4.Electrónica Aplicada A Redes FICCT (13/10/2021)

Introduction to Potentiometers and Resistors

Overview of Class Topics

• The class begins with a recap of the previous lesson on passive elements, specifically focusing on resistors and their role in circuits.

• Introduction of variable resistors known as potentiometers, which will be explained using a blank whiteboard for clarity.

Understanding Resistors

• Resistors are represented in circuits as rectangles with two contacts labeled 'R'. They are often cylindrical ceramic components marked with color bands indicating their resistance value.

• Resistance values are measured in ohms (Ω), and while traditional methods require memorization, modern technology allows for easy measurement using instruments.

Functionality of Potentiometers

• Potentiometers allow for adjustable resistance without needing to replace components; they can vary resistance continuously within a circuit.

• The third contact in a potentiometer enables users to adjust the resistance by moving an internal needle, effectively short-circuiting parts of the resistor to change its value.

Practical Application

• When adjusting the potentiometer, if the needle moves left (lowering resistance), most current bypasses through the short-circuited section, allowing for greater control over circuit behavior.

Visual Representation and Components

Drawing Potentiometers

• A larger drawing of a potentiometer is created for better visibility during explanations. It illustrates how varying one part affects overall resistance.

Physical Characteristics

• In real life, potentiometers resemble small drums or cylinders that rotate to adjust settings. This physical manipulation directly influences electrical properties.

Connections and Circuit Integration

Terminal Connections Explained

• The three terminals on a potentiometer include two outer contacts and one central contact (the needle). These connections determine how voltage is divided across the resistor.

Misconceptions Clarified

• It's important not to confuse input/output designations; unless specific conditions apply (like connecting positive voltage), these terms may not accurately describe function.

Conclusion: Utilizing Potentiometers

Final Thoughts on Usage

Understanding Electric Current and Its Analogies

Comparison of Electric Circuits to Hydraulic Systems

• The concept of electric current is compared to a hydraulic system, where the flow direction is from positive to negative, similar to water flowing through pipes.

• In an electrical circuit, when an electron jumps from one energy level to another, it creates a vacancy or "hole" that can be filled by another electron.

Movement of Electrons and Holes

• The analogy of a person moving from one chair to another illustrates how electrons move; when one electron occupies a hole, it creates a new hole in its previous position.

• This movement can be viewed as either the person moving or the vacancy moving, emphasizing that what we perceive as current is actually the movement of holes left by electrons.

Directionality of Current Flow

• Despite electrons having negative charge and flowing in the opposite direction (from negative to positive), conventional current flow is described as positive to negative for practical understanding.

• The discussion highlights that while we cannot see electrons directly in a copper wire, their behavior at a quantum level influences our understanding of electric current.

Resistance and Circuit Behavior

• When discussing electric circuits with resistance, it's noted that resistance limits current flow similarly to how narrowing pipes restrict water flow.

• A comparison is made between direct current (DC) and alternating current (AC), noting that AC flows in both directions while DC flows in one direction only.

Practical Examples with Calculations

• An example circuit using 5 volts and 150 milliamperes demonstrates how adding resistance changes the expected current flow; with 100 ohms resistance, the actual flow reduces significantly.

• The role of capacitors in circuits is introduced; they are represented symbolically similar to voltage sources but require careful distinction during drawing.

Capacitors in Hydraulic Analogy

• In hydraulic systems, components like pumps and pipes represent various elements within electrical circuits. Resistance affects water flow just as it does electrical currents.

Understanding Capacitors and Their Functionality in Circuits

The Basics of Capacitors

• A capacitor functions as a storage unit for electrical energy, similar to a bucket that collects water. When the flow of water (current) stops, the stored energy can still be utilized.

• The process of charging a capacitor involves filling it with energy until it reaches capacity, at which point it can release this energy back into the circuit.

Charging Process

• Capacitors store electrical charge between two conductive plates, allowing electrons to accumulate. This accumulation occurs on a microscopic scale.

• The time taken for a capacitor to charge is significant; it does not fill instantaneously but rather follows an exponential curve based on its size and the current supplied.

Voltage and Current Dynamics

• As voltage increases across the capacitor during charging, this relationship is also reflected in current over time. The charging time is crucial for understanding how capacitors behave in circuits.

• For example, when connected to a 5V source, reaching full charge takes time due to the exponential nature of the charging process.

Circuit Behavior Upon Disconnection

• If the power source is removed from the circuit (opened), while no new current flows, the voltage remains present temporarily as stored energy begins to deplete.

• Even after disconnection, there will still be some current flowing from the capacitor until it fully discharges.

