Fundamental Circuits Analysis Lab LAB Electromechanical Devices It is important for you to understand basic electromechanical devices such as switches, relays, and electronic switches. These devices are used in many mechatronics applications and you as a technician need a solid understanding of these devices both for design, maintenance, and troubleshooting. Objectives After completing this lab, you should be able to: • • • • • Construct basic switching circuits Design switching circuits Explain basic switching circuits Test basic switching circuits Use basic laboratory instruments Materials Required The following are quantities and descriptions of materials need in this lab experiment. 1 1 X X 1 1 2 1 Power Supply 0 – 12V DC at 1 Ampere DMM or VOM Miscellaneous resistors, (values to be determined) Assorted switches DPDT 12V DC relay 2N3904 or 2N2222 NPN Bipolar Transistor 12V DC incandescent Lamps Protoboard and connecting wire Discussion Fundamental Circuits Analysis Lab T he purpose of this laboratory experiment is to give you an introduction to some of the basic electromechanical devices used in industry. In this experiment we will investigate basic mechanical switches, electrical relays, and a transistor used as a solid-state switch. A switch is a device for making or breaking an electric circuit or they maybe used for selecting between multiple circuits. Switch types are mechanical, electrical, or electronic. In this experiment we will be looking at the mechanical, electrical, and electronic switches. Each switch type can come in many different switching arrangements which are defined by their contacts. Some contacts are normally open until closed by the operation of the switch, while others are normally closed and then opened by the operation of the switch. There are also two important terms pole and throw which are used to describe switches. A pole refers to the movable arm located inside the switch. A throw is one or more positions/contacts that the switch can select. There terms are usually abbreviated along with additional letters and numbers to define the switching modes. Let’s take a look at a few examples to get a better understanding of switch labeling. The simplest switch configuration is the single-pole-single-throw switch and that would be abbreviated as SPST. This switch type is used to simply open or close a circuit and its schematic symbol is show in Figure 4-1 (a) along with a picture of a SPST toggle switch shown in Figure 4-2. A single-pole-double-throw is shown in Figure 4-1 (b). We can see that this switch has one moving arm, (single pole) and two contact positions, (double throw). The switch shown in Figure 4-1 (c) is a DPST and the dashed line between the two arms indicate that they are mechanically but not electrically linked. This indicates that the actuator or lever on the switch moves both arms at the same time. It is common to use a slightly different labeling scheme for rotary switches and they also add another term known as timing. The switch shown in Figure 4-1 (g) is a rotary switch and would be labeled as a SP4P switch. The SP4P abbreviation means single-pole-four-position. The timing terms are nonshorting – break before make or shorting – make before break. What this means is as the movable arm is going to the next position it will either make an electrical connection to the new contact before disconnecting from the old contact, (make before break) or it will first disconnect from the old contact before making an electrical connection to the new contact, (break before make). Different applications require different timing switches and in instrumentation and control this can be a very important factor. The rotary switches used in this experiment are non-shorting to control the intensity of the lamp. Fundamental Circuits Analysis Lab (a) (e) (b) (f) (d) (c) (g) Figure 4-1 Switch Symbols Toggle Slide Rotary Push-button DIP Figure 4-2 Sample Switches Fundamental Circuits Analysis Lab Once we have made a particular switch type selection we should consider at a minimum, the voltage and current rating of that switch. As an example the following miniature toggle switch has the following characteristics: SPDT, ON-OFF, 5A-120V AC / 28V DC, 2A-250 AC, Initial Contact resistance 10 mΩ [Note: That is milli lowercase m not Mega uppercase M.], Insulation Resistance 1000 MΩ, Silver Plated Contacts. Notice