11:11
Relay Basics
Brian Slota
Overview
This video explains the fundamental principles of how electrical relays function as switches. It uses an "ice cube" relay as a visual aid to demonstrate the internal components, including the coil, common contact, normally closed (NC) contact, and normally open (NO) contact. The explanation covers how applying voltage to the coil creates an electromagnet that changes the position of the common contact, thereby switching the circuit between NC and NO states. The video also illustrates how relays are represented in ladder logic diagrams and highlights their utility in controlling larger loads with smaller control signals.
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Chapters
- Relays are essentially electrically operated switches used to control circuits.
- They allow a low-power control signal to switch a higher-power circuit.
- The "ice cube" relay is a common type, easily identifiable by its clear casing and distinct pin arrangement.
Understanding relays as switches is crucial because they are fundamental components in electrical control systems, enabling complex operations with simple inputs.
The 'ice cube' relay with markings like '20 AC 24V' indicating its coil specifications.
- The coil acts as an electromagnet when voltage is applied, pulling a movable plate (common contact).
- The common contact switches between two sets of stationary contacts: normally closed (NC) and normally open (NO).
- In its de-energized state (resting state), the common contact is typically connected to the NC contact due to a spring mechanism.
- Applying voltage to the coil energizes it, causing the common contact to move and connect to the NO contact, breaking the connection with the NC contact.
Knowing the function of each component and how they interact explains the core mechanism by which relays control electrical flow.
Observing the common plate physically move between the NC and NO contacts when voltage is applied to the coil, often visible through the clear casing of an 'ice cube' relay.
- Normally Closed (NC) contacts are connected to the common contact when the relay coil is de-energized.
- Normally Open (NO) contacts are not connected to the common contact when the relay coil is de-energized.
- The terms 'common,' 'normally closed,' and 'normally open' refer to the default states before the coil is activated.
Distinguishing between NC and NO contacts is essential for correctly wiring circuits and predicting how a load will behave when the relay is activated or deactivated.
Using a multimeter to check for continuity: there is continuity between common and NC when de-energized, and between common and NO when energized.
- Relays are represented by symbols in ladder logic diagrams, with the coil typically labeled 'CR' (Control Relay).
- The diagram shows the relay coil being energized by a control circuit (e.g., a switch).
- Contacts associated with the relay (CR) are shown in their de-energized state on the diagram.
- Normally closed contacts are depicted as a closed line, while normally open contacts are shown as an open line.
Understanding ladder logic symbols allows electricians to read and interpret control system schematics, which are vital for troubleshooting and design.
A diagram showing a switch closing to energize a coil labeled 'CR', which in turn controls other components through CR's NC and NO contacts.
- Relays enable a low-voltage or low-current control signal to switch a high-voltage or high-current load.
- They provide electrical isolation between the control circuit and the load circuit.
- Relays can be used to control various types of loads, such as lights, motors, or other devices.
Relays are versatile components that enhance safety and efficiency by allowing complex control logic and the management of powerful electrical loads using simple, safe control signals.
Using a 24V control signal to operate a relay that switches on a 120V red or green light, demonstrating the ability to control a higher voltage load with a lower voltage input.
Key takeaways
- Relays function as electrically controlled switches, using a low-power signal to operate a higher-power circuit.
- The core components of a relay are the coil (electromagnet) and the movable common contact that switches between normally closed (NC) and normally open (NO) stationary contacts.
- The state of NC and NO contacts is determined by whether the relay coil is energized or de-energized.
- Ladder logic diagrams represent relays and their contacts using specific symbols to illustrate control sequences.
- Relays are essential for isolating control circuits from power circuits and for controlling heavy loads with light-duty switches.
- The 'ice cube' relay provides a clear visual of the internal switching mechanism, making it ideal for learning basic relay operation.
Key terms
RelaySwitchCoilElectromagnetCommon ContactNormally Closed (NC)Normally Open (NO)De-energized StateEnergized StateLadder Logic Diagram
Test your understanding
- How does applying voltage to a relay's coil change the state of its contacts?
- What is the difference between a normally closed (NC) contact and a normally open (NO) contact on a relay?
- Why are relays useful for controlling high-power loads with low-power control signals?
- How are relays typically represented in a ladder logic diagram, and what do their symbols signify?
- Explain the role of the 'common' contact in a relay's operation.