Fundamental Electronic Circuits and Components

Posted on May 6, 2025 in Technology

Understanding Electronic Circuits and Components

Electronic circuits involve components designed to modify electric current. This document covers fundamental concepts, devices, and their principles of operation in electronics.

Resistors: Controlling Current Flow

Resistors are components that impede the flow of electrons. This opposition to current flow helps protect other parts of a circuit from damage due to excessive current. Resistance is measured in Ohms (Ω).

  • Ammeter: An instrument used to measure electric current; it is connected in series with the circuit component whose current is being measured.
  • Voltmeter: An instrument used to measure electric potential difference (voltage); it is connected in parallel across the component whose voltage is being measured.

Ohm’s Law defines the fundamental relationship between voltage (V), current (I), and resistance (R) in an electrical circuit: V = I * R. According to this law, for a given voltage, a greater resistance results in a lower current flow.

Fixed Resistors

Fixed resistors have a constant resistance value. They are commonly made from carbon compounds or metal film. Their resistance value and tolerance are often indicated by a series of colored bands printed on the resistor’s body. The physical composition and dimensions of the resistive material determine its specific resistance.

Variable Resistors (Potentiometers)

Variable resistors, also known as potentiometers or rheostats, allow their resistance value to be manually adjusted. The resistance can typically be varied from nearly zero Ohms up to a maximum value specified by the manufacturer.

Specialized Resistor Types

Certain resistors are designed to change their resistance value in response to variations in physical parameters:

  • LDR (Light Dependent Resistor): Its resistance decreases as the intensity of incident light increases, and conversely, increases in darkness.
  • NTC (Negative Temperature Coefficient) Thermistor: Its resistance decreases as its temperature increases.
  • PTC (Positive Temperature Coefficient) Thermistor: Its resistance increases as its temperature increases.

Capacitors: Storing Electric Charge

Capacitors are passive electronic components that store electrical energy in an electric field. They consist of two conductive plates separated by an insulating material called a dielectric. The ability of a capacitor to store charge is known as capacitance, measured in Farads (F).

Transistors: Amplifying and Switching

Transistors are semiconductor devices fundamental to modern electronics, used for amplifying electronic signals or acting as electrically controlled switches. They are typically made from materials like silicon or germanium (boron can be used as a dopant). A common type, the Bipolar Junction Transistor (BJT), has three terminals: Base (B), Collector (C), and Emitter (E).

Transistor Operating States:

  • OFF (Cut-off Region): If there is no, or insufficient, current flowing into the base terminal, the transistor does not allow significant current to flow from the collector to the emitter. It acts like an open switch.
  • SATURATION Region: If a sufficient current flows into the base, the transistor allows maximum current to flow from the collector to the emitter. It acts like a closed switch.
  • ACTIVE Region: When the base current is between the values that cause cut-off and saturation, the transistor operates in the active region. In this state, the collector current is proportional to the base current (amplified). This is the region used for signal amplification.

Power Supply Components

Power supply circuits convert electrical power from a source (e.g., mains AC) into a form suitable for powering electronic devices (e.g., stable DC voltage).

  • Transformer: Reduces (or increases) AC voltage from the input (e.g., mains network) to a desired AC voltage level for the secondary circuit.
  • Rectifier (often Diodes): Converts AC voltage to pulsating DC voltage. (Implicitly needed before a regulator)
  • Voltage Regulator: An integrated circuit (IC) or discrete circuit that eliminates ripple from the rectified DC signal and provides a stable, continuous DC output voltage, even if the input voltage or load conditions vary.
    • Fixed Regulator: Provides a specific, constant output voltage (e.g., +5V, +12V).
    • Variable Regulator: Allows the output voltage to be adjusted to a desired level within a certain range.
  • Filter Capacitor: Connected after the rectifier and often before/after the regulator, this capacitor smooths out voltage variations (ripple) and helps eliminate transient voltage spikes, providing a more stable DC voltage.

Relays: Electromechanical Switches

Relays are electromechanical devices that use an electromagnet to operate a set of switch contacts. They allow a low-power signal to control a higher-power circuit, providing electrical isolation between the control and controlled circuits. They can also function as safety elements.

A relay typically consists of two main circuits:

  • Electromagnetic Circuit (Control Circuit): This part is operated by a relatively small current and includes the coil (electromagnet) and its core. When current flows through the coil, it generates a magnetic field.
  • Electric Circuit (Load Circuit): This part includes the switch contacts that are moved by the magnetic force. These contacts handle the larger currents of the circuit being controlled.

