Exercise 2 Switches - · PDF fileSwitches Exercise 2 ... Toggle switch The toggle switch is operated manually using a lever or a handle. A toggle switch ... Toggle switches

© Festo Didactic 89688-00 19

When you have completed this exercise, you will be familiar with the operation of switches. You will be introduced to the various types of switches and to the various configurations. You will also be introduced to the Indicator Lights, Switches, and Push Buttons modules.

The Discussion of this exercise covers the following points:

Introduction to switches

Switch typesToggle switch. Push-button switch. Selector switch.

Switch configurationsSingle-pole single-throw switch. Double-pole single-throw switch. Single-pole double-throw switch. Double-pole double-throw switch.

Introduction to the indicator light

Training system modulesPush Buttons module. Switches module. Indicator Lights module.

Introduction to switches

Switches are one of the basic components of electrical circuits. The main use of switches is to either prevent or allow the flow of electrical current at a particular point of a circuit. When the switch is in open state, it completely prevents the flow of electrical current. In other words, the circuit containing the switch is open. On the other hand, when the switch is in closed state, it allows the flow of electrical current. In this case, the circuit containing the switch is closed. An example of an electrical switch is a standard light switch. When this switch is set to its open state, the light is off. When the switch is set to its closed state, the light is on.

Instead of simply preventing the flow of current, some switches also allow the diversion of current. This way, it is possible to use the switch in order to select which load of a circuit is powered on and which is not.

Switch types

There are many types of switches, classified according to their mechanism of operation. The most common switch types are the toggle switch, push-button switch, and selector switch. These switches are described in the following subsections.

Switches

Exercise 2

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

Exercise 2 – Switches Discussion

20 © Festo Didactic 89688-00

Toggle switch

The toggle switch is operated manually using a lever or a handle. A toggle switch generally allows selection between only two states, determined by the position of the lever. The most common example of a toggle switch is the light switch. Table 3 shows the circuit diagram symbol for a toggle switch.

Table 3. Toggle switch and associated symbol.

Component Symbol

Push-button switch

The push-button switch is operated by a push button. The state of the push button determines the state of the switch. When the button is not pressed, the switch is said to be in normal mode. The normal mode of a push-button switch may be open or closed.

When the button of a normally open (NO) push-button switch is not pressed, the switch is open. When the button is pressed, the switch is closed. For example, a car horn is controlled using an NO push-button switch. When the steering wheel button is not pressed, the circuit is open and no current flows in the horn. However, when the push button is pressed, the circuit closes, current flows in the horn and a sound is produced. Table 4 shows an example of a normally open push-button switch, and the associated symbols in closed and open states.

Table 4. Normally open push-button switch and associated symbol in open and closed states.

Component Symbol

Normally open push-button switch

Open state

Closed state



When the button of a normally closed (NC) push-button switch is not pressed, the switch is closed (hence the name). When the button is pressed, the switch is open. For example, the light in a refrigerator is activated by an NC push-button switch. When the refrigerator door button is pressed (i.e., when the door is closed), the circuit is open and no current flows in the refrigerator light. However, when it is no longer pressed (i.e., when the door opens), the circuit closes and current flows in the refrigerator light. Table 5 shows an example of a normally closed push-button switch, and the associated symbols in closed and open states.

Table 5. Normally closed push-button switch and associated symbol in closed and open states.

Component Symbol

Normally closed push-button switch

Closed state

Open state

Selector switch

The selector switch is manually operated using a rotating knob. It allows selection between two or more positions, with each position making contact at a different branch in the circuit. Table 6 shows an example of a three-position selector switch and the associated symbols. As the symbol shows, from a single circuit branch, the selector switch allows connection to any of three different circuit branches.

Table 6. Selector switch and associated symbol.

Component Symbol

Switch configurations

In addition to the different working mechanisms described in the previous section, switches can also be classified according to their configuration. These configurations are differentiated by the number of poles and the number of throws of the switch.

The number of poles of a switch indicates the number of circuits that are controlled by the switch. For instance, a single-pole switch controls only one circuit, while a double-pole switch controls two circuits. Basically, a double-pole switch is equivalent to two single-pole switches that are controlled using the same switch mechanism (e.g., lever, push button, and knob).

Emergency buttons are examples of an NC push-button switch. When the push button is not pressed, the circuit is closed and current flows normally. How-ever, when the push button is pressed (i.e., in the case of an emergency), the circuit becomes open and current ceases to flow in the cir-cuit (© Siemens AG 2014, all rights reserved).



The number of throws of a switch indicates the number of contacts to which the switch can be connected. In other words, it represents the number of circuit paths that the switch can make contact with. For instance, a single-throw switch can make contact with only one circuit path, while a double-throw switch can make contact with two circuit paths.

