Detailed Explanation of the Working Principle of Resistors: A Beginner's Guide

Resistors are one of the most fundamental and frequently used electronic components. They can be found in both simple circuit experiment boards and complex computer motherboards. For beginners who are just getting into electronic technology, understanding the working principle of resistors is a crucial step in laying a solid foundation for their electronic learning.

What is a resistor?

Detailed Explanation of the Working Principle of Resistors

1. Basic principle

When current flows through a resistor, free electrons in the conductor collide with lattice atoms. These collisions impede the movement of electrons, thereby restricting the current and converting part of the electrical energy into heat energy.
This process reflects the physical essence of resistance - resistance is caused by the obstruction of electron movement due to the microscopic structure within the conductor.

2.Ohm’s Law in Action

Ohm's Law is one of the most fundamental and crucial tools for analyzing and designing circuits. It is not only the key to understanding the working principle of resistors, but also widely applied in various scenarios in actual circuit design. Below, we will elaborate on the practical applications of Ohm's Law from multiple perspectives:

（1）Current-limiting design

When connecting components such as LED lights, sensors and motors, it is necessary to control the current passing through these components by means of resistors to prevent them from being burned out. For example:

• The working voltage of an LED lamp is 2V, the power supply provides 5V, and the required working current is 20mA.
• The required resistance can be calculated using Ohm's Law:R = (Vpower supply - VLED) / I = (5V - 2V) / 0.02A = 150Ω
• This implies that a current-limiting resistor of 150Ω needs to be connected in series to ensure the stable operation of the LED.

（2） Voltage Divider

Voltage division is the process of dividing a higher voltage into multiple lower voltages through two or more series-connected resistors, which is used in scenarios such as analog signal input, ADC sampling, and voltage detection. 

Example: Reduce the 12V voltage to 6V for the sensor to use.

• If two resistors of equal value (such as 1kΩ) are used, the midpoint voltage is:Vout = Vin × R2 / (R1 + R2) = 12V × 1k / (1k + 1k) = 6V

(3) Resistor selection and power matching

After determining the resistance value, it is also necessary to calculate the power it consumes according to Ohm's Law to avoid burning out the components:

• Power P = V × I, or P = I² × R or P = V² / R can also be used
• If the voltage across the resistor is 10V, the resistance value is 100Ω :P = V²/ R = 100V²/ 100Ω= 1W
• In practice, a resistor with a power rating of 1.5 to 2 times should be selected, that is, a 2W resistor is safer.

(4) Circuit debugging and analysis 

Ohm's Law is also often used for:

• Analyze open circuit and short circuit: Determine whether the resistor is damaged by measuring the voltage/current
• Fault location: Based on the deviation between the expected current and the actual measured value, determine whether the component is abnormal
• Virtual simulation calculation: Input the Ohm's Law formula in circuit simulation software (such as Multisim, LTspice) to quickly simulate the impact of resistance changes on the system

(5) Combined application with other laws
In more complex circuits, Ohm's law is often used in combination with Kirchhoff's laws (KVL, KCL) :

• Analyze the distribution of voltage, current and resistance in parallel and series circuits
• Solve the voltage distribution of complex power grid nodes
• Optimize the circuit design to enhance the efficiency of power utilization

The types and structures of resistors

Resistors can be classified into various types based on their applications, regulation methods, material compositions and structural features. Understanding these different types is of great significance for circuit design and component selection. Starting from the common classification methods, the following comprehensively introduces the main types of resistors and their structural characteristics.

1.Fixed Resistor

Fixed resistors are the most common type, and their resistance values are determined during manufacturing and cannot be adjusted. It is often used to limit and distribute current and voltage, and has the advantages of high stability, low price and rich variety.

Common subtypes:

（1）Carbon Film Resistor

• Production method: Deposit a layer of carbon film on the surface of the ceramic substrate
• Features: Low cost and wide application, but poor stability and accuracy
• Suitable for: general household appliances, low-precision circuits

（2）Metal Film Resistor

（3）Wire-wound Resistor

• Structure: Wind the resistance wire (such as nickel-chromium alloy) around the ceramic frame
• Features: High power, high-temperature resistance, but not suitable for high-frequency applications
• Application: Power amplifier, motor control, braking circuit

2.Variable Resistor / Potentiometer

Variable resistors allow users to manually adjust the resistance value and are widely used in control circuits, such as volume knobs, brightness adjustments, and potential fine-tuning.

