What are Resistors?
A resistor is one of the electronic components which is used to regulate the flow of electric current in a circuit. They are also known as passive components, meaning instead of generating energy, it dissipates the energy as heat. The main function of a resistor is to divide voltage, limit the current and adjust signal levels which makes them essential in almost every electronic device.
Resistors work on Ohm's law, which says that the voltage across a resistor is directly proportional to the current passing through it. This resistance is measured in ohms. Resistors play a vital role in ICs, microcontrollers, sensors, wireless modules, and relays by controlling current flow, setting voltage.
Unit of Resistor
The German physicist Georg Simon Ohm formulated the Ohm’s Law named the unit of resistor as the Ohm. The resistance defines how much a resistor opposes the flow of electric current. When we apply the voltage of 1 volt, the one ohm resistance allows a current of 1 ampere through the circuit.
Resistance values vary from milliohms to megaohms or giga-ohms. Resistor values are also identified by using resistor color codes or by using the numerical markings in surface-mount devices.
Types of Resistors
Resistors are classified on the basis of the material composition, adjustability and application. Below are some different types of resistors:
1.Fixed Resistors
This type of resistors have predefined resistance values which cannot be changed. They are commonly used in building electronic circuits. Fixed resistors are further classified in following types.
Metal Film Resistors
This type of resistors has high accuracy, offers low noise and has high temperature stability which makes them ideal for precision applications.
Carbon Film Resistors
This type of resistors provide better stability than carbon composition resistors because they use a thin carbon layer on a ceramic substrate.
Carbon Composition Resistors
These resistors are cost effective but have high noise levels. They are made from a mixture of carbon particles and a binder.
Wire Wound Resistors
They are made by winding the metal wire around an insulating core. These resistors handle high power applications with low noise.
Thick and Thin Film Resistors<h5>
These resistors provide high reliability and miniaturization. They are mostly used in SMD (Surface Mount Device) applications .
2.Variable Resistors
These types of resistors allow manual adjustment of resistance values and are used for tuning and calibration.
Potentiometers
This is a three terminal resistor used for voltage division in volume controls and sensor adjustment.
Rheostats
These are two terminal variable resistors mainly used for current control in high power applications.
Trimmers
These are small preset resistors designed for fine tuning circuits.
3. Special Purpose Resistors
These resistors are used for specific functions.
Thermistors
These are temperature sensitive resistors. They are classified as NTC (Negative Temperature Coefficient) in which resistance decreases with rising temperature which are mostly used in temperature sensors and PTC (Positive Temperature Coefficient) in which resistance increases with rising temperature.
LDR (Light Dependent Resistor)
These resistors change resistance based on light intensity used in automatic lighting systems.
Varistors (VDR- Voltage Dependent Resistor)
These resistors change resistance based on voltage levels.
Fusible Resistors
These resistors work as a fuse under excessive current flow and protects the circuit from damage.
4. Power Resistors
These resistors are designed to dissipate large amounts of power while maintaining stability.
Cement Resistors
These resistors are encased in ceramic material to withstand high power loads.
Metal Oxide Resistors
These types of resistors handle high voltage and temperature conditions very effectively.
Shunt Resistors
These resistors are used in current sensing applications to measure high currents with low resistance.
Functions of Resistors
1. Current Limiting
Limiting the flow of electric current is one of the most important functions of a resistor. To prevent the excessive current in the circuit, the resistors are placed in series with active components like LED/LCD display modules or microcontrollers.
2. Voltage Division
To generate specific voltage levels from higher voltage source resistors are used in voltage divider circuits. This phenomena is very useful in analog circuits, sensor applications and microcontroller interfacing. In this we use two resistors in series and derive lower voltage output based on the ratio of their resistance values.
3. Signal Conditioning
To ensure signals are suitable for processing by other components resistors help in shaping and refining signals. They are used in pull-up and pull-down resistors which ensures stable logic levels in digital circuits by preventing floating inputs.
They are used in Biasing resistors to establish proper operating conditions and are also used in RC filters to help signal smoothing and noise reduction.
4. Power Dissipation and Heat Generation
For preventing sensitive components resistors convert excess electrical energy into heat. This function is important in power supply circuits and high power applications.
Properties of Resistors
Below are some properties of resistors which determine their performance in different applications.
1. Resistance
Resistance is the main property of resistors which determines how much it opposes the flow of electric current. Resistance varies based on material composition, length and cross-sectional area.
2. Tolerance
This tells us how much actual resistance can vary from the stated value. It is crucial for precision circuits. +- 5% is the standard resistor.
3. Temperature Coefficient
It measures how much a resistor’s resistance changes with temperature fluctuations. Low TCR resistors are used in precision applications and high TCR resistors may cause performance issues in extreme environments.
4. Noise Characteristics
Resistors like carbon composition resistors generate electrical noise due to irregularities in their structure. Metal film and wire wound resistors offer lower noise levels and are preferred in high precision audio and RF circuits.
Working Principle of Resistors
The working principle of the resistor is based on Ohm’s law which states that the voltage across a resistor is directly proportional to the current passing through it.
V = I X R
Free electrons in the conductor collide with atoms when voltage is applied across a resistor’s terminals which causes resistance to flow the current. The level of opposition depends on the material, length, cross-sectional area and temperature of the resistor.
Applications of Resistors
Resistors are used in Automotive electronics where ECU (Electronic Control Unit) circuits for accurate signal processing.
They are used in medical devices like ECG and EEG machines where precision resistors ensure accurate bio-signal measurements.
Resistors, along with capacitors, create time delay effects in various applications like oscillators and clock circuits for microcontroller timing.
Resistors, when combined with capacitors, form RC filters to eliminate noise and smooth signals in audio, RF, and communication systems.
Frequently Asked Questions
1. What are the common types of resistors?
The common types of resistors include fixed resistors (carbon film, metal film, wire-wound, thick/thin film), variable resistors (potentiometers, rheostats, trimmers), and special resistors (thermistors, LDRs, varistors, fusible resistors).
2. What is the symbol and unit of a resistor?
The symbol of a resistor is "R" and its unit is the Ohm (Ω).
3. What are the types of fixed resistors?
Types of fixed resistors include carbon composition, metal film, thick film, thin film, wire-wound, and metal oxide resistors.
4. What is a thermistor?
A thermistor is a temperature-sensitive resistor whose resistance changes with temperature.
5. What is the difference between a resistor and a capacitor?
A resistor opposes the flow of current by dissipating energy as heat, while a capacitor stores and releases electrical energy in the form of an electric field.
6. What happens if a resistor fails in a circuit?
If a resistor fails open, the circuit may stop working; if it fails short, excessive current can flow, potentially damaging components.
7. Can a resistor increase voltage?
No, a resistor cannot increase voltage; it only reduces or limits current and drops voltage.
8. How do you choose the right resistor for a circuit?
Choose a resistor based on required resistance value, power rating, tolerance, and temperature coefficient suitable for the circuit's voltage and current conditions.
