the resistance of the blower motor. Blower resistors consist of several resistors, which are used to control the speed of the fan as when we change the resistance the current passing through the motor reduces, which lowers the fan speed. Various designs are used for the construction of blower resistors that involve different sized wire-wound resistors are placed in series for controlling each fan speed, and in other designs, integrated circuits are mounted on the PCB (printed circuit boards).
14. Filter Circuits
Resistors are used in filtering circuits of cell phones and computers that help in damping out unwanted electrical signals. Passive components like a capacitor or inductor are used along with resistors in these filter circuits. The filter circuit acts as a low or high-pass filter (a low-pass filter permits frequencies of lower range to pass through the circuit while high-pass filter permits frequencies of high range to pass through the circuit), depending upon the position of the resistor in the circuit. These circuits block unwanted frequency ranges and only allow the desired frequency ranges to pass through the electric circuit.
15. Fusible Resistors
The fusible resistor is actually a wire-wound resistor. It works as a normal resistor that limits the electric current at the given power supply, but if the power supply exceeds the normal value, then it acts like a fuse, and it gets burned, and it is turned into an open circuit, protecting the devices from the short-circuit. So, fusible resistors can be used to perform dual functions, i.e., as a fuse and as normal resistors in the electric circuits.
Fusible Resistors