Based on the above notions, the most common switch configurations are described in the following subsections.

Single-pole single-throw switch

A single-pole single-throw (SPST) switch is the most simple switch configuration. Figure 13 shows an example of an SPST toggle switch. As the figure shows, the switch controls only one circuit and can make contact with only one circuit path. An SPST switch is basically an on/off type of switch allowing the circuit to be either open or closed.

Figure 13. Circuit containing an SPST toggle switch.

Double-pole single-throw switch

A double-pole single-throw (DPST) switch operates just as two SPST switches actuated using the same mechanism operate. Figure 14 shows an example of a DPST toggle switch. The dotted line in the figure is used to indicate that the two switch symbols are in fact a single DPST toggle switch. The figure also shows that the same switch is used to control both indicator lights at the same time. Just like the SPST switch, the DPST switch is basically an on/off switch with the difference being that it controls more than one circuit branch at the same time.

Power source Indicator light

SPST toggle switch



Figure 14. Circuit containing a DPST toggle switch.

Single-pole double-throw switch

A single-pole double-throw (SPDT) switch allows selection between two different circuit branches. Figure 15 shows an example of an SPDT toggle switch. As the figure shows, the SPDT switch can make contact with two circuit branches. This means that the SPDT switch can either make contact with the branch containing

indicator light or with the branch containing indicator light , but not with both at the same time.

Figure 15. Circuit containing an SPDT toggle switch.

Double-pole double-throw switch

A double-pole double-throw (DPDT) switch operates just as two SPDT switches actuated using the same mechanism operate. Figure 16 shows an example of a DPDT toggle switch. As the figure shows, when the switch is in its upper position, current flows through indicator lights and , while indicator

lights and are off. On the other hand, when the switch is in its lower position, current flows through indicator lights and , while indicator lights and are turned off.

Power source

Indicator

light

DPST toggle switch

Indicator

light

Power sourceIndicator

light

SPDT toggle switch

Indicator

light



Figure 16. Circuit containing a DPDT toggle switch.

Introduction to the indicator light

An indicator light is a basic type of component often found in electrical circuits. It is designed to produce light from electricity. Table 7 shows a typical incandescent light bulb with its associated symbol (photo courtesy of KMJ).

Table 7. Indicator light and associated symbol.

Component Symbol

There are different types of indicator lights, differentiated by the way each type produces light. The most common type is the incandescent indicator light. In this type of indicator light, light is produced by making current flow through the filament wire (usually a tungsten wire) in the indicator light. As the wire heats due to the current flowing through it, it begins to glow. This phenomenon is called incandescence.

Power source

Indicator

light

DPDT toggle switch

Indicator

light

Indicator

light

Indicator

light



Training system modules

Push Buttons module

Figure 17. Push Buttons module.

The Push Buttons module of your training system consists of one normally open (NO) push-button switch and one normally closed (NC) push-button switch. Each push-button switch is designed to operate at either 24 V ac using the 2 mm terminals, or at line voltage using the 4 mm terminals. The Push Buttons module is also equipped with four fault switches and two ground terminals.

Switches module

Figure 18. Switches module.

DPST toggle switch

DPST toggle switch

DPDT toggle switch

IEC symbols for DPST and DPDT switches

NO push button

NC push button

IEC symbols for NO and NC push-button switches

Exercise 2 – Switches Procedure Outline


The Switches module of your training system consists of two double-pole single-throw (DPST) toggle switches, and one double-pole double-throw (DPDT) toggle switch. Each switch is designed to operate at either 24 V ac using the 2 mm terminals, or at line voltage using the 4 mm terminals.

The Switches module is also equipped with four fault switches and two ground terminals.

Indicator Lights module

Figure 19. Indicator Lights module (220-240 V version).

The Indicator Lights module of your training system has six indicator lights of the incandescent type. The indicator lights at the left are designed to operate at 24 V using the 2 mm terminals, and the indicator lights at the right are designed to operate at line voltage using the 4 mm terminals.

The Indicator Lights module is also equipped with four fault switches and two ground terminals. The use of the fault switches is explained later in this manual.

The Procedure is divided into the following sections:

Setup

Push-button switchesNormally open push-button switch. Normally closed push-button switch.

Toggle switchesSingle-pole single-throw toggle switch. Single-pole double-throw toggle switch.

High voltages are present in this laboratory exercise. Do not make or modify any

banana jack connections with the power on unless otherwise specified.

PROCEDURE OUTLINE

PROCEDURE

Indicator light

Ground terminal

Fault switch

2 mm terminal 4 mm terminal

IEC symbol for an indicator light

Exercise 2 – Switches Procedure


Setup

In this section, you will install the equipment required in the workstation.

