Lecture on Relay Logic Circuits

Presenter

EJ Dagel, Director of Robotics and Manufacturing at Dunwoody College Technology

Learning Objectives

• Describe the physical construction and operation of a relay
• Describe how to use relays to control electronic devices
• Wire a simple relay control circuit

Types of Relays

• Contactors: Larger relays used to start motors and control large current devices.
• Ice Cube Relays: Smaller and generally used in control logic circuits.
• Solid State Relays & Transistor-Based Relays: Other types not extensively discussed.

Physical Construction of a Relay

• Coil Terminals: Designated for coil wiring, can be DC or AC.
• DC Relay Example:
  • Wired from DC source (e.g., 24V DC) through a switch, providing current through the coil.
• Electromagnet Function: The coil creates a magnetic field that moves the Armature.
• Armature Movement:
  • Moves between normally open (NO) and normally closed (NC) contacts based on coil energization.
  • NC Contact: Closed when no power is supplied.
  • NO Contact: Closed when power is supplied.
• Common Terminal: Used to deliver power through contacts to a control device.

Control Circuit Example

• Push Buttons:
  • Start Push Button: Normally open (closed when pressed).
  • Stop Push Button: Normally closed (open when pressed).
• Ladder Diagram: Schematic with power rails and rungs, representing circuits.

Operation Steps

1. Initial State: Stop button closed, no power to start button and relay coil.
2. Press Start Button:
   • Makes contact, delivering power to the relay coil.
   • Relay coil energizes and closes NO contacts.
3. Buzzer: Sounds when NO contacts close.
4. Release Start Button:
   • Holding circuit keeps relay energized and buzzer on due to closed NO contacts.
5. Press Stop Button:
   • Opens circuit, de-energizing relay coil.
   • NO contacts return to open position, stopping the buzzer.

Practical Application

• In the lab, students will wire and test a relay control circuit.