Relays, Circuits, and Electromagnetism Explained

Posted on Feb 26, 2025 in Electromechanical Engineering

Relays: How They Work

A relay consists of two primary mechanisms:

  • An electromagnet that can be controlled by low-intensity current.
  • Contacts that open and close due to the electromagnet’s action.

Low-intensity current is used to control a larger current. A relay is composed of the electromagnet (coil and core), contacts (open and closed), springs, coil connections, and a common terminal.

Relay Operation Example

Consider a system with two lamps, two batteries, and two relays:

  1. When electricity is supplied to a coil, it becomes a magnet and attracts a plate. This action switches one contact connected to a lamp off, and another one on.
  2. Each relay has closed and open contacts, and a coil. When voltage is applied to the coil, the contacts move.

Resistance

Resistance is the opposition to the flow of electricity. Its unit of measurement is the ohm (Ω).

Diodes

Diodes allow the passage of electricity from the anode to the cathode. Direct polarization occurs when current flows from the anode (P) to the cathode (N).

Light and Temperature Dependent Resistors

  • LDR (Light Dependent Resistor): Resistance decreases as light intensity increases.
  • NTC (Negative Temperature Coefficient) Thermistor: Resistance decreases as temperature increases.
  • PTC (Positive Temperature Coefficient) Thermistor: Resistance increases as temperature increases.

Transistors

Each circuit has two transistors with a gate. The first, smaller transistor, when opened by a small number of electrons (like a lever), opens the second, larger transistor. This allows a small number of electrons to control a large amount. The terminals are:

  • E: Emitter
  • B: Base
  • C: Collector

Magnetic Fields and Reluctance

Magnetic field lines always go from north to south without crossing. Reluctance is the opposition to the passage of these magnetic field lines.

Electromagnetic Induction

  1. Faraday’s Law (Generator): If a coil moves into and out of a magnet, an electrical current is generated. Examples: flashlight generator, bicycle dynamo, car alternator.
  2. Motor Effect: If a coil is placed within a magnetic field and current is supplied, a force is generated. Examples: doorbell, car door lock, gate opener.

Properties of Magnetic Field Lines

  1. Magnetic field lines always travel from north to south and never cross.
  2. They always take the path of least reluctance.

Magnetic Permeability

Magnetism can be transmitted. Therefore, a non-magnetic material can become magnetized.

Electromagnets

Three important factors affect the strength of an electromagnet:

  1. Increasing the number of turns (spins) in the coil increases the magnetic field intensity.
  2. Increasing the current intensity increases the magnetic field intensity.
  3. Adding an iron core increases the magnetic field strength by reducing reluctance.

Electromagnetic Induction (Rotating Coil)

When a coil rotates within a magnetic field, an electrical current is generated at the ends of the coil; this is electromagnetic induction.

Electric Motor Components

  • Rotor: The rotating part of the motor.
  • Stator: The stationary part, linked to the casing.
  • Inductor: The coil and magnet that generate the magnetic field.
  • Induced (Armature): One or more coils within which the magnetic field is generated.
  • Collectors: Connect the current to the coils.
  • Brushes: Transmit current to the collectors, supporting the rotor and preventing cable tangling.

How an Electric Motor Works

Current flows through the collectors to the rotor coil, turning it into a magnet. The magnetic attraction causes the rotor to turn halfway. The brushes lose contact, but inertia keeps the coil rotating. Then, the polarity changes in the collector, the magnetic poles of the coil change, and the other half of the rotation occurs. This process repeats continuously.

Transformers

A transformer receives electrical current from one circuit and increases or decreases the voltage, returning the current to another circuit. It consists of two isolated copper coils wound around a steel core:

  • Primary Coil
  • Secondary Coil

Steel Core: Made of treated silicon steel plates.

Function: A transformer changes the voltage from one side to the other.