Capacitors and Electronic Circuits
3. Electronic Components
Learning outcomes
- I can identify common electronic components.
- I can describe the function of diodes, LEDs, transistors, and integrated circuits.
- I can interpret electronic circuit symbols.
- I can compare passive and active components.
- I can explain how electronic devices control current.
Electronic Components
What Are Electronic Components?
Electronic components are the individual parts used to build electronic circuits.
Different components can:
- control current
- limit current
- store energy
- produce light
- detect environmental changes
- amplify signals
- switch circuits
- process information
Modern electronic devices may contain anywhere from a few components to billions of microscopic components integrated onto semiconductor chips.
Electrical Circuits vs Electronic Circuits
An electrical circuit provides a path through which electric current can flow.
An electronic circuit uses components to deliberately control electrical signals and current.
Simple electrical circuits may contain:
- cells
- switches
- lamps
- resistors
Electronic circuits may additionally contain:
- diodes
- LEDs
- transistors
- sensors
- integrated circuits
Electronic circuits can therefore perform much more complex functions.
Circuit Symbols
Circuit diagrams use standardized circuit symbols rather than realistic pictures of components.
Symbols make circuits:
- easier to draw
- easier to understand
- less cluttered
- internationally recognizable
Students should learn to recognize both:
the physical component
and:
its circuit symbol
Resistors
A resistor limits or controls current.
Resistance is measured in:
ohms (Ω)
The relationship between voltage, current, and resistance is:
V = IR
Therefore:
I = V/R
Increasing resistance reduces current when voltage remains constant.
Why Resistors Are Important
Resistors can be used to:
- limit current
- divide voltage
- protect components
- control transistor circuits
- set operating conditions
- produce controlled heating
For example, an LED often requires a resistor to prevent excessive current.
Capacitors
A capacitor stores separated electric charge and energy in an electric field.
Its capacitance is:
C = Q/V
Capacitors are used for:
- energy storage
- timing
- filtering
- smoothing power supplies
- reducing electrical noise
- coupling signals
Capacitors and resistors are both extremely common electronic components.
Diodes
A diode is a semiconductor component designed primarily to allow current to flow much more easily in one direction than the other.
It acts approximately like an electrical one-way device.
A diode has two terminals:
- anode
- cathode
Its behavior depends on the direction of the applied voltage.
Forward Bias
A diode is forward biased when connected in the direction that allows significant current to flow.
Once the appropriate forward voltage is reached:
current can increase rapidly
The diode then behaves approximately like a low-resistance conducting path, although real diodes still have a voltage drop.
Reverse Bias
A diode is reverse biased when voltage is applied in the opposite direction.
Under normal operating conditions:
very little current flows
Therefore, a diode can be used to control the direction of current.
Diode Behavior
A simplified model is:
forward direction → conducts
reverse direction → blocks
This is useful for understanding many circuits.
However, real diodes are not perfect switches.
They have:
- forward voltage drops
- small reverse currents
- maximum current ratings
- maximum reverse-voltage ratings
Diode Circuit Symbol
The diode circuit symbol indicates its direction of conduction.
The line side corresponds to the:
cathode
When interpreting a circuit, identify the diode orientation before predicting whether current can flow.
The symbol allows us to determine whether the diode is forward or reverse biased.
Rectification
One important diode application is rectification.
Rectification converts:
AC → one-directional current
Because AC repeatedly reverses direction, a diode can block one half of the alternating cycle.
More complex arrangements of several diodes can produce full-wave rectification.
Rectifiers are commonly found in power supplies and chargers.
Light-Emitting Diodes
An LED is a:
light-emitting diode
It is a semiconductor diode that emits light when an appropriate forward current passes through it.
LEDs are used in:
- indicator lights
- displays
- lighting
- screens
- remote controls
- communication systems
- decorative lighting
LED Circuit Symbol
The LED symbol is based on the ordinary diode symbol but includes arrows pointing outward.
The arrows represent:
light being emitted
Like ordinary diodes, LEDs are polarized.
They must normally be connected in the correct direction.
LEDs Need Current Control
An LED should not normally be connected directly across a voltage supply without appropriate current control.
