Unit 4: Electrical and Electronic Skill

Multimeter

A Multimeter (or Volt-Ohm-Milliammeter, VOM) is an all-in-one electronic measuring instrument that combines several measurement functions in one unit. It is the most essential tool for working with electronics.

Use of Multimeter as...

• Ammeter (to measure Current):
  • Connection: Must be connected in SERIES with the component.
  • How: You must "break" the circuit and insert the multimeter into the path, so the current flows *through* the meter.
  • Principle: The meter has a very *low* internal resistance, so it doesn't affect the current it is trying to measure.
• Voltmeter (to measure Voltage):
  • Connection: Must be connected in PARALLEL (across) the component.
  • How: You touch the two probes (red and black) to the two sides of the component you want to measure. The circuit is *not* broken.
  • Principle: The meter has a very *high* internal resistance, so it draws almost no current from the circuit, thus not affecting the voltage.
• Ohmmeter (to measure Resistance):
  • Connection: Must be used on a component that is OUT of the circuit.
  • How: Turn all power OFF. Remove the resistor from the circuit (or at least one leg of it). Touch the probes to each end of the resistor.
  • Principle: The meter sends a small, known current from its own battery through the resistor and measures the resulting voltage, calculating resistance using Ohm's Law (R = V/I).

Specifications of a good Multimeter

• Accuracy: How close the reading is to the true value (e.g., ±0.5%).
• Resolution: The smallest change it can detect (related to the number of "counts" or digits on the display).
• Input Impedance: A good voltmeter has a very high input impedance (e.g., > 10 MΩ) so it doesn't "load" the circuit.
• True RMS: For measuring AC voltage, "True RMS" meters give an accurate reading even for non-sinusoidal waves (like from a dimmer switch), while cheaper meters are only accurate for perfect sine waves.
• Safety Rating (CAT): A "CAT III" or "CAT IV" rating means it is safe to use on high-energy mains-voltage circuits.

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Electronic Components

Resistor

• Symbol: `---[ ]---` (a zigzag line is also common)
• Function: To resist or limit the flow of electric current.
• Governing Law: Ohm's Law (V = IR)
• Identification: Its value (in Ohms, Ω) is printed on it or indicated by a "color code" (a series of colored bands).

Capacitor

• Symbol: `---||---` (a straight line and a curved line is also common)
• Function: To store electrical energy in an electric field. It acts like a tiny, very fast rechargeable battery.
• Property: It blocks steady DC current but passes changing AC current.
• Use: Filtering, timing, and smoothing voltage supplies. (Unit: Farad, F)

Diode

• Symbol: `--->|---` (an arrow pointing at a bar)
• Function: A "one-way valve" for electricity. It allows current to flow easily in one direction (the direction of the arrow) but blocks it completely in the other direction.
• Use: Converting AC to DC (rectification), protecting circuits from reverse voltage.
• LED (Light Emitting Diode): A special type that emits light when current flows through it.

ICs (Integrated Circuits)

• Description: A "chip." It is a complex circuit containing millions or even billions of tiny transistors, resistors, and capacitors all fabricated on a single, small piece of silicon.
• Function: Acts as the "brain" of a device. (e.g., a microcontroller, a processor, or an amplifier).

PCB (Printed Circuit Board)

• Description: The green (or blue) board that all the components are soldered onto.
• Function: It provides mechanical support for the components and electrical connections between them using a pattern of copper "traces" (the shiny lines).

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Cathode Ray Oscilloscope (CRO)

A CRO (or just "oscilloscope") is a test instrument that displays a graph of an electrical signal (Voltage) versus time. It allows you to "see" electricity.

Electron Gun

This is the part that creates the "pen" for drawing the graph.

1. A cathode (a metal filament) is heated, boiling off electrons (thermionic emission).
2. A high positive voltage at the anodes (electrostatic acceleration) pulls these electrons and accelerates them into a very fast, narrow electron beam.
3. A control grid (electrostatic focusing) changes the voltage to control the *number* of electrons, which changes the brightness of the dot on the screen.

Uses of CRO

The beam then passes through two sets of "deflection plates" before hitting the fluorescent screen.

• Y-Plates (Vertical): The input signal (the "voltage" you want to see) is applied here. A positive voltage deflects the beam *up*, a negative voltage deflects it *down*.
• X-Plates (Horizontal): The CRO's internal "timebase" generator applies a "sawtooth" voltage to these plates. This sweeps the beam from left-to-right at a constant speed, then instantly resets it to the left.

Result: The combination of the vertical (Y) signal and the horizontal (X) time sweep "draws" the graph of Voltage vs. Time on the screen.

Primary Uses:

• Measuring voltages (AC and DC)
• Measuring frequencies and time periods
• Viewing the shape of a wave (e.g., sine, square, sawtooth)
• Troubleshooting and debugging electronic circuits.

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Circuit Control and Safety Devices

Regulated Power Supply

A device that converts the high-voltage, alternating current (AC) from the wall socket (e.g., 230V AC) into a steady, low-voltage, direct current (DC) (e.g., 5V DC) that electronics need. It "regulates" the output, meaning the voltage stays constant even if the input AC voltage or the load changes.

Relays

A relay is an electrically operated switch.

• Principle: It uses a *low-power* circuit to control a *high-power* circuit, while keeping them electrically isolated.
• Working: A small current (e.g., from a 5V logic chip) energizes an electromagnet (a coil). This magnet pulls a metal lever (an armature), which physically closes a switch in a separate, high-voltage circuit (e.g., a 230V light bulb or motor).

Fuses

A fuse is a simple safety device designed to protect a circuit from overcurrent (too much current).

• Working: It is just a thin metal wire or filament enclosed in a non-combustible body.
• If the current in the circuit exceeds the fuse's rating (e.g., "5 Amps"), the thin wire gets extremely hot and melts ("blows").
• This breaks the circuit, cutting off all power and protecting the expensive components from being destroyed.
• Fuses are "one-time-use" and must be replaced.

Switches

A mechanical device that manually opens (breaks) or closes (completes) an electrical circuit. (e.g., a light switch).

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Electronic switch using transistor

A transistor (specifically, a Bipolar Junction Transistor or BJT) can be used as a solid-state switch, which has no moving parts and is much faster than a mechanical relay.

A transistor has three terminals: Base (B), Collector (C), and Emitter (E).

The principle is that a small current flowing into the Base (I_B) controls a large current flowing from the Collector to the Emitter (I_C).

Two States of the Switch:

1. OFF State (Cut-off):
   • Action: We apply zero voltage/current to the Base (I_B = 0).
   • Result: The transistor acts like an open switch. No current can flow from the Collector to the Emitter (I_C = 0).
   • Example: The LED or motor connected to the Collector remains OFF.
2. ON State (Saturation):
   • Action: We apply a small positive voltage/current to the Base (e.g., from a logic chip).
   • Result: The transistor "turns on" fully and acts like a closed switch. A large current is allowed to flow from the Collector to the Emitter (I_C = max).
   • Example: The LED or motor connected to the Collector turns ON.

This is the fundamental principle behind all digital logic and computing. The "ON" and "OFF" states represent "1" and "0".