that the current rating for the switch is different depending on whether the switch will be operating in an AC or DC circuit. We will be discussing this difference in a later experiment and for now we are only interested in the DC rating because we will be using a DC power source. Another factor that affects the current rating of the switch is what type of load the switch is controlling. If it is purely resistive we can use the specified value. However if it is an inductive load (motor) or an incandescent lamp we need to de-rate the value by 25% or 75% respectively. Most damage to switches is due to overheating which is caused by too high a contact resistance and/or too large of a current following through the switch. Also too high a voltage that causes arcing when the switch breaks the circuit can damage the switch contacts. We can check a switch by measuring its on and off resistance using an ohmmeter. The contact resistance in the on position should be very low… much less than one ohm while the off position should show an extremely high resistance usually beyond the measurement capabilities of the ohmmeter. Many digital ohmmeter will display this by either a flashing display, the letter OL, or an ∞ sign. Electromechanical relays are devices that complete or interrupt a circuit by physically moving electrical contacts into contact with each other. A relay involves two circuits: the energizing circuit and the contact circuit. The coil is on the energizing side and the relay contacts are on the contact side. When a relay coil is energized, current flow through the coil creates a magnetic field. Whether in a DC unit, where the polarity is fixed, or in an AC unit where the polarity changes 120 times per second, the basic function remains the same: the magnetic coil attracts a ferrous plate, which is part of the armature. One end of the armature is attached to the metal frame that is formed so that the armature can pivot, while the other end opens and closes the contacts. Relay contacts are designed, built and specified for the type of application for the relay. No single voltage and current rating applies to a given set of contacts under all circumstances. In this laboratory experiment we will be using a miniature 12V DC, DPDT relay. This relay can be plug directly into your protoboard. Transistors have many uses, and one is the ability for bipolar or field effect transistors to turn devices on and off. While there are limitations as to what the transistor can switch on and off, transistor switches offer generally lower cost and substantial reliability over conventional mechanical relays. Single transistor by themselves do not have a latching characteristic and when the transistor input control signal is removed they will stop conducting. Latching is the characteristic of memory and in this instance means that once the device is triggered or turn on it will normally stay on even if the control signal is removed. A complete discussion of transistors operation is beyond the scope of this lab manual and you will need to take an additional class in solid-state devices. For this lab experiment we just need to know that when sufficient voltage and current are applied to the Base (B-termial) of the transistor it will conduct current like a closed switch through its Collector (C-terminal) and Emitter (E-terminal). When the voltage and current are removed from the Base circuit the transistor will stop conducting between the C-E terminals. Fundamental Circuits Analysis Lab Pre-Lab 1. Review Ohms Law and the power formulas. 2. Using the Internet go to Mouser Electronics at: http://www.mouser.com/ or your instructor may request that you visit another site. At the Mouser home page select electromechanical for the products links area. Locate an Alpha Rotary switch that is shorting, ¼-inches round shaft w/solder terminals, and has 4 poles and 3 positions. Recorded the Mouser Stock Number of this switch: ______________ 3. Using the Internet or a transistor manual located in the laboratory determine the three lead designations for a 2N3904 or a 2N2222 NPN bipolar transistor. 4. Visit GlobalSpec at:: http://relays.globalspec.com and read their learn more information on electromechanical relays. Procedure Section I Switches 1. Reference the textbook or the Internet and complete Table 4-1. 