Laboratory Exercise Nine

Transcription

1 Basic D.C. AVIM 121 Lab 9 Page 1 of 7 rev Laboratory Exercise Nine Objectives 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. Multimeter 2. VPS (Variable Power Supply) 3. 1 No. 47 lamp 4. 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 E S E S R L R L (a) Switch Open with no current flow. (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.

2 Basic D.C. AVIM 121 Lab 9 Page 2 of 7 rev 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) E S Figure 9-2 Single-pole, single-throw switch Lamp 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.

3 Basic D.C. AVIM 121 Lab 9 Page 3 of 7 rev 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)

4 Basic D.C. AVIM 121 Lab 9 Page 4 of 7 rev 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

5 Basic D.C. AVIM 121 Lab 9 Page 5 of 7 rev 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 Figure 9-9 No. 47 Lamp 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

6 Basic D.C. AVIM 121 Lab 9 Page 6 of 7 rev 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.

7 Basic D.C. AVIM 121 Lab 9 Page 7 of 7 rev Switches Name: Data Tables Date: Section A: Contact Arrangements 1. The schematic symbols Section B: Switch Circuits 2. Instructor Approval 3. Instructor Approval