Introduction to the Laboratory

Transcription

1 Memorial University of Newfoundland Department of Physics and Physical Oceanography Physics 2055 Laboratory Introduction to the Laboratory The purpose of this lab is to introduce you to some of the equipment which you will be using throughout the semester. By the end of this session you should know how to: 1. use the low voltage power supply and breadboard to construct a simple circuit; 2. use the digital multimeters to measure voltage, current and resistance at various points in a circuit; 3. debug a circuit if anything appears not to work. Power Supplies The B+K Precision Triple Output DC Power Supplies, shown in figure (1), provide two main supplies with a 0 30 V dc output and a third with a V dc output. The main supplies are adjustable with both coarse and fine voltage controls and are capable of current output 0 2 amps. Two LED meter displays can monitor either the output current or output voltage of each supply. The third has a current output up to 5 amps, allowing it to handle digital logic circuitry. The two main supplies can be operated independently or in one of two tracking modes. In the series tracking mode, the A and B supplies are connected in series, allowing a single output of 0 60 V at up to 2 amps. In the parallel tracking mode, the two supplies are connected together in parallel, allowing a single 0 30 V output at up to 4 amps. Connection is made using either a banana plug inserted into the appropriate + or terminal or by connecting a short wire from the terminal to the breadboard. The central green terminal (GND) will normally not be used. Breadboards The name breadboard comes from the days when electrical components were attached by screws to large wooden boards. Modern breadboards are small plastic rectangles with 1

2 Figure 1: Low voltage power supply used in the laboratory holes for connecting wires. They come in different sizes; the most common size used in this laboratory has two rows each of 50 holes along the top and bottom, and 64 columns of ten holes, separated into two groups of five. Behind each hole is a small spring which contacts any wire pushed into the hole. This spring is connected to neighbouring holes so that there is an electrical connection between wires pushed into adjacent holes. Fig (2) shows a section of the breadboard. Note that the holes are usually connected in groups of five, so that Figure 2: Diagram to show the connections between the holes in a breadboard. you can connect up to five wires at one point. The rows of holes at the top and bottom of the breadboard are normally used to provide high voltage and ground connections. The breadboards are provided with eight dummy terminals which allow you to connect a power supply to the circuit which you have constructed. 2

3 Multimeters A multimeter is the most useful measuring instrument which you will find in the laboratory. It allows you to measure voltage, current and resistance in both AC and DC circuits. Be aware that in AC mode, the multimeter will give a null reading when connected to a DC signal, since there is no time-varying component. Figure 3: Photograph of the Fluke 83V multimeter There are many different kinds of multimeter in common use (digital, analogue, pushbutton or rotary switches). Figure (3) shows the Fluke model 83V meters used in this laboratory. It is important to setup the meter correctly before use to prevent damage to the meter or to the circuit. To measure voltage and resistance simply turn the dial to the required position. The meter automatically chooses the range which gives the most accurate value. Current measurement often causes the most trouble. The ideal current meter has zero resistance, which means that if you put it directly across a power supply, you will burn something up. You must always put the current meter in series with the element through which the current you wish to measure is going. This means that you have to break the circuit and insert the meter so that the current can go through the meter as well as the circuit element of interest. Also, you need to make sure that the + and terminals are connected correctly: when the meter displays a negative value, it means that the direction of the current is opposite to what you thought it was. 3

4 Using the multimeter as a voltmeter when the ammeter function is selected must be avoided. The internal fuse will blow if such an ammeter is hooked across a potential difference of more than a few volts. This includes power supplies. A resistance measurement uses electrical power provided by the meter itself. Do not attempt to measure the resistance of a component when it is still connected to a circuit. To do so would either give a meaningless result, or worse, damage the meter. You must disconnect an individual component from the circuit before measuring its resistance otherwise you will measure the equivalent resistance of the whole circuit. Uncertainties in Multimeter Readings The accuracy of the meter is stated in the form ±([%of reading] + [number of least significant digits]). For example, if the accuracy for resistance is given as ±(0.2%+2), then a reading of kω is uncertain by (0.2%) (2 digits) or ±0.003 kω. Thus the value to be recorded is 673 ± 3 Ω. The uncertainties for the Fluke meters are given in the following tables. Fig (4) shows the errors that should be applied to dc volts and resistance readings, Fig (5) should be used for ac voltages and Fig (6) is used for both ac and dc current measurements. 4

