Question 1
 Don’t just sit there! Build something!!

Learning to analyze relay circuits requires much study and practice. Typically, students practice by working through lots of sample problems and checking their answers against those provided by the textbook or the instructor. While this is good, there is a much better way.

You will learn much more by actually building and analyzing real circuits, letting your test equipment provide the “answers” instead of a book or another person. For successful circuit-building exercises, follow these steps:

1. Draw the schematic diagram for the relay circuit to be analyzed.
2. Carefully build this circuit on a breadboard or other convenient medium.
3. Check the accuracy of the circuit’s construction, following each wire to each connection point, and verifying these elements one-by-one on the diagram.
4. Analyze the circuit, determining all logic states for given input conditions.
5. Carefully measure those logic states, to verify the accuracy of your analysis.
6. If there are any errors, carefully check your circuit’s construction against the diagram, then carefully re-analyze the circuit and re-measure.

Always be sure that the power supply voltage levels are within specification for the relay coils you plan to use. I recommend using PC-board relays with coil voltages suitable for single-battery power (6 volt is good). Relay coils draw quite a bit more current than, say, semiconductor logic gates, so use a “lantern” size 6 volt battery for adequate operating life.

One way you can save time and reduce the possibility of error is to begin with a very simple circuit and incrementally add components to increase its complexity after each analysis, rather than building a whole new circuit for each practice problem. Another time-saving technique is to re-use the same components in a variety of different circuit configurations. This way, you won’t have to measure any component’s value more than once.

Question 2

What is an electromechanical relay?

Question 3

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

Question 4

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

Question 5

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?

Question 6

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?

Question 7

Complete the schematic diagram for a SPDT relay circuit that energizes the green light bulb (only) when the pushbutton switch is pressed, and energizes the red light bulb (only) when the pushbutton switch is released:

Question 8

Small relays often come packaged in clear, rectangular, plastic cases, which had led to the name ice cube relay to be commonly applied to them. These so-called “ice cube” relays have either eight or eleven pins protruding from the bottom, allowing them to be plugged into a special socket for connection with wires in a circuit:

Draw the necessary connecting wires between terminals in this circuit, so that actuating the normally-open pushbutton switch will energize the relay, which will in turn supply electrical power to the motor.

Question 9

Suppose that an electromechanical relay has a coil voltage rating of 5 volts, and a coil resistance of 250 Ω. However, you desire to energize this relay with a 24 volt power supply. You know that a series-connected resistor might enable the 5-volt relay coil to be powered by the 24-volt supply without damage, but you do not know what size of resistor would be appropriate for the task.

Draw a schematic diagram for such a circuit, showing the power supply (battery symbol), relay, and resistor. Explain how the resistor’s value would be calculated, along with the calculated value in ohms.

Question 10

Design an experimental circuit for determining the pull-in and drop-out current values for an electromagnetic relay coil. Why would these statistics matter to us, when working with relay circuits?

Question 11

Find one or two relays and bring them with you to class for discussion. Identify as much information as you can about your relays prior to discussion:

• Coil voltage
• Coil resistance
• Number of switch poles
• Number of switch throws
• Voltage rating of contacts
• Current rating of contacts

Also, calculate the amount of current that will be drawn by the coil when energized.

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