Discharge Characteristics

• The discharge process also follows an exponential decay pattern; as voltage decreases over time, so does current until both reach zero.

• Importantly, discharge duration depends solely on capacitor size rather than initial current since no new charge enters once disconnected.

Understanding Capacitors and Their Applications

The Basics of Capacitors

• Electrons in a poor material begin to move, indicating that the discharge time depends on the capacitor's size, measured in farads.

• A large capacitor would be comparable to an oil storage tank used in workshops for changing car oil, illustrating the impracticality of very large capacitors.

Common Capacitor Values

• Standard values used in applications are typically microfarads (10^-6 F), nanofarads (10^-9 F), and picofarads (10^-12 F).

• In digital electronics, where voltages do not exceed 5 volts, components must be designed to handle these lower voltage levels without exceeding tolerances.

Circuit Design Considerations

• Digital circuits require strict adherence to voltage specifications; exceeding 5 volts can lead to malfunction or damage.

• Understanding passive components like resistors and capacitors is crucial when designing electronic circuits.

Parasitic Capacitance

• Cables close together can act as capacitors due to air acting as a dielectric; proper spacing is essential to avoid unintended capacitance.

• When connecting multiple cables within a circuit, care must be taken to prevent parasitic capacitance from affecting performance.

Circuit Functionality and Components

• Each component in a circuit has specific connections: resistors have two leads, capacitors have two terminals, and switches control current flow.

• Proper arrangement of wires is necessary to minimize parasitic effects while ensuring effective circuit operation.

Building Basic Circuits

• The basic elements of a simple circuit include resistors and capacitors; understanding their interactions is key for functionality.

• Drawing out circuits helps visualize how components connect; this includes identifying power sources and ground connections.

Current Flow and Storage

• When closing a circuit with a switch, current flows through the capacitor until it reaches its maximum charge capacity.

• The stored charge allows for controlled current release when needed; this principle underlies many electronic devices' operations.

Importance of Signal Generation

• This fundamental circuit design generates signals critical for powering electronic devices; it serves as the foundational pulse generator for various systems.

Reset Signal and Its Functionality

Understanding the Reset Signal

• The reset signal is referred to as "reset" and is activated at level 0, where voltage drops from 5 volts to 0 and then returns to 5 volts after a certain time. This behavior is illustrated in time diagrams.

• The duration of the zero pulse (pulse 0) is determined by the size of the capacitor used in the circuit, which influences the charging time.

Importance of RC Circuits

• The RC (resistor-capacitor) pair is fundamental in applied electronics, serving as a cornerstone for generating initial signals when devices like computers or electronic controls are powered on.

Inductance and Its Role

Introduction to Inductors

• The third passive element discussed is the inductor (bobina), which generates an electromagnetic field when current flows through a coiled conductor. This phenomenon occurs even though the field itself remains invisible.

Electromagnetic Induction

• When alternating current passes through an inductor, it creates an electromagnetic field that can induce current in nearby conductors, demonstrating principles of electromagnetic induction. This process involves primary and secondary circuits where voltage can be transformed based on coil turns.

Transformers: Practical Applications

Transformer Functionality

• A transformer consists of primary and secondary coils; it reduces high voltages (e.g., from 220 volts) down to lower levels suitable for electronic applications (e.g., 5 volts). This conversion does not alter the nature of current but merely adjusts its voltage level for safe use in digital electronics.

Types of Transformers

Power Supply and Electronic Components Overview

Understanding Power Requirements

• The discussion begins with the importance of knowing how much current a power supply must handle, specifically mentioning a requirement for a 5-volt output capable of supporting 1500 to 2000 milliamperes.

• A typical phone charger is cited as an example, often rated at 2 amperes (2000 milliamperes), which indicates its capacity to support devices effectively.

Internal Components of Chargers

• The speaker describes opening a phone charger to reveal its internal components, including a small circuit board and a yellow transformer that converts 220 volts from the wall outlet down to 5 volts.

• It is noted that while the output voltage is reduced, it remains alternating current (AC), highlighting the need for further study on active components involved in this conversion process.

Introduction to Active Components

• The next class will focus on active electronic components such as diodes and transistors, which are fundamental in electronics.

• Emphasis is placed on understanding diodes first, as they serve as essential building blocks in electronic circuits.

Class Interaction and Resources

• A light-hearted moment occurs when students interact with the instructor about their appearances during class discussions.

• Students inquire about where to find transformers locally; the instructor suggests various electronics shops around town known for selling components necessary for repairs.

Practical Applications and Further Learning

• The instructor encourages students to explore local electronics stores for parts needed in practical applications or projects they may undertake.