Why use a Relay?

MAAM for the question: What will happen when the pushbutton switch is actuated in this circuit?

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?


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.

FIG. 1 ANSWER:

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


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!