Basic D.C. AVIM 121 Lab 9
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Laboratory Exercise – Nine
Objectives
•
•
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Construct a circuit from a schematic
Use SPDT, SPST and DPDT switches
Learn how to control a load with several switches
Reading Assignment
Chapter 11 , Basic Electronics
List of Materials
1.
2.
3.
4.
Multimeter
VPS (Variable Power Supply)
1 No. 47 lamp
Three DPDT switches
Procedural Notes
•
•
•
•
To minimize wiring errors, exercise care when constructing the circuits of this experiment.
The various switch types can be connected from a DPDT switch.
Read each exercise section carefully before starting the laboratory work.
Record all measurements and calculations in the data table.
Introduction and Objectives
The hand operated switch is the most basic electronic circuit control device. The switch controls the
electric current by opening a set of contact to stop current from flowing or closing a set of contacts to
allow current to flow. The open and closed switch conditions are shown in Figure 9-1.
Switch Open
Switch Closed
ES
ES
RL
(a) Switch Open with no current flow.
RL
(b) Switch Closed with current flow.
Figure 9-1 Schematics show switch (a) open with no current flow and (b) closed with current flow.
Lights and other electrical devices are turned ON and OFF with a switch. In fact switches are used in
practically every electrical and electronic device. There are dozens of switches in every home on such
items as electric stoves, dish televisions, radios, etc. In this exercise you will be introduced to switch
contact arrangements, ratings, and applications in circuits.
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Once you have read through the theory section and worked the laboratory exercises you will be able to:
• Identify various switch contact arrangements
• Use a switch to control electric current
Switches
Switch manufacturers produce a variety of switches with different electrical and physical configurations.
Most of them have adopted a "standard line" which answers the needs of hobbyists, maintenance
personnel, students and the original equipment manufacture (OEM). Standard line items can be
purchased from local electronic parts supply stores. Switches are available in a variety of styles such as
toggle, rotary, rockers and push-button, to mention only a few.
Since the primary function of a switch is to control current in an electric circuit, the selection of the switch
must be based on contact arrangement and contact rating. While the physical size of the switch is an
influencing factor it cannot always be a basis for selection because contact arrangements and ratings
determine the minimum physical size.
Pole (a)
SPST Switch
Throw (b)
ES
Lamp
Figure 9-2 Single-pole, single-throw switch
Figure 9-3 Flashlight schematic
Contact Arrangements
Switch action is defined in terms of poles (pairs of mating contacts operated by the switch control) and
throws (number of positions the switch control provides the poles). Figure 9-2 shows the symbol for a
single-pole, single-throw switch in the open condition.
Since there is only one moving element (Point A) in the switch of Figure 9-2 the switch has one pole. The
only active position that pole can be placed in is the contact indicated as Point B. For this reason, the
switch is called a single-pole single-throw switch and it is abbreviated to SPST. (One use for this type of
switch is in the flashlight such as that shown in the schematic circuit diagram of Figure 9-3. Here the
switch turns the light on and off.
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Figures 9-4 through 9-6 show other kinds of switch arrangements. The two switch symbols in Figure 9-4
are single-pole, double-throw (SPDT) switches. In this design, the switch has two possible positions and
can close one of two circuits.
Pole (a)
Pole (a)
Throw (b)
Throw (b)
Figure 9-4 Single-pole, double-throw switch (SPDT)
Figure 9-5 Double-pole, double-throw switch (DPDT)
The switches shown in Figure 9-5 are double-pole, double-throw (DPDT). The dashed lines indicate that
the poles are physically connected together so that all poles can move simultaneously from one position
to another. With these switches the two poles can be thrown in two positions to control two of four circuits.
The switches in Figure 9-6 are three-pole, double-throw (3PDT).
Figure 9-5 Triple-pole, double-throw switch (3PDT)
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Procedure
In this exercise switch contact arrangements and applications are investigated.
Section A: Contact Arrangements
1. Set the multimeter to measure resistance (R x 1).
2. Connect the multimeter to the DPDT switch as shown in figure 9-7(a).
3. Throw the switch lever open (multimeter will indicate3 infinite ohms), and then close it (multimeter
will indicate zero ohms). By using one center terminal and one outside terminal you have made
an SPST switch.
4. Connect the multimeter to the DPDT switch as shown in figure 9-7(b), position A.
5. Close the switch so that the multimeter indicates zero ohms. Now move the multimeter to position
B. The multimeter should indicate open (infinite ohms).
6. With the multimeter still in position B throw the switch; the meter should indicate a closed switch
(zero ohms). By using one center terminal and two outside terminals you have made a SPDT
switch.
7. Draw DPST, SPDT and DPDT switch symbols in the data sheets.
(a
(b
(a) Using a DPDT switch to make a SPST switch
(b) Using a DPDT switch to make a SPDT switch
Figure 9-7
Section B: Switch Circuits
1. Connect the switch circuit of figure 9-8 so the lamp will light when the switch is closed. The
Double-Pole, Double-Throw (DPDT) switch can be used as: (a) a SPST switch by using one
center terminal and one outside terminal, and (b) a SPDT switch by using one center terminal and
two outside terminals.
SPST Switch
6V
No. 47 Lamp
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2. Using schematic of figure 9-9 construct the circuit so either switch will control the light. Have
instructor approve the circuit prior to attaching VPS.
SPDT
SPDT
6V
No. 47 Lamp
Figure 9-9
3. Using figure 9-10 wire the circuit so each switch controls the light. Have instructor approve the
circuit prior to attaching VPS.
SPDT
DPDT
SPDT
6V
No. 47 Lamp
Figure 9-10 Controlling a lamp with any of three switches
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Applications
The switch finds applications in practically every electronic and electrical device including those found in
aviation, automotive, industrial, electrical appliances, and household installations.
Problems
1. Compute the inrush current of a 220V, 8A lamp if the inrush current is 5 times the specified
amount.
2. Design and draw the schematic of a circuit that can control one light from four places by using
two SPDT and two DPDT switches, a number 47 lamp and a 6V power source.
Basic D.C. AVIM 121 Lab 9
9–
Switches
Data Tables
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Name:
Date:
Section A: Contact Arrangements
1. The schematic symbols
Section B: Switch Circuits
2. Instructor Approval
3. Instructor Approval
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