Lab Guidance: Measuring Voltages and Currents in Electrical Circuits - Prof. Timothy M. Jo, Lab Reports of Physics

Download Lab Guidance: Measuring Voltages and Currents in Electrical Circuits - Prof. Timothy M. Jo and more Lab Reports Physics in PDF only on Docsity! PHYSICS 536 GENERAL INSTRUCTIONS FOR LABORATORY A. INTRODUCTION A teaching laboratory has special problems. You cannot assume that components and instruments are working correctly. A preceding student may have returned a faulty item to storage without recognizing the problem. Diodes, transistors, integrated circuits, and electrolytic capacitors are usually reliable Always report suspected instrument problems to the instructor. Learn the limitations and capabilities of the instruments so that you get good results. The equipment is not fragile, but it can be damaged. The following are common mistakes. 1. Instruments pushed of tables. 2. Dropped scope probes. 3. Ignoring voltage and current limitations of instruments. For example, be sure the analog meter is on the correct scale before it is connected to the circuit. 4. A bright, stationary spot on a scope face will leave a permanent mark. Since there are a large number of students using the laboratory, everyone must help keep the lab in order. B. LAB REPORTS. Lab reports should be brief and concise. Often it is sufficient to write the expected and observed value without comment. Arrange the results clearly to help the grader. Other sources should be identified; for example, tables in the text, manufacturers specifications, general reference values from the lecture notes, etc. Formal error analysis is not required. However, you need to learn the accuracy of various components so that you know when your results are reasonable. An unreasonable measurement should be recognized in the lab and resolved with the aid of the instructor, if necessary. If the problem is not resolved, it should be described in your report. Graphs, sketches, and tables should be included when requested. The following material is not required. 1. Repeat of homework calculation. 2. Instruction form the experiment description. 3. Circuit diagrams. 4. Description of what you did, e.g. I connected the green wire to the blue terminal, etc. You may include materials of this type if you wish, but it will not improve your grade. C. GENERAL INSTRUCTIONS. The general instructions given below will be used in many experiments. They are numbered for easy reference in the experiment instructions. These references are essential, and you should not proceed in an experiment without understanding the assigned instructions. Instructions that are not needed in the first lab period are marked with an asterisk. You can study these instructions when they are referenced in subsequent experiments. 1.1 CIRCUIT CONSTRUCTION. Most circuits are assembled using sockets mounted on a metal box. The white sockets running across the short dimension of the board are connected together in sets of five, separated by the center division. Connection errors are the most common problem in the lab. Be sure you understand the socket arrangement, and check the connections first if the circuit does not work. Arrange the circuit similar to the diagram in the instructions to avoid confusion. This also helps the instructor to find errors if you need assistance. In most experiments, it is not necessary to cut leads if the circuit is neat and safe. However, you may cut leads to improve the arrangement of components and avoid accidental contact between components. Some circuits require special component arrangements, which will be specified. Do not force wire into the plug-in sockets. The wire can go between the metal part of the sockets and the plastic wall, which damages the socket. Try wiggling the wire in the socket until it goes in easily. With practice, you can tell when the wire is going in correctly. You should not insert large diameter wire into the sockets, for example, the meter probes. Connect the meter probe to a component lead of a short wire. Large wire can be forced into the socket, but then it will not make good contact when a normal size wire is used. 1.2 VOLTAGE AND COMMON. There are three sets of colored posts with sockets (red, green, and black). The post and all sockets of the same color are connected together. In addition, the black set is connected to the metal box. The red and green are used to distribute voltages, and the black is used as the circuit common. 1.3 SIGNAL CONNECTORS. Two coaxial connectors are mounted on the box. Each has a wire that can be connected to the white sockets. These connectors are used to connect the circuit to the signal generator and scope. 2.1 METERS. There are several types of meters used in the lab; a hand-held digital multimeter and a benchtop digital multimeter. Familiarize yourself with the various measurement options and ranges for each of these options. The best measurements are made when the appropriate range is selected. These meters are described in more detail in the First Laboratory instructions. The clips on the meter leads should be positioned carefully so that they do not cause connections between components. The T can be disconnected form the scope without affecting the signal going to the circuit if both scope channels are needed to observe signals in the circuit. 4.4* EXTERNAL SOURCE RESISTANCE. The model used to represent a voltage source has a constant voltage applied to a series resistor (rs). We create that pattern in real circuits by connecting the signal generator to the circuit through a real resistor Rs. The amplitude of the signal applied to Rs is adjusted (if necessary) to keep it constant. 5.1 OSCILLOSCOPE. The oscilloscope requires more operator skill than the other lab instruments. You will have ample opportunity to become familiar with a scope during the first lab period. Study the written material, experiment with the controls, and ask questions until you can use the scope correctly. 