Relays 101: How Electromechanical and Solid-State Relays Work

relays-explained-in-brief

Relays are electrically operated switches that provide isolation between control and load circuits. In this video Paul from Engineering Mindset explains how relays work, the two main families electromechanical relays and solid state relays, and common configurations such as normally open and normally closed contacts. It introduces latching relays, dual pole dual throw arrangements, and explains how back EMF from the coil is managed with suppressors. The discussion includes practical examples such as turning a fan or a pump on and off from a low-power signal, and how different relay types are chosen for simple control and more complex logic.

overview-of-relays

Relays are electrically operated switches with two distinct circuits: the primary side that carries a control signal at low voltage and the secondary side that powers the load such as a fan, pump, or lamp. Energising the coil generates a magnetic field that attracts an armature, moving a movable contact to complete or break the circuit on the secondary side. When the current is removed, a spring returns the armature to its original position. Relays can have one or more poles and throws, and the terms normally open (NO) and normally closed (NC) describe the default state of the contacts. The video uses simple diagrams to show how these configurations affect circuit design and safety.

electromechanical-relays-components-and-operation

This section describes the physical parts: the coil, the armature, and the contactor. In an energised coil, the electromagnet attracts the armature, pulling the movable contact to touch the secondary terminals and complete the load circuit. When the current is removed, the spring returns the armature to the original position, breaking the circuit if the contact is NO. If NC, the contact is closed when the coil is de-energised and opens when energised. Relays can have single or multiple poles, allowing several secondary circuits to be controlled from a single primary signal. The difference between SPST, SPDT, DPDT, and other configurations is explained, along with practical examples such as a temperature-triggered fan or a water-level pump.

normally-open-vs-normally-closed-relays

Normally open relays keep the secondary circuit open until the coil is energised, at which point the magnetic field draws the armature to close the contact. In normally closed relays, the secondary circuit is closed by default and energising the coil opens the circuit. The choice depends on whether you prefer a fail-safe, off state or a normally-on condition. The video provides simple illustrations of each and discusses how this choice affects circuit design.

latching-relays-and-memory

Latching relays hold their position after the activating signal is removed, providing positional memory without continuous power. The video describes an elevator-call example where a lamp stays on after a button press until a reset action moves the piston and turns it off. Latching designs can be implemented with magnetic latching or mechanical latches and are useful for reducing power consumption or creating self-hold logic in control systems.

relay-variants-and-contacts

Relays may use single or double poles and may be configured as DPDT to switch two circuits with two states. The video explains the concept of throws as the number of contact positions, with double-throw relays acting as changeover devices that route power between different outputs in response to the primary signal. The DPDT example shows how one input can power different indicators and loads depending on state, enabling compact control for multiple devices like a fan and an LED indicator.

back-emf-and-protecting-switching-circuits

When the coil energises it stores energy in the magnetic field; when it is switched off the field collapses and a back electromotive force (back EMF) can generate voltage spikes. To protect the circuit, a diode or other snubber is placed across the coil to give a path for current as the field collapses, dissipating energy safely. The principle is explained with simple diagrams showing current flow through the diode during turn-off, preventing damage to transistors or other devices.

solid-state-relays

Solid state relays replace moving parts with semiconductor devices. The primary side uses an LED to optically couple to a photosensitive transistor on the secondary side, achieving input-output isolation and switching through semiconductors. The text notes the underlying principle of PN junctions and how light excites carriers to make the output conduct, allowing a control signal to switch a load without mechanical movement.

practical-uses-and-examples

Many examples illustrate typical relay applications: a bi-metal strip on the primary side acting as a temperature sensor to turn on a cooling fan; a water storage tank level control using a pump; and elevator call light memory using a latching relay. The video invites viewers to share their own experiences with relays and brainstorm ideas for new projects where relays could simplify control and isolation in electrical systems.

summary-and-key-takeaways

Relays provide a simple way to control high-power loads with low-power signals and guarantee isolation between circuits. You can mix different pole and throw configurations to achieve multiple loads from a single control line, and you can choose electromechanical or solid state designs depending on speed, life, and mechanical wear considerations. When designing a circuit, remember to account for back EMF and use appropriate suppression to protect devices, especially when driving coils from microcontrollers or switching transistors. The overall lesson is that relays are versatile, reliable switching devices that remain fundamental in automation and control engineering.