Once conducting, an LED's current can rise rapidly.
A series resistor is commonly used to limit current.
LED Resistor Example
Suppose:
Supply voltage = 9 V
LED voltage ≈ 2 V
Desired current = 20 mA
Convert:
20 mA = 0.020 A
Voltage across resistor:
V_R = 9 − 2
V_R = 7 V
Use:
R = V/I
R = 7/0.020
R = 350 Ω
A suitable practical resistor value near this calculated value can be selected while ensuring the LED current remains within its allowed range.
Why LEDs Are Useful
LEDs are widely used because they can offer:
- low energy consumption
- long operating life
- rapid switching
- small size
- low heat production compared with many older lighting technologies
- many available colors
LED technology has largely replaced older indicator lamps in many electronic devices.
Transistors
A transistor is a semiconductor device used to control electrical current.
Transistors are among the most important components in modern electronics.
They can act as:
- electronic switches
- amplifiers
Modern integrated circuits may contain enormous numbers of microscopic transistors.
Transistor as an Electronic Switch
A transistor allows a relatively small electrical signal to control a larger current.
This makes it possible to switch devices electronically without requiring a person to operate a mechanical switch.
For example:
sensor → transistor → motor
A small sensor signal can control whether current flows through the motor.
BJT Transistors
One common transistor type is the bipolar junction transistor (BJT).
A BJT has three terminals:
- base
- collector
- emitter
A small base current can control a much larger collector-emitter current.
A simplified idea is:
small base current → controls larger collector current
Transistor Switch Example
Imagine a circuit containing:
- temperature sensor
- transistor
- fan
When temperature becomes high enough, the sensor produces a signal.
The transistor switches on.
Current then flows through the fan.
When the temperature falls:
the transistor can switch off.
This allows automatic electronic control.
Transistors as Amplifiers
A transistor can also amplify an electrical signal.
A small input signal can control a larger output signal.
Amplification is important in:
- microphones
- speakers
- radios
- sensors
- communication systems
- audio equipment
The transistor does not create energy. Energy for the larger output comes from the circuit's power supply.
MOSFETs
Another extremely important transistor is the MOSFET.
A MOSFET commonly has three terminals called:
- gate
- drain
- source
A voltage applied to the gate controls current between the drain and source.
MOSFETs are widely used in:
- computers
- power electronics
- motor controllers
- switching power supplies
- digital circuits
Transistor vs Mechanical Switch
A mechanical switch requires physical movement.
A transistor can switch electronically.
This provides advantages such as:
- very fast switching
- no mechanical moving parts
- automatic control
- small size
- easy integration into computer circuits
Billions of transistor switching operations can occur every second in modern digital electronics.
Integrated Circuits
An integrated circuit (IC) is an electronic circuit containing many components manufactured together on a small semiconductor chip.
ICs may contain:
- transistors
- diodes
- resistors
- capacitors
- interconnections
Instead of constructing every circuit from separate components, complex circuits can be built into a single chip.
What Integrated Circuits Can Do
Integrated circuits can perform functions such as:
- calculations
- data storage
- signal processing
- timing
- amplification
- communication
- motor control
- sensor processing
- power management
Examples include:
- microprocessors
- memory chips
- amplifiers
- timer ICs
- voltage regulators
- microcontrollers
Microprocessors
A microprocessor is a complex integrated circuit designed to process instructions and data.
Microprocessors contain huge numbers of transistors operating as electronic switches.
They are used in:
- computers
- phones
- vehicles
- appliances
- industrial systems
Digital information is represented using electrical states corresponding to binary values such as:
0 and 1
Microcontrollers
A microcontroller is an integrated circuit designed to control electronic systems.
A microcontroller may combine:
- processor
- memory
- input/output connections
- timers
- communication systems
on one chip.
Microcontrollers are common in:
- washing machines
- robots
- toys
- thermostats
- automotive systems
- smart devices
Active and Passive Components
Electronic components are often classified as:
passive
or:
active
This distinction is useful, although precise definitions can vary depending on the engineering context.
Passive Components
Passive components do not normally provide signal amplification or active electronic control.