1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise.

Install the equipment required in the workstation as shown in Figure 20.

Figure 20. Suggested location of the modules in the workstation (220-240 V version).

2. Make sure that all fault switches are set to the O (off) position.

3. Make sure that the main power switch on the Power Source module is set to the O (off) position, then connect it to an ac power outlet.

Push-button switches

In this section, you will connect circuits containing push-button switches and indicator lights. In each circuit, you will observe and record the operation of the indicator lights depending on the state of the switches.

Normally open push-button switch

4. Set up the circuit shown in Figure 21. The wiring diagram of the circuit is shown in Figure 22.



a In this exercise, the circuits that you need to set up are given as both a wiring diagram and a circuit diagram. This is done to allow familiarization with the use of circuit diagrams. Note, however, that in later exercises, only circuit diagrams are given.

Figure 21. Circuit diagram of a power source supplying power to an indicator light controlled using an NO push-button switch.

(a) Wiring diagram (120 V).

(b) Wiring diagram (220-240 V).

Figure 22. Wiring diagram of the circuit in Figure 21.

Indicator light

NO push-button switch

24 V

0 V



To reduce the risk of electrical shock, connect all ground (green) terminals of the

modules in series with the ground (green) terminal of the power source.

5. Turn the power source on.

Press on the push button for about five seconds, then release it. Observe what happens as you do so. Repeat this step a few times.

6. Describe the indicator light operation depending on the state of the NO push-button switch. Indicate for each state of the switch whether the resulting circuit is open or closed.

7. Turn the power source off.

Normally closed push-button switch

8. In the circuit of Figure 21, replace the NO push-button switch with a normally closed (NC) push-button switch. This results in the circuit shown in Figure 23 and Figure 24.

Figure 23. Circuit diagram of a power source supplying power to an indicator light controlled using an NC push-button switch.

NC push-button switch

24 V

Indicator light

0 V







Press on the push button for about five seconds, then release it. Observe what happens as you do so. Repeat this step a few times.

10. Describe the indicator light operation depending on the state of the NC push-button switch. Indicate for each state of the switch whether the resulting circuit is open or closed.



11. Briefly explain how the circuit in Figure 23 operates just like the circuit for the light in a refrigerator.


Toggle switches

In this section, you will connect circuits containing toggle switches and indicator lights. In each circuit, you will observe and record the operation of the indicator lights depending on the state of the switches.

Single-pole single-throw toggle switch

13. Replace the Push Buttons module with the Switches module.

14. Set up the circuit shown in Figure 25. When you connect the SPST toggle switch, make sure that it is in its open state (O position).

Figure 25. Circuit diagram of a power source supplying power to an indicator light controlled using an SPST toggle switch.

24 V

SPST toggle switch

Indicator light

0 V







Toggle the SPST toggle switch between its open state and its closed state, waiting a few seconds before each change. Observe what happens as you do so. Repeat this step a few times.

16. Describe the indicator light operation depending on the state of the SPST toggle switch. Indicate for each state of the switch whether the resulting circuit is open or closed.




Single-pole double-throw toggle switch

18. Set up the circuit shown in Figure 27. In this circuit, the SPST toggle switch is replaced by a single-pole double-throw (SPDT) toggle switch and a second branch (also containing an indicator light) is added. When you connect the SPDT toggle switch, make sure that it is in the A position.

Figure 27. Circuit diagram of a power source supplying power to two indicator lights controlled using an SPDT toggle switch.




Indicator

light Indicator

light

SPDT toggle switch

24 V

0 V

Exercise 2 – Switches Conclusion



Switch the SPDT toggle switch back and forth between the A position and the B position, waiting a few seconds before each change. Observe what happens as you do so.

20. Describe the operation of indicator light and indicator light depending on the position of the SPDT toggle switch.


Disconnect your circuit.

Return the leads to their storage location.

In this exercise, you became familiar with the use of switches in electrical circuits, as well as with the most common types of switches. You learned the possible configurations of switches in electrical circuits. You were introduced to the Push Buttons, Switches, and Indicator Lights modules.

1. What is the main use of switches in electrical circuits?

2. Name one example of a common toggle switch. What happens when this toggle switch is in its open state or its closed state?

3. Explain the operation of a normally open (NO) push-button switch. When is it open and when is it closed?

CONCLUSION

REVIEW QUESTIONS

Exercise 2 – Switches Review Questions


4. Explain the operation of a single-pole double-throw (SPDT) switch.

5. Of what type are the indicator lights included in the training system? How does this type of lamp operate? Briefly explain.

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Exercise 2 Switches - · PDF fileSwitches Exercise 2 ... Toggle switch The toggle switch is operated manually using a lever or a handle. A toggle switch ... Toggle switches