5 80 Series III Users Manual Table 13. DC Voltage, Resistance, and Conductance Function Specifications Accuracy 1 Function Range Resolution Model 83 Model 85 Model 87 L V V V 1000 V V 0.01 V 0.1 V 1 V ±(0.1% + 1) ±(0.1% + 1) ±(0.1% + 1) ±(0.1% + 1) ±(0.08% + 1) ±(0.08% + 1) ±(0.08% + 1) ±(0.08% + 1) ±(0.05% + 1) ±(0.05% + 1) ±(0.05% + 1) ±(0.05% + 1) F mv mv 0.1 mv ±(0.3% + 1) ±(0.1% + 1) ±(0.1% + 1) e ns Ω kω kω kω MΩ MΩ ns 0.1 Ω kω 0.01 kω 0.1 kω MΩ 0.01 MΩ 0.01 ns ±(0.4% + 2) 2 ±(0.4% + 1) ±(0.4% + 1) ±(0.7% + 1) ±(0.7% + 1) ±(1.0% + 3) ±(1.0% + 10) ±(0.2% + 2) 2 ±(0.2% + 1) ±(0.2% + 1) ±(0.6% + 1) ±(0.6% + 1) ±(1.0% + 3) ±(1.0% + 10) ±(0.2% + 2) 2 ±(0.2% + 1) ±(0.2% + 1) ±(0.6% + 1) ±(0.6% + 1) ±(1.0% + 3) ±(1.0% + 10) 1. See the first sentence in Table 11 for a complete explanation of accuracy. 2. When using the REL function to compensate for offsets. Figure 4: Accuracy of the meter for dc volts and resistance 44 Specifications Table 12. Model 83 AC Voltage Function Specifications Function Range Resolution Accuracy 1 K mv V V V 1000 V 0.1 mv V 0.01 V 0.1 V 1 V 50 Hz - 60 Hz 45 Hz - 1 khz 1 khz - 5 khz ±(0.5% + 4) ±(0.5% + 2) ±(0.5% + 2) ±(0.5% + 2) ±(0.5% + 2) 1. See the first sentence in Table 11 for a complete explanation of accuracy. 2. Below a reading of 200 counts, add 10 counts. 3. Frequency range: 1 khz to 2.5 khz. ±(1.0% + 4) ±(1.0% + 4) ±(1.0% + 4) ±(1.0% + 4) ±(1.0% + 4) ±(2.0% + 4) ±(2.0% + 4) ±(2.0% + 4) ±(2.0% + 4) 3 unspecified Figure 5: Accuracy of the meter for ac voltages 5 43

6 Specifications Table 14. Current Function Specifications Accuracy 1 Function Range Resolution Model 83 2 Model 85 3, 4 Model 87 3, 4 Burden Voltage (typical) ma \ (45 Hz to 2 khz) ma [ ma ma 4000 ma A ma ma 4000 ma A ma 0.1 ma 1 ma 0.01 A 0.01 ma 0.1 ma 1 ma 0.01 A ±(1.2% + 2) 6 ±(1.2% + 2) 6 ±(1.2% + 2) 6 ±(1.2% + 2) 6 ±(0.4% + 4) ±(0.4% + 2) ±(0.4% + 4) ±(0.4% + 2) ±(1.0% + 2) 6 ±(1.0% + 2) 6 ±(1.0% + 2) 6 ±(1.0% + 2) 6 ±(0.2% + 4) ±(0.2% + 2) ±(0.2% + 4) ±(0.2% + 2) ±(1.0% + 2) ±(1.0% + 2) ±(1.0% + 2) ±(1.0% + 2) ±(0.2% + 4) ±(0.2% + 2) ±(0.2% + 4) ±(0.2% + 2) 1. See the first sentence in Table 11 for a complete explanation of accuracy. 2. AC conversion for Model 83 is ac coupled and calibrated to the rms value of a sinewave input. 3. AC conversions for Models 85 and 87 are ac coupled, true rms responding, and valid from 5% to 100% of range. 4. See note 3 in Table W 10 A continuous; 20 A for 30 seconds maximum; >10 A: unspecified. 6. Below a reading of 200 counts, add 10 counts. 1.8 mv/ma 1.8 mv/ma 0.03 V/A 0.03 V/A 1.8 mv/ma 1.8 mv/ma 0.03 V/A 0.03 V/A Figure 6: Accuracy of the meter for ac and dc current Resistor Colour Code 45 The resistance of a resistor is indicated by four coloured bands, as shown in Fig (7). Reading from left to right, the first two bands represent the first two digits, the third the multiplier and the fourth band, which is gold or silver, represents the tolerance of the resistor. A B C D Figure 7: Four coloured bands enable the resistance to be determined Denoting the bands by A, B, C and D, then the value of the resistor (in Ω) is AB 10 C where the numerical values of A, B and C are given by the colour code in the table. 6