A relay can function as a simple ON/OFF switch or as a changeover switch (commutator), often utilizing three contacts: Common (COM), Normally Open (NO), and Normally Closed (NC).

Relay Characteristics (Examples):

  • Coil Voltage (e.g., 6V DC): The nominal DC voltage required to energize the electromagnet and activate the relay.
  • Coil Resistance (e.g., 75 Ω): The internal DC resistance of the relay coil.
  • Contact Voltage Rating (e.g., 250V AC/DC): The maximum voltage that the switch contacts can safely switch or carry.
  • Contact Current Rating (e.g., 6A): The maximum current that the switch contacts can safely carry when closed.

Diodes: One-Way Current Flow

A diode is a semiconductor component that allows electric current to pass predominantly in only one direction. It has two terminals: an Anode (+) and a Cathode (-). Current flows easily from the anode to the cathode (forward bias), while flow in the opposite direction (reverse bias) is largely blocked.

Switches and Push Buttons

Push Buttons

A push button is a type of switch that opens or closes an electrical circuit only for the duration it is physically pressed by an operator. When released, it typically returns to its default state due to an internal spring mechanism.

  • NO (Normally Open) Push Button: The circuit is open (contacts are not connected) when the button is not pressed. Pressing the button closes the circuit (contacts connect).
  • NC (Normally Closed) Push Button: The circuit is closed (contacts are connected) when the button is not pressed. Pressing the button opens the circuit (contacts disconnect).

Switches

A switch is a component used to make or break an electrical circuit, or to divert the flow of electric current from one conductor to another. Unlike momentary push buttons, switches typically maintain their state (on or off, or selected path) until manually changed again.

Electronic Systems: Input, Process, Output

Most electronic systems can be understood as having three fundamental stages:

  1. Input Stage: This stage collects data or signals from the external environment or another part of the system.
    • Information is often gathered using sensors. Sensors are devices that detect physical phenomena (e.g., light, temperature, pressure, sound) and convert them into electrical signals (e.g., changes in voltage, current, or resistance).
    • Voltage Division: A common circuit technique used with resistive sensors. It takes advantage of changes in a sensor’s resistance to produce corresponding changes in voltage. A set of components that converts one form of energy or signal to another (like a sensor combined with a voltage divider circuit) is often part of a transducer.
  2. Process Stage: This stage receives the electrical signals from the input stage and interprets or manipulates them. It might involve amplification, filtering, comparison, calculation, or decision-making based on the input data.
  3. Output Stage: This stage takes the processed signal from the process stage and produces a tangible result or action.
    • This often involves actuators, which are devices that convert electrical signals back into a physical action (e.g., lighting an LED or bulb, turning a motor, displaying information on a screen, sounding an alarm).
    • If the output stage requires more current or power than the processing stage can directly supply, a power amplifier (which might utilize transistors) is often needed to boost the signal to the required level.

Digital Electronics: The World of 0s and 1s

Digital electronics is a branch of electronics that deals with signals that have only two discrete states. These states are typically represented as ON / OFF, HIGH / LOW, TRUE / FALSE, or numerically as 1 / 0. This binary system is the foundation of computers and most modern electronic devices.

Truth Tables

A truth table is a mathematical table used in logic—specifically in connection with Boolean algebra, Boolean functions, and propositional calculus—which sets out the functional values of logical expressions on each of their functional arguments. For digital circuits, a truth table systematically lists all possible combinations of input values (0s and 1s) and shows the corresponding output value (0 or 1) for each combination.

Logic Gates

Logic gates are the fundamental building blocks of digital circuits. They are electronic circuits, often implemented as Integrated Circuits (ICs), that perform basic logical operations on one or more binary inputs to produce a single binary output.

NOT Gate (Inverter)

A NOT gate, or inverter, has one input and one output. The output is always the logical inverse of the input. If the input is HIGH (1), the output is LOW (0), and if the input is LOW (0), the output is HIGH (1).

OR Gate

An OR gate has two or more inputs and one output. The output of an OR gate is HIGH (1) if any of its inputs are HIGH (1). The output is LOW (0) only when all of its inputs are LOW (0).

AND Gate

An AND gate has two or more inputs and one output. The output of an AND gate is HIGH (1) only if all of its inputs are HIGH (1). If any input is LOW (0), the output is LOW (0). This operation is analogous to logical multiplication.