Common passive components include:
- resistors
- capacitors
- inductors
They can:
- dissipate energy
- store energy
- filter signals
- limit current
- divide voltage
but they do not normally provide amplification.
Resistors as Passive Components
A resistor limits current and transfers electrical energy into thermal energy.
It cannot amplify a signal.
Therefore, it is classified as:
passive
Capacitors as Passive Components
A capacitor can:
- store energy
- release energy
- filter signals
- create timing effects
However, it does not produce sustained amplification.
Therefore, a capacitor is also considered:
passive
Active Components
Active components can control current or electrical signals in more sophisticated ways.
Examples commonly include:
- transistors
- integrated circuits
Depending on the classification system, semiconductor devices such as diodes are also commonly grouped with active components.
Active components are essential for:
- switching
- amplification
- signal processing
- logic
- electronic control
Passive vs Active Components
| Passive Components | Active Components |
|---|---|
| Resistors | Transistors |
| Capacitors | Integrated circuits |
| Inductors | Amplifier devices |
| Store or dissipate energy | Can actively control signals |
| Cannot normally amplify | Can provide amplification/control |
| Often simpler | Often semiconductor-based |
A practical electronic circuit usually contains both.
Electronic Control of Current
Electronic devices control current in several different ways.
Resistor
Limits current using resistance.
Diode
Controls the direction of current.
LED
Controls current direction while converting some electrical energy into light.
Transistor
Uses one electrical signal to control another current.
Integrated circuit
Combines many electronic components to perform complex control.
Sensors and Electronic Control
Sensors can detect environmental conditions and produce electrical signals.
Examples include:
- light sensors
- temperature sensors
- pressure sensors
- motion sensors
- sound sensors
A sensor signal can be processed by a transistor or integrated circuit to control another device.
For example:
light sensor → electronic circuit → lamp
or:
temperature sensor → electronic circuit → fan
Light-Dependent Resistors
A light-dependent resistor (LDR) changes resistance depending on light intensity.
Typically:
more light → lower resistance
An LDR can be used in:
- automatic lighting
- light sensors
- alarms
- brightness controls
The LDR itself is a passive sensing component, but it can be combined with active components to control a circuit.
Thermistors
A thermistor is a resistor whose resistance changes significantly with temperature.
A common type is an NTC thermistor.
For an NTC thermistor:
temperature increases → resistance decreases
Thermistors can be used in:
- temperature sensors
- thermostats
- battery protection
- electronic equipment
From Sensor to Output
Consider an automatic cooling system.
Input
Thermistor detects temperature.
Processing
A transistor or integrated circuit responds to the sensor signal.
Output
A fan switches on.
The system can be represented as:
input → processing → output
This basic pattern appears throughout electronic control systems.
Relays
A relay is an electrically controlled switch.
A small control current can cause another circuit to open or close.
Relays can provide electrical isolation between:
- control circuit
- load circuit
Modern circuits may instead use transistors for many switching applications, but relays remain useful when physical contacts or electrical isolation are required.
Diodes and Inductive Loads
Components such as motors and relay coils store energy in magnetic fields.
When their current is suddenly interrupted, they can produce a large voltage.
A diode can sometimes be placed across the coil to provide a safe path for the current as the magnetic field collapses.
This type of diode is often called a:
flyback diode
It helps protect transistors and other electronic components.
Voltage Dividers
Resistors can be arranged to produce a fraction of a supply voltage.
For two series resistors:
V_out = V_in × R₂/(R₁ + R₂)
This arrangement is called a voltage divider.
Voltage dividers are often combined with:
- thermistors
- LDRs
- transistor circuits
- sensor inputs
to produce voltage signals that change with environmental conditions.
Example Voltage Divider
Suppose:
V_in = 12 V
R₁ = 2 kΩ
R₂ = 4 kΩ
Then:
V_out = 12 × 4/(2 + 4)
V_out = 12 × 4/6
V_out = 8 V
Changing one resistance changes the output voltage.
This allows resistive sensors to produce useful control signals.
Reading Electronic Circuit Diagrams
When interpreting a circuit diagram:
- Identify the power supply.