2. Use an ohmmeter and measure the resistance of the miniature SPDT toggle switch labeled 1 in its position. Note: Position is with the lever of the switch in the up position view from the back. If the resistance reading indicates no continuity record the word “open” in row 1 of Table 4-2. 3. Repeat step 2 for the switch in row 2 of Table 4-2. 4. Repeat step 2 for the switch in row 3 of Table 4-2. For this switch you will rotate the selector shaft clockwise looking from the back and verify the resistance readings supports good continuity for lugs A through D with respect to E. 5. Repeat step 2 for the switches in rows 4 and 5 of Table 4-2. For these switches position is with the button released or not pressed and position is with the button pressed in. 6. Using the switches provided and with the B1 power supply off and disconnected, construct the first circuit shown in Figure 4-3. 7. With the power supply still disconnected turn on the supply and adjust it for 12V DC and set the power supply’s current limiter control to its 12:00 o’clock position. Turn the power supply off and then connect it to the circuit. Set the SPST switch S1 in the circuit to position 1, open and then turn the power supply on. 8. Measure the voltage across the switch terminal lugs and then across the L1 lamp. Record your voltage measurements and the lamp’s state on or off in Table 4-3. 9. Close switch S1 by setting it to position 2 and repeat the above step. 10. Turn the power supply off and then replace SPST switch S1 with a PBNO switch as shown in Figure 4-4. Turn the power supply on and record your results in Table 4-3 for the PBNO switch. Position 1 is with the switch not depressed and position 2 is with the switch depressed. Fundamental Circuits Analysis Lab 11. Turn the power off and replace the PBNO switch with a PBNC switch then turn the power supply on and record your results in Table 4-3 for the PBNC switch. Position 1 is with the switch not depressed and position 2 is with the switch depressed. 12. Disassemble the circuit in Figure 4-4 and construct the circuit in Figure 4-5. Given that L1 is a 12 V DC, 40mA Lamp solve for the values of R1 and R2 such that R1 limits the series circuit current to 10 mA and R2 limits the series circuit current to 20 mA. S1 is a four position non-shorting rotary switch. When you have solved for the resistor values determine the close %5 tolerance resistor values and then build and apply 12v DC to the circuit. In S1’s position 1 the circuit is open and no current flows. In S1’s position 2 the current is limited and the lamp glows dimly. In S1’s position 3 the lamps brightness increases and finally in position 4 the lamp is at normal brilliance. Confirm the proper operation of the circuit and then record voltage measurements across the resistors and lamps in positions 2 and 3 in Table 4-3. In the Lamp state column comment on the brightness of the L1 lamp. When you are finished with this step disassemble the circuit and proceed to the Section I design tasks. Section I Design Tasks 1. Using a DPDT ON-ON switch design a circuit that will reverse the direction of rotation of a 12V DC miniature motor. Devise a method for determine motor shaft rotation. When you have completed the design build and verify the proper operation of the circuit. Demonstrate the circuit operation to your instructor. On a separate sheet of paper draw the schematic circuit diagram and be sure and include reference designators and values for each component. Briefly describe in written on the paper the basic operation of the circuit. Instructor: ________________ 2. Using as a minimum, two SPDT switches design and build a three-way switched circuit to turn on a 12 V DC lamp. When you have completed the design build and verify the proper operation of the circuit. Demonstrate the circuit operation to your instructor. On a separate sheet of paper draw the schematic circuit diagram and be sure and include reference designators and values for each component. Briefly describe in written on the paper the basic operation of the circuit. Instructor: ________________ Section II Relays 1. Construct the circuit in Figure 4-6 and then apply 12V DC and 24 volts DC to the circuit from the laboratory tri-power supply. Make sure that both current limit controls are at 12 o’clock. Operate S1 and verify that the relay closes and supplies power to the two series wired lamps L1 and L2 . Fundamental Circuits Analysis Lab 2. Do not