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

Preview a portion of this course before purchasing it.

Credit: 5 PDH

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.

Specific Knowledge or Skill Obtained

This course teaches the following specific knowledge and skills:

• The history and inventors of relays and the first applications of relays
• The advantages of relays over early mechanical systems used for control and calculations
• How relays are used today in modern systems
• The advancements in relay technology that are making the world better and more efficient
• The use of electromagnetism in relays to control remote switches
• The way inductance of the magnetic coil affects the operation speed of relays
• The making of a basic relay and how it works
• How relays are protected from dirt and corrosion
• The method used to determine the pickup and dropaway voltage and current for relays
• What relay hysteresis is and how it affects relay operation
• The water analogy of relay coil induction and how it is different from electrical explainations
• The transient response, startup inductive current delay, and inductive current discharge of a relay
• What causes an inductive current spike and its water analogy
• How electromagnets work in relays
• The different schematic symbols used to represent relays
• How a relay schematic is drawn with an example
• How multiple contact relays are used and represented in schematics
• Vital relays and what design and inspection rules they have to meet to keep people and equipment safe
• The differences between shelf mount vital relays and plug in relays
• Vital relays must be designed to be efficient and use so little energy
• Why shelf mount relays are still produced and used even though plug in relays are easier to install
• How vital relay schematics are drawn and how they differ from standard relay schematics
• The differences in how open and closed contacts are depicted in most common schematic drawings
• What the different relay symbols that are typically used in most circuit schematics
• What repeater relays are and how are they wired
• What relay busses are and how ladder logic is used
• What stick circuits are and how they are created
• What the difference is between generic relays, contactors, and solenoid operated relays
• How double coil relays are used
• How protection relays such as overcurrent, undercurrent, over voltage, and under voltage are used and what their schematic symbols are
• How other types of relays are schematically represented and how they are used
• What solid state relays are and how they are different from conventional relays
• What a stepping relay is and where it is used
• How remote control relays are used
• What remanance relays and intermittant relays are and how they are used
• How an AC rely is symbolized
• How an on/off time delay relay, mechanical resonance relay, and interlocking relay are drawn and applied
• How reed relays are used and their advantages over conventional relays
• The inrush currents that occur with motors and incandescent lamps
• How to use diodes for surge protection and the water analogy that shows how they work
• How resistors and capacitors are used for relay surge protection
• How online catalogues are used to select the right relay for a circuit design
• How to use a schematic diagram to establish relay requirements
• What information is included in relay manufacturers data sheets
• What parts and labor are 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. PDH credits are not awarded until the course is completed and quiz is passed.