7 Colour Value Multiplier Tolerance (D) black = 1 gold ±5% brown = 10 silver ±10% red = 100 no band ±20% orange = 1000 yellow green blue violet grey white For example, if the bands on a resistor are coloured yellow (4), violet (7), brown (1) and silver, the nominal resistance is 470 Ω and the tolerance is ±10%. If the third band (band C) is gold or silver, then the value of the multiplier is 10 1 or 10 2, respectively. Five-Band Resistors Modern resistors may have five coloured bands, shown in Fig (8). 1st 2nd 3rd mult tol Figure 8: Five coloured bands are also used to indicate resistance 7

8 Colour 1st band 2nd band 3rd band Multiplier Tolerance (D) black = 1 brown = 10 ±1% red = 100 ±2% orange = 1000 yellow green ±0.5% blue ±0.25% violet ±0.1 % grey ±0.05% white gold 0.1 ±5% silver 0.01 ±10% 1. Hold the resistor so that the tolerance band is on the right. Identify the first three bands write down the numbers associated with those colours. For example, red, green, white corresponds to The fourth colour band is the multiplier. Black corresponds to In practise you can ignore it and simply read the resistance in Ohms. If the multiplier band is different than Black, multiply the reading by the appropriate value. 3. The 5th coloured band corresponds to the tolerance. Brown corresponds to ±1%. Thus red, green, white, black, brown is 259 Ω ± 1% 4. If the resistor has one more band past the tolerance band it is a quality band, specified as the % Failure rate per 1000 hour. Alligator Clips An alligator clip (or crocodile clip) is a spring-loaded clip with serrated jaws, often used to make temporary electrical connections. One of the jaws may have a wire attached or a 4 mm banana plug inserted into it. Alligator clips are useful for connecting components in a circuit to a multimeter for a quick measurement of resistance, voltage or current. However, they should not normally be used in a circuit where you will be taking repeated measurements 8

9 Cat. No. Description Net Price l BU-30TBO Plated Steel $.56 NO. 60 ALLIGATOR CLIP Standard alligator clip in plated steel or solid copper. Outstanding for a wide in an experimental setup. In this case you will need to use proper connecting wires to make range of test work. Convenient knurled thumb grip, barrel connection for banana the necessary electrical contacts. plug. Has small lip for soldering, or wire may be staked in barrel. Strong hinge prevents jaw offset. Standard barrel diameter.161 I.D., 5 16 jaw opening. Length Cat. No. Description Net Price l BU-60 Plated Steel $.31 l BU-60C Solid Copper.61 NO. 60-S SCREW CONNECTION The most popular configuration. A 6-40 set screw eliminates soldering or crimping. Same as No. 60 except for screw jaw opening. Length Cat. No. Description Net Price l BU-60S Plated Steel $.43 l BU-60CS Solid Copper.67 9 NO. 60PR-2 SPECIAL BARREL FOR TEST PROD OR

10 Procedure You will need to think about the best way of presenting your data in your lab notebook. Ideally, somebody should be able to take your lab notebook and duplicate your results using your book only as a guide. Be careful to write everything down in your lab notebook as you perform each step. Don t miss anything out! V A R Using the above diagram as a guide, construct a series circuit using a 5 volt dc source as the power supply and one of the resistors provided as the load. What is the value of the resistance according to the colour code? A represents a digital multimeter configured to measure current. What is the current (in ma)? Use another digital multimeter to obtain accurate values for: 1. the load resistance. Does this agree with the value indicated by the colour code? 2. the source voltage, 3. the voltage across the resistor. Do you expect this to be the same as the source voltage? If not, why not? 4. the current in the circuit. Determine also the uncertainty in each measurement. According to Ohm s Law, the ratio of voltage to current will be equal to the load resistance. How well does your data comfirm this? Increase the source voltage to 12 volts dc and repeat the above measurements. 10

11 Resistors in Series and Parallel Keeping the 12 volt dc source, add a second resistor (of different value) to the circuit so that you have two resistors in series. Calculate the new resistance, given by R = R 1 + R 2, and check it with the DMM. Measure the new current in the circuit. Also measure the voltage across the series combination of resistors and the voltage across each resistor. This is an example of a potential divider (or voltage divider). The voltage across each resistor can be calculated from the potential divider formula, so that the voltage across R 2 is given by V out = V in R 2 R 1 + R 2 (1) Calculate the voltages you expect and compare these with your measured values. Reconfigure the circuit so that the resistors are now in parallel. The new resistance is given by R = R 1 R 2 /(R 1 + R 2 ). As before, measure the total resistance, the voltage across the parallel combination, the current in the circuit and the current in each resistor - you will need to think carefully about where to connect the ammeter in the circuit for these last measurements. Results and Discussion Tabulate your results carefully, and discuss as fully as possible the relationship between the source voltage, resistance, the voltage across each resistor and the current(s) in the circuit for both the series and parallel configurations. 11