disassemble the circuit and proceed to the Section II design task. Section II Design Tasks 1. Using the circuit in Figure 4-6 redesign it using two momentary action pushbutton switches so that one switch latches the lamps on while the other switch latches them off. You are encouraged to work in teams and solicit help from the instructor. [Hint: Use the additional contacts on the relay.] On a separate sheet of paper draw the schematic circuit diagram and be sure and include reference designators and values for each component. Briefly describe in written on the paper the basic operation of the circuit. Instructor: ________________ Section III Solid State Switch 1. Construct the circuit in Figure 4-7 and then apply 5V DC and 24 volts DC to the circuit from the laboratory tri-power supply. Make sure that both current limit controls are at 12 o’clock. Operate S1 and verify that the transistor conducts and supplies power to the two series wired lamps L1 and L2 . Fundamental Circuits Analysis Lab Post-Lab 4 Name: _____________________________________ Score: _______________ Date: _______________ Team: _______________ Questions: 1. In your own words and using sketches explain what a shorting rotary switch is. 2. What does SP12P mean for a switch? 3. A ¼ HP, 48V DC motor is controlled by a SPST toggle switch. What should the minimum current rating of the switch be for safe operation? (Hint: 1 HP = 746 watts) 4. List several advantages of using a switch/s and relay to control a circuit load. 5. Why would a non-shorting switch be the best choice in Figure 4-5? 6. For a complete circuit how many wires are needed between two 3-way switches? 7. Name a disadvantage of using a solid-state switch. 8. What is a latching relay or switch? 9. Why is silver or a silver alloy used on many switch contacts? 10. What part of a switch is sometimes referred to as the common? 11. In section 1 - design task 1 how could the circuit be change to allow for a motor off state as well as also selection rotational direction of the shaft? Fundamental Circuits Analysis Lab 12. What is an advantage of a transistor or solid-state switch over an electromechanical relay? 13. Refer to Figure 4-7. Is the transistor Q1 acting as a latching device? Briefly explain your answer. 14. In terms of push-button switches what does “NO” and “NC” represent? 15. List several disadvantages of using incandescent lamps over Light Emitting Diodes in control panel displays? Conclusion: Table 4-1 Component Identification Switch Label Type Number Style 1 SPDT Toggle 2 DPDT Toggle 3 SP4P Rotary 4 NOPB Push-button 5 NCPB Push-button Schematic Symbol Fundamental Circuits Analysis Lab Table 4-2 Switch Resistance Testing Switch Resistance Label Position Number Resistance Position Pictorial Symbol Position 1 Lug A to B = __________Ω Lug A to B = __________ Ω Lug B to C = __________Ω Lug B to C = __________ Ω Lug A to C = __________Ω Lug A to C = __________ Ω Lug A to B = __________ Ω Lug A to B = __________ Ω Lug B to C = __________Ω Lug B to C = __________ Ω Lug A Lug B Lug C Back View Position 2 Lug A to C = __________ Ω Lug A to C = __________ Ω Lug D to E = __________ Ω Lug D to E = __________ Ω Lug E to F = __________ Ω Lug E to F = __________ Ω Lug D to F = __________ Ω Lug D to F = __________ Ω Lug D Lug B Lug E Lug C Lug F Back View Lug B Lug A to B = __________ Ω 3 Lug A Lug C Lug E Lug B to C = __________ Ω Lug A Lug D Lug A to C = __________ Ω Back View Lug D to E = __________ Ω 4 Lug A to B = __________ Ω Lug A to B = __________ Ω Position Lug A Lug B Back View Fundamental Circuits Analysis Lab Switch Label Number Resistance Position Resistance Position 5 Pictorial Symbol Lug A to B = __________ Ω Lug A to B = __________ Ω Position Lug A Lug B Bottom View Table 4-3 Circuit Measurements with 12V DC Power Supply Circuit Switch Voltage Across Voltage Across Figure Type Switch S1 Lamp L1 Position 1 Fig. 4-3 SPDT Position 2 Fig. 4-4 Position 1 PBNO Position 2 Fig. 4-4 Position 1 PBNC Position 2 Fig. 4-5 Position 2 Rotary Position 3 Lamp State Fundamental Circuits Analysis Lab S1 1 2 B1 12V L1 12V Figure 4-3 S1 1 2 B1 12V Figure 4-4 L1 12V Fundamental Circuits Analysis Lab R1 OFF 1 2 S1 3 R2 B1 12V COM 4 L1 12V Figure 4-5 S1 B1 12V 4 6 8 13 11 9 1 L1 12V L2 12V Relay 16 Figure 4-6 B2 24V Fundamental Circuits Analysis Lab L1 12V S1 L2 12V R1 2.2k C B1 5V B E Figure 4-7 Q1 2N2222 / 2N3904 B2 24V