Common types:

（1）Potentiometer

• With a rotating or sliding structure, the resistance value can be continuously adjusted
• Structure: Three terminals, with a sliding contact connected at the middle end
• Application: Audio volume, display brightness control, etc

（2）Trimmer / Trimpot

• It is used for fine-tuning circuits and is generally adjusted with a screwdriver
• Application: Calibration, bias adjustment

3.Thermistor 

Thermistors are a type of resistor that is sensitive to temperature, and their resistance values change significantly with temperature.

（2）PTC (Positive Temperature Coefficient) thermistor

• When the temperature rises, the resistance increases
• Application: Automatic fuse reset, motor protection

4.LDR / Photoresistor

A photoresistor is a component that is sensitive to the intensity of light.

• Working principle: Enhanced light → Decreased resistance
• Material: Generally cadmium sulfide (CdS)
• Applications: Light-controlled lamps, brightness adjustment, security systems

5.Varistor

A varistor is a voltage-sensitive component. When the voltage exceeds a certain value, its resistance drops sharply, playing a role in surge absorption and overvoltage protection.

• Common material: Zinc oxide
• Application: Power lightning protection modules, surge protectors, household appliances

6.Special packaging and structural forms

（1）Classified by packaging:

• Through-Hole resistor: Suitable for traditional PCB soldering, it is convenient for replacement and maintenance

• Surface mount Resistor (SMD Resistor) : Suitable for surface mount technology (SMT), it is small in size and has high efficiency in automated production

（2）By shape and structure:

• Axial Resistor: The two pins extend symmetrically, facilitating the use of the plug-in board
• Radial Resistor: The pins are led out from the same side and are mostly used in small circuit boards

The main function of a resistor

Resistors play an indispensable role in circuits. It can not only limit the current and distribute the voltage, but also provide a stable working environment and signal processing capabilities. Understanding the function of resistors is helpful for mastering the core principles of electronic circuit design. The following is a detailed introduction to the main functions and application scenarios of resistors in circuits.

1.Current Limiting

This is one of the most fundamental functions of a resistor. By connecting resistors in series in the circuit, the current flowing through the circuit can be limited, preventing excessive current from damaging the components.

Common applications include:

• In an LED circuit, series current-limiting resistors can control the current passing through the LED and extend its service life
• In the motor or relay starting circuit, prevent excessive starting current from causing impact

2.Pressure division effect

Resistors can proportionally distribute high voltages into lower voltages for use in subsequent circuits. This method is called voltage division and is very common in many low-voltage control systems.

Common applications include:

• Voltage matching at the analog signal input terminal
• Provide appropriate reference voltages for sensors and voltage detection chips

3. Pulling up and pulling down effects

In digital circuits, signal lines are sometimes in a suspended state, making it impossible to determine their logic levels. By connecting pull-up resistors or pull-down resistors, the level can be stabilized at a high or low state to avoid logical errors.
Common applications include:

• The signals at the input pins of the single-chip microcomputer are stable
• Prevent false triggering in the key circuit

4. Signal Conditioning and Attenuation

In analog circuits, resistors can be used to adjust the amplitude of the signal or limit the size of the signal. For example, in combination with capacitors, filters can be constructed to adjust the frequency response; The gain can be set in combination with the amplifier.

Common applications include:

• The volume control knob uses a potentiometer for signal adjustment
• Analog filters are used to remove noise signals

5.Thermal energy release and load simulation

Heat is generated when current passes through a resistor. In some scenarios, resistors are specifically used as heating elements or for simulating loads for testing.

Common applications include:

• High-power resistors are used for load simulation and power supply testing
• Heating resistors are used in electric heaters, low-power heaters, etc

6.Detection and Sensing

Some special resistors can change their resistance values according to the variations of external physical quantities and thus are used in sensors. For example, thermistors are used for temperature detection, photoresistors for light detection, and shunt resistors for current measurement.

Common applications include:

• Temperature detection and control (such as thermistors)
• Automatic adjustment of light intensity (such as photoresistor)
• Current monitoring and protection (such as current sampling resistors

7.Summary

The main functions of resistors in circuits include:

• Control the magnitude of the current and protect the devices
• Distribute voltage to meet the requirements of different circuits
• Stabilize the logic level to prevent false triggering
• Regulate the signal and participate in filtering and amplification
• Release electrical energy to serve as a heating element or dummy load
• Participate in the sensing function to achieve the detection of temperature, light, current, etc

 Precautions for the selection of resistors

When choosing the appropriate resistor, the following key parameters need to be noted:

• Resistance value (Ω) : Calculated based on circuit requirements
• Power (W) : It must be higher than the actual power consumed in the circuit, and redundancy of more than twice is often selected
• Accuracy (±%) : The deviation range that determines the resistance value
• Temperature coefficient: Affects stability in high-temperature environments
• Packaging type: such as Through Hole or surface mount (SMD)

Common application examples of resistors