5.2 INPUT OPTIONS. The vertical inputs have three options; AC, DC, and ground. AC places a capacitor in series with the signal. The scope displays changed of the input voltage but is insensitive to the average voltage. This mode is convenient for most sine wave measurements. It must be used to observe small variations of large voltages. The capacitor will produce a DC level shift for pulses (see notes Section 3.04). 5.3* DC INPUT. The DC mode uses a direct connection rather than a capacitor in series with the input signal. Therefore the scope displays the total voltage relative to its common line. This mode is convenient when you want to observe variations and the voltage relative to a fixed reference. The input is connected first to the reference, and the vertical position control is adjusted until the trace is on a convenient horizontal line. Then the scope input is connected to the signal. The relationship between the signal and the reference is given by the trace relative to the selected horizontal line. The ground mode is provided for convenience when the reference is zero volts. This mode switches the vertical input from the front panel to the scope common and opens the connection to the front panel inside the scope after the vertical position is adjusted. When the zero volt reference is set, the vertical position of the scope can be used as a DC voltage meter. 5.4 PROBES. The signal can be connected to the scope through a probe or a coaxial cable. The probe has the advantage of adding only a little capacitance to the circuit (<10pf), but it attenuates the signal by a factor of ten. The equivalent resistance of the probe is 10M. The attenuation is necessary to reduce the capacitance at the probe tip as explained in text appendix A and in notes 3.10.2. Amplitude measurements would be very inconvenient if the attenuation of the probe depends on the frequency of the signal. The probe is tested by using it to observe a square wave on the scope. (A suitable signal is provided at a front panel connector on the scope.) If the attenuation is independent of frequency, the square wave will not be distorted as shown below. A capacitor in the probe is adjusted to remove the distortion. If the ground lead of the probe is not connected to the ground of the circuit, a large 60 cycle signal can be observed on the scope. Unfortunately, the ground leads tend to break inside their insulation, which can produce this effect. Check the ground lead if you see 60 cycle on the scope. 5.5* COAXIAL CABLE. When the signal observed is very small, a probe cannot be used because it reduces signal amplitude by a factor of 10. (Non attenuating probe can be purchased, but we do not use them in lab). For small signals, a coaxial cable is used to connect the scope to the circuit through one of the connectors on the metal box. The disadvantage of the cable is that it adds capacitance to the circuit, typically 20 pf per foot of cable. Since this capacitance can have several adverse effects, a cable should not be used unless it is essential because of small signal amplitude. 5.6* SINE WAVE AMPLITUDE normally is specified as peak-to-peak, because that is the easiest quantity to measure on the scope. If effective or peak values are used, they will be labeled clearly. Adjust the vertical sensitivity of the scope so that the sine wave covers several centimeters on the display to improve accuracy when the amplitude is measured. When both channels of the scope are used, connect them to one signal initially to insure that they have the same gain. Sine wave amplitude measurements are inconvenient when the peaks are far apart. The horizontal and vertical position must be adjusted to get the peaks close to the grid marks on the scope face. It is more convenient to use a slow horizontal sweet to bring the peaks close together so that there is always some peak close to the grid marks. At very slow speed the signal looks like a continuous band across the scope, which is a convenient display for amplitude measurements. However, you must change the sweet to check the form of the sine wave often enough that you are sure it is not distorted by some fault in the circuit. 5.6A* THE BREAK FREQUENCY is determined by observing the gain (vo/vi) as a function of signal frequency. (vi and vo are the amplitude of the input and output signals respectively.) The gain is constant in the mid-frequency region, but it decreases by 30% at the break frequency. Use a mid-frequency signal to adjust the vertical gains of the scope until the vo amplitude is three divisions and the vi amplitude is 3 divisions (leaving one division to separate the two signals). The fine gain knobs can be adjusted because we are interested in the change in vo/vi, not the actual amplitudes vo and vi. Change the input signal frequency until vo drops from 3 to 2.1 divisions, i.e., by 30% which is the break frequency. If the signal generator is working correctly, the amplitude of vi will be constant when the frequency is changed. Nevertheless, it is good practice to monitor the input signal. (The amplitude of vi will appear to decrease above 1MHz because of the high-frequency attenuation in the model 1222 scope. This attenuation is the same in both scope channels, hence it does not affect the ratio vo/vi. The most convenient way to deal with this scope attenuation is to increase the amplitude of the signal from the generator to maintain the three division display of vi on the scope). When the break frequency measurement is completed, return the vertical fine gain adjustments to the calibrated position, so that the scope is ready for actual (rather than relative) amplitude measurements. 