magnetism and electromagnetism

A relay is an electrically operated switch that uses an electromagnet to mechanically operate a switch, thereby opening or closing a circuit. Its primary purpose is to control a high-power circuit with a low-power signal.

Components of a Relay

  1. Electromagnet Coil: When current flows through this coil, it generates a magnetic field. The strength of this field depends on the number of turns in the coil and the current flowing through it.

  2. Armature: This is the moveable portion of a relay. It is a ferrous (iron-containing) piece that is attracted by the electromagnetic force generated by the coil. When attracted, it pivots or moves to make or break contact.

  3. Contacts: These are the switching elements of the relay, typically made of conductive material to ensure low resistance. They open (normally open - NO) or close (normally closed - NC) to control the flow of current in a different part of the circuit.

    • Normally Open (NO) Contacts: These contacts are open when the relay coil is de-energized, meaning no current can flow through them. They close when the coil is energized.

    • Normally Closed (NC) Contacts: These contacts are closed when the relay coil is de-energized, allowing current to flow. They open when the coil is energized.

    • Changeover/Double-Throw (CO or DT) Contacts: These have both NO and NC contacts, providing a switch between two different circuits.

  4. Spring: A spring provides a restoring force that returns the armature to its original position when the current to the coil is removed, ensuring the contacts return to their normal state.

Types of Relays

Relays are classified by their contact arrangements:

  • Single Pole Single Throw (SPST): Has one input terminal (pole) and one output terminal (throw). It acts as a simple on/off switch.

    • Can be SPST-NO (Normally Open) or SPST-NC (Normally Closed).

  • Single Pole Double Throw (SPDT): Has one common input terminal and two output terminals (one NO, one NC). It can switch current to one of two paths.

  • Double Pole Single Throw (DPST): Has two input terminals and two output terminals. Essentially two SPST relays controlled by a single coil.

  • Double Pole Double Throw (DPDT): Has two input terminals and four output terminals (two NO, two NC). Two SPDT relays controlled by a single coil.

How a Relay Works

  • When an electrical current is applied to the coil, it becomes magnetized.

  • This magnetic field attracts the armature, causing it to move.

  • The movement of the armature either closes normally open contacts or opens normally closed contacts, thereby switching the secondary circuit on or off.

  • When the current to the coil is interrupted, the magnetic field collapses, and the spring pulls the armature back to its original position, resetting the contacts.

Applications of Relays

Relays are widely used in various applications due to their ability to control high-power circuits with low-power signals and provide electrical isolation. Some common applications include:

  • Motor Control: Starting and stopping electric motors.

  • Automotive Systems: Controlling headlights, wipers, power windows, and central locking systems.

  • Industrial Automation: Interfacing control circuits with power circuits, safety interlocks.

  • HVAC Systems: Controlling compressors, fans, and heating elements.

  • Telecommunications: Switching circuits in older telephone exchanges.

  • Home Appliances: In washing machines, refrigerators, and ovens for switching different functions.

Principles of Magnetism in Relays

  • Electromagnets: Magnets created by the flow of electricity through a coil are called electromagnets. The magnetic field produced is temporary and can be controlled by varying the current.

  • Magnetic Lines of Force: These lines model the direction and strength of the magnetic field. Outside a magnet, they extend from the North pole to the South pole and form continuous closed loops, known as a magnetic circuit, passing through the magnet from South to North.

  • Domain Theory of Magnetism: This theory explains how magnetic materials behave. In an unmagnetized material, magnetic domains (regions where atomic magnetic moments are aligned) are randomly oriented. When an external magnetic field is applied, these domains align, leading to magnetization.

  • Permanent Magnets: These magnets retain their magnetism without an external magnetic field. They can be demagnetized or damaged by strong opposing magnetic fields, high temperatures, or severe mechanical shock.

  • Ceramic Magnets: Also known as ferrite magnets, these are hard, brittle permanent magnets made of ceramic materials. They are commonly used in applications requiring strong magnetic fields, such as motors, loudspeakers, and in specific braking systems like on escalators, due to their resistance to demagnetization and cost-effectiveness.