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 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.

Learning Objectives

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

• Summarize the history and inventors of relays and the first applications of relays.
• Explain the advantages of relays over early mechanical systems used for control and calculations.
• Describe how relays are used in modern systems.
• Identify advancements in relay technology that improve efficiency and system capabilities.
• Explain the use of electromagnetism in relays to control remote switches.
• Describe how the inductance of a magnetic coil affects relay operating speed.
• Explain the construction and operation of a basic relay.
• Identify methods used to protect relays from dirt and corrosion.
• Explain the method used to determine relay pickup and dropaway voltage and current.
• Describe relay hysteresis and how it affects relay operation.
• Explain the water analogy for relay coil induction and how it differs from the electrical explanation.
• Describe the transient response, startup inductive-current delay, and inductive-current discharge of a relay.
• Explain the causes of an inductive-current spike and the corresponding water analogy.
• Describe how electromagnets operate in relays.
• Identify the schematic symbols used to represent relays.
• Draw a relay schematic using an example.
• Explain how multiple-contact relays are used and represented in schematics.
• Describe the design and inspection rules that vital relays must meet to protect people and equipment.
• Distinguish shelf-mount vital relays from plug-in relays.
• Explain why vital relays are designed for efficiency and low energy use.
• Describe why shelf-mount relays continue to be produced and used even though plug-in relays are easier to install.
• Compare vital-relay schematics with standard relay schematics.
• Distinguish how open and closed contacts are depicted in common schematic drawings.
• Identify the relay symbols typically used in circuit schematics.
• Explain repeater relays and how they are wired.
• Describe relay buses and the use of ladder logic.
• Explain stick circuits and how they are created.
• Distinguish among generic relays, contactors, and solenoid-operated relays.
• Describe how double-coil relays are used.
• Explain how protective relays, such as overcurrent, undercurrent, overvoltage, and undervoltage relays, are used and represented schematically.
• Identify how other types of relays are represented schematically and how they are used.
• Distinguish solid-state relays from conventional relays.
• Describe a stepping relay and where it is used.
• Explain how remote-control relays are used.
• Identify remanence relays and intermittent relays and describe how they are used.
• Interpret the schematic symbol for an AC relay.
• Describe how on/off time-delay relays, mechanical-resonance relays, and interlocking relays are drawn and applied.
• Explain how reed relays are used and their advantages over conventional relays.
• Identify the inrush currents that occur with motors and incandescent lamps.
• Apply diodes for surge protection and explain the water analogy that illustrates their operation.
• Explain how resistors and capacitors are used for relay surge protection.
• Use online catalogs to select the appropriate relay for a circuit design.
• Interpret a schematic diagram to establish relay requirements.
• Identify the information included in relay manufacturers’ data sheets.
• Describe 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. PDH credits are not awarded until the course is completed and quiz is passed.