- Identify each component symbol.
- Trace possible current paths.
- Check diode and LED orientation.
- Identify series and parallel sections.
- Identify input components such as sensors.
- Identify control components such as transistors or ICs.
- Identify output devices such as lamps, LEDs, motors, or speakers.
Do not rely only on how components are positioned on the page.
Follow the actual electrical connections.
Connections and Junctions
Circuit diagrams often use lines to represent wires.
When several wires are electrically connected, a junction may be indicated by a dot.
Crossing lines do not always mean that the wires are electrically connected.
The exact symbol convention should therefore be interpreted carefully.
Circuit Example: LED Indicator
Consider:
battery → resistor → LED → battery
The battery provides electrical energy.
The resistor limits current.
The LED emits light when forward biased.
If the LED is reversed:
little or no current flows under normal conditions
and the LED does not light.
Circuit Example: Automatic Light
A more complex system could contain:
- LDR
- resistors
- transistor
- LED or lamp
The LDR detects light level.
Its resistance changes.
This changes a control voltage.
The transistor responds to the voltage and switches the output.
This allows the lamp to turn on automatically when conditions meet the circuit's designed threshold.
Circuit Example: Temperature-Controlled Fan
A temperature-control circuit may contain:
- thermistor
- resistors
- transistor
- motor
- protection diode
As temperature changes:
thermistor resistance changes
This produces a changing electrical signal.
The transistor then controls current through the motor.
The diode can protect the transistor from voltage produced when motor current is interrupted.
This demonstrates how several components work together rather than independently.
Why Semiconductor Components Are Important
Modern electronics depends heavily on semiconductors.
Semiconductor materials have electrical properties between those of good conductors and insulators, and their behavior can be carefully controlled.
Silicon is one of the most important semiconductor materials.
Semiconductor technology allows engineers to manufacture:
- diodes
- LEDs
- transistors
- solar cells
- sensors
- integrated circuits
From Individual Transistors to Computer Chips
Early electronic systems used large individual components.
Modern manufacturing allows enormous numbers of microscopic transistors to be produced on small pieces of semiconductor material.
This has allowed electronic devices to become:
- smaller
- faster
- more powerful
- more reliable
- less expensive per function
Component Ratings
Electronic components have operating limits.
Important ratings can include:
- maximum voltage
- maximum current
- maximum power
- temperature range
- capacitance
- resistance
- polarity
Exceeding these ratings can cause:
- overheating
- incorrect operation
- permanent damage
Correct circuit design requires components to operate within their ratings.
Resistor Power Rating
Suppose a resistor carries:
I = 0.20 A
and has:
R = 100 Ω
Power:
P = I²R
P = 0.20² × 100
P = 0.04 × 100
P = 4 W
A small resistor rated for only 0.25 W would not be suitable.
The resistor would require a power rating safely above the expected dissipation.
Component Failure
Electronic components can fail because of:
- excessive current
- excessive voltage
- overheating
- incorrect polarity
- electrostatic discharge
- mechanical damage
- aging
- manufacturing defects
Good circuit design includes protection against predictable faults.
Electronic Devices as Systems
Most electronic devices can be understood as systems containing:
input → processing/control → output
For example:
Automatic light
LDR → transistor/control circuit → lamp
Digital thermometer
temperature sensor → integrated circuit → display
Audio system
microphone → amplifier → speaker
Computer
keyboard/sensors → processor → display/output
Understanding individual components helps explain how these larger systems operate.
Common Mistakes
Mistake: Saying all electronic components simply resist current.
Different components perform very different functions.
Mistake: Saying a diode conducts equally in both directions.
A diode strongly favors current in one direction.
Mistake: Treating an LED like an ordinary lamp.
An LED is a polarized semiconductor diode and normally requires current control.
Mistake: Connecting an LED directly to a high enough voltage supply without limiting current.
Excessive current can damage the LED.
Mistake: Saying a transistor creates electrical energy.
A transistor controls energy supplied by another source.
Mistake: Saying a transistor is only an on/off switch.
Transistors can also amplify signals.
Mistake: Assuming an integrated circuit contains only one component.
