Electronics You Might not Have Learned in College Lesson 5: Introduction to Relays

In Electronics you Might not have Learned in College Lesson 5: Introduction to Relays, you'll learn ...

• The physics of relay electromagnetic coils, the inductive effects of relay windings, and the design of relay contacts and armatures
• Electrical relay specifications such as pickup and drop away voltages and currents, relay hysteresis, and effect of contact gap distance
• Transient response, startup current delay, as well as inductive discharge, and current spikes
• How to specify and select a relay

Overview

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Credit: 5 CPD

Length: 75 pages

Relays are the hidden workhorses of the electric age. When electromagnetism was first discovered, it was just a novelty for entertainment of college students for years until it was discovered that contacts could be attached to an iron plate and drawn together through a couple of wires. The contacts could be used to switch on lights, motors, and power. Telegraphs, computation machines, and telephone systems were born. Eventually, mechanical gears for adding machines were replaced by wires and coils of relays. In the pre-semiconductor age, relays became king of electrical systems.

Relays appeared everywhere. They were used in household appliances, automobiles, heating and air-conditioning systems, for electrical equipment monitoring and protection, and remote-control systems. The many advancements in modern relays makes them still heavily used in spite of competition by solid-state devices. Millions of relays are still sold and used every year. One major market is for switching of heavy currents in areas where rugged, easy to monitor relays are trusted more than microcircuits. The future of relays where they will be extensively used in renewable energy sources and monitoring electrical systems is a good reason to know more about how they work and are used.

In Introduction to Relays, the physics of relay electromagnetic coils, the inductive effects of relay windings, and the design of relay contacts and armatures are covered extensively. Illustrations are used to explain normally open and closed contacts as well as the magnetic coil and magnetic structure of relays.

This lesson explains electrical relay specifications such as pickup and drop away voltages and currents, relay hysteresis, and effect of contact gap distance. Water analogies are used to explain the physics of the effects of magnetic induction in relays. The time constant of magnetic decay is also explained as well as the time delay on some relays. Transient response, startup current delay, as well as inductive discharge, and current spikes are covered by water analogies and current and voltage plots. Schematic symbols for relays and examples of circuits are also shown and explained. Various parts of relays are described, as are multiple contact relays.

One early use of relays was for protective signal systems for railroads and later for automotive traffic systems. A section of this lesson is devoted to describing in detail the design and rules for making vital relays. Because of the safety that depends on these relays, they have very special regulations and test procedures that are described in detail. Pictures and illustrations are included to help better understand the design of these extremely useful devices. Examples are provided of some of the basic rules of vital relay circuit design and contact symbols are demonstrated in simple examples. This lesson presents several cases of the many schematic symbols for relays and contacts as well as examples of repeater relays, computer ladder logic using relay symbols, and relay stick circuits. There are also descriptions of common relays and high current devices such as contactors and solenoid relays that are commonly used in power circuitry.

A listing is included that covers the symbols and short descriptions of many different special relays that perform different functions in circuitry. Also explained are relays that protect against over voltage and under current situations as well as interlocking relays and reed relays.

Brief discussions of other devices that can cause current surges such as motors and incandescent lighting systems are also included. Presentations are made for surge suppression devices such as resistors, capacitors, and diodes. Their use and benefits as well as detriments are illustrated and defined.

Finally, instruction is provided on how to specify and select a relay. A sample circuit is created, an online catalog example is shown, and its information explained to aid students to correctly apply relays to build their own projects. This is an extremely useful lesson for anyone who wants more knowledge about the hidden but massive world of relays.

Learning Objectives

Upon completion of this course, participants will be able to:

• Summarize the historical development, key inventors, and earliest applications of relays.
• Compare the advantages of relays with early mechanical control and calculation systems.
• Describe modern applications of electromechanical and electronic relays.
• Identify relay-technology advancements that improve system efficiency and capability.
• Explain how electromagnetism enables a relay coil to control remote contacts.
• Analyze how coil inductance affects relay operating speed.
• Illustrate the construction and operation of a basic relay.
• Assess methods used to protect relays from dirt and corrosion.
• Determine relay pickup and dropaway voltage and current from test data.
• Define relay hysteresis and relate it to relay operating behavior.
• Contrast the water analogy for relay-coil induction with the corresponding electrical model.
• Trace relay transient response, including startup current delay and inductive-current discharge.
• Relate inductive current spikes to the stored energy in relay coils.
• Demonstrate how relay electromagnets actuate switching contacts.
• Recognize common schematic symbols used for relays.
• Diagram a basic relay schematic using standard conventions.
• Model multiple-contact relays in schematic diagrams.
• Evaluate vital-relay design and inspection rules used to protect people and equipment.
• Differentiate shelf-mount vital relays from plug-in relays.
• Justify low-energy design requirements for vital relays.
• Critique practical reasons shelf-mount relays remain in service despite easier-to-install alternatives.
• Interpret vital-relay schematics and distinguish them from standard relay schematics.
• Classify open- and closed-contact depictions used in common schematic conventions.
• Match common relay symbols to their corresponding relay functions.
• Configure repeater-relay wiring for a representative control circuit.
• Apply ladder-logic concepts to relay-bus circuits.
• Develop a stick circuit from stated control requirements.
• Categorize generic relays, contactors, and solenoid-operated relays by function.
• Examine applications of double-coil relays.
• Document the functions and schematic symbols of overcurrent, undercurrent, overvoltage, and undervoltage protection relays.
• Associate additional relay types with their schematic representations and applications.
• Characterize solid-state relays and distinguish them from conventional electromechanical relays.
• Locate appropriate applications for stepping relays.
• Review common uses of remote-control relays.
• Outline the functions and applications of remanence and intermittent relays.
• Select the correct schematic symbol for an AC relay.
• Specify schematic representations and applications for time-delay, mechanical-resonance, and interlocking relays.
• Recommend reed relays for applications where their advantages over conventional relays are beneficial.
• Calculate inrush-current implications for motors and incandescent lamps.
• Use diodes to provide relay-coil surge protection and explain the associated water analogy.
• Design resistor-capacitor networks for relay surge protection.
• Choose a relay from an online catalog based on stated circuit requirements.
• Derive relay requirements from a schematic diagram.
• Extract required selection data from relay-manufacturer data sheets.
• Estimate the parts and labor required to assemble a modern relay.

Certificate of Completion

You will be able to immediately print a certificate of completion after passing a multiple-choice quiz consisting of 25 questions. CPD credits are not awarded until the course is completed and quiz is passed.