Basic Electricity

Basic Electromagnetic Relays

11 questions By Tony R. Kuphaldt

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Question 4 of 11

What will happen when the pushbutton switch is actuated in this circuit?

Reveal answer

The light bulb will de-energize when the pushbutton switch is actuated.

Follow-up question: is the contact inside the relay normally-open or is it normally-closed?

Notes:
There is a sequence of events to the final result of the pushbutton’s actuation. Be sure to ask you students to explain all the steps, from beginning to end, of this relay circuit’s operation. Test their comprehension of this circuit, to ensure they fully understand what is taking place.

A logical question your students may ask is, “What is the point?” After all, a circuit with no relay at all (just a switch, battery, and lamp) could accomplish the same task! What is the point of having an extra battery and this device called a relay? Resist the temptation to tell them why, and let them figure out some possible reasons for using a relay.
Question 5 of 11

Suppose an engineer designed a fire alarm system for a large warehouse, with multiple pushbutton switches connected to a single alarm siren:

Actuating any of the normally-open switches results in the alarm sounding. So far, so good.

Now, suppose that one of the wires connecting one side of several switches to the rest of the circuit breaks open. What will the result of this failure be?

Reveal answer

The result of this wiring failure will be that the lower four switches will be non-functional.

Notes:
The immediate and obvious conclusion is that four out of the five switches will be non-functional. However, it isn’t this consequence in itself that is the greatest problem. Rather, it is the silence of the failure - no one knows anything has failed, until they try to use one of the non-functional switches! Ask your students to explain what the essential nature of the problem is: what is it about this circuit’s operation that makes an “open” failure in the field wiring so dangerously inconspicuous?
Question 6 of 11

Suppose an engineer designed a fire alarm system for a large warehouse, with multiple pushbutton switches connected to a single alarm siren:

Actuating any of the normally-closed switches results in the alarm sounding. So far, so good.

Now, suppose that one of the wires connecting one side of several switches to the rest of the circuit breaks open. What will the result of this failure be?

Reveal answer

The alarm will immediate sound - a false alarm.

Notes:
Ask your students what essential task the relay performs in this system. Is there any way to build this circuit to have the same functionality without using a relay?