An IC can contain enormous numbers of interconnected components.
Mistake: Confusing passive and active components.
Resistors and capacitors are passive; transistors and ICs are active control components.
Mistake: Assuming the physical position of components on a circuit diagram determines their connections.
Follow the circuit lines and junctions.
Mistake: Ignoring component ratings.
Voltage, current, power, temperature, and polarity limits are important.
Key Terms
Electronic component — individual device used to perform a function within an electronic circuit.
Circuit symbol — standardized graphical representation of an electrical or electronic component.
Diode — semiconductor device that allows current much more readily in one direction than the other.
Anode — one terminal of a diode.
Cathode — the other terminal of a diode, identified by the line side of the basic diode symbol.
Forward bias — voltage condition that allows significant diode current.
Reverse bias — voltage condition that normally prevents significant diode current.
Rectification — conversion of AC into one-directional current.
LED — light-emitting diode that produces light when appropriately forward biased.
Transistor — semiconductor device used for switching or amplification.
BJT — transistor with base, collector, and emitter terminals.
MOSFET — transistor commonly using gate voltage to control current between drain and source.
Amplification — use of a small signal to control a larger signal using energy supplied by a power source.
Integrated circuit (IC) — circuit containing many components manufactured together on a semiconductor chip.
Microprocessor — integrated circuit designed to process instructions and data.
Microcontroller — integrated circuit containing processing, memory, and control functions for embedded systems.
Passive component — component such as a resistor or capacitor that does not normally provide amplification or active signal control.
Active component — component capable of active signal control, switching, or amplification.
Sensor — component that responds to a physical condition and produces an electrical response.
LDR — resistor whose resistance changes with light intensity.
Thermistor — resistor whose resistance changes significantly with temperature.
Relay — electrically operated switch.
Voltage divider — resistor arrangement that produces a fraction of an input voltage.
Semiconductor — material whose electrical properties can be controlled to produce electronic devices.
Key Takeaways
- Electronic circuits use components to control current and electrical signals.
- Circuit symbols provide standardized representations of components.
- Resistors limit current and are passive components.
- Capacitors store energy in electric fields and are passive components.
- Diodes allow current much more readily in one direction than the other.
- Diodes can be used for rectification and circuit protection.
- LEDs are light-emitting diodes.
- LEDs are polarized and normally require current limiting.
- Transistors can act as electronic switches.
- Transistors can also amplify electrical signals.
- A small transistor input can control a much larger output current.
- Integrated circuits combine many electronic components on a single semiconductor chip.
- Microprocessors and microcontrollers are important types of integrated circuits.
- Active components provide electronic switching, amplification, or control.
- Passive components store, dissipate, or modify electrical energy without providing active amplification.
- Sensors allow electronic circuits to respond to environmental conditions.
- LDRs respond to light and thermistors respond to temperature.
- Voltage dividers can convert changing sensor resistance into changing voltage.
- Modern electronic devices combine inputs, processing, and outputs.
- Semiconductor technology makes modern digital electronics possible.
Check Your Understanding
- What is an electronic component?
- Why are circuit symbols used instead of realistic pictures in circuit diagrams?
- Describe the main function of a resistor.
- Describe the main function of a capacitor.
- Explain how a diode controls current.
- What is meant by forward bias?
- What is meant by reverse bias?
- Explain how diodes can be used to convert AC into one-directional current.
- What is an LED?
- Why is a resistor often connected in series with an LED?
- A 12 V supply powers an LED with a 2 V forward voltage at 20 mA. Calculate the required series resistance.
- Describe two functions of a transistor.
- Explain how a small transistor signal can control a motor.
- What is an integrated circuit?
- Compare passive and active electronic components.
- Classify a resistor, capacitor, transistor, and integrated circuit as passive or active.
- Explain how an LDR and transistor could be used to create an automatic light.
- Explain how a thermistor could be used in a temperature-controlled fan.
- A 100 Ω resistor carries 0.10 A. Calculate the power transferred by the resistor.
- Describe how sensors, transistors, and integrated circuits allow modern electronic devices to control electrical current automatically.