5.6B* PULSE TIME CONSTANTS. You can use the time for a pulse to change by 60% to measure the pulse time constant. when /1 0.6, 1.09te t /1 0.6, 1.09te t Adjust the scope so that the pulse amplitude is five divisions, and the horizontal trace is on a line three divisions below the scope center. Then t is the time for the pulse to rise to the scope center. 5.7* CONTINUOUS VERTICAL GAIN. This adjustment is a common source of error. A previous user may have left the control in the uncalibrated position, so get in the habit of checking before you make measurements. The continuous control can also be rotated accidentally if the concentric knobs on the gain control are improperly mounted. If you observe this condition, report it to the instructor. 5.8* DUAL TRACE. Dual trace permits you to see two signals on the scope at the same time. Before making measurements, the same signal should be observed on both channels to be sure that they have the same gain. The trigger is taken from the A channel, or external trigger can be used. 5.9* SUM AND DIFFERENCE. The signals applied to the two input channels can be added inside the scope and displayed. One channel can be inverted so that the display is the difference between the two input signals. Some scopes only provide the differential mode, so the inversion is automatic. On others the inversion is manual. 5.10* EXTERNAL TRIGGER. Internal trigger is normally used so that the horizontal sweep is started by the observed signal. However, time relations can be effected in this mode by variations of the observed signal. This problem is avoided my carrying a stable signal from the generator directly to the trigger through the external connector. The 1V sine wave from out generator is suitable. The trigger level and slope controls have their usual functions in the external mode. 5.11* PHASE MEASUREMENTS. Use the step and continuous controls to adjust the sweep until one complete cycle covers eight horizontal divisions. Then each division represents 45º of phase shift. 5.12* OVERLOAD DISTORTION. A large input signal can deflect the oscilloscope trace off of the screen. If the input signal is very large (approximately 10 times larger than can be displayed), the trace will be distorted when it returns to the scope face. The The BJT should be checked on the medium resistance scale. (On the high resistance scale, the emitter-base junction will conduct in both directions because the meter battery voltage is too large. The meter battery is too small on the low resistance scale.) 10.3* JFET TESTS. The gate-to-channel junction should be tested on the high resistance scale like a diode. The source or drain lean can be used for the channel. The polarity of the two types of JFETs is shown below. The channel between the source and drain should act like a resistor. Connect the gate to source using one of the clips on the meter lead. Then connect the other lead to the drain. Use the x100 meter scale. The resistance of the channel should be approximately 200 ohms or less because vgs=0. The channel is not polarized. 11.* The arrangement of the IC pins is shown in the top view below. A notch or a small circle are used to distinguish the two ends. 12.1* A COMMON REFERENCE is needed between voltage sources and measuring instruments. The following illustration shows a positive voltage applied to the circuit through the red terminals. The negative terminal of the source must be connected to the circuit common, which is the black terminals and the metal box (see GI-1.2). A voltage in the circuit can not be measured by connecting the input lead of the meter to a point in the circuit. The meter-common and circuit-common also must be connected as shown by the dotted line. 12.2* GROUND. When the metal box is sitting on the table by itself, it is as the same potential as the earth. There is sufficient conduction through the table and building to remove and charge from the box. Under normal conditions, this situation is unchanged when the voltage source is attached! The voltage source created a potential between the positive and negative terminals, not between terminals and ground. The charge from the source flows around a closed loop (out through the positive terminal and in through the negative), so none is collected on the circuit or the box. However, there is a potential flaw in the system. A leak through the electrical insulation in the voltage source can produce a potential between the source and ground. This is prevented by providing a good path from the source to ground through the third wire on the AC plug. When the source is protected in this way, the voltage between box and earth is not affected by the presence of the source. A wire between circuit common and ground is not needed, although it does no harm. 12.3* AN OSCILLOSCOPE is shown in the next illustration. The common of the scope must be connected to the circuit common. If two probes are used, only once connection to common is needed. (Although this is satisfactory for our lab, a connection to common should be used for each probe when high-frequency signals are observed). The scope is grounded through the third wire of the AC plug to protect against leakage in its power supply. The scope common is grounded through the scope, hence the circuit common also is grounded through the scope. 13.1* BY-PASS CAPACITORS. One voltage source usually is connected to several parts of the circuit as shown below. Current always flows around closed loops. If C were not present, the current from part-B would have to flow completely around part-A to complete a loop. For DC and low frequency, current from part-B does no effect the voltage across part-A, because the voltage source and connecting wires have negligible resistance. At high frequency, however, the inductance of the long wires creates impedance, and the long path serves as an antenna to send and receive high-frequency noise. A by-pass capacitor from the voltage line to common solves the problem by providing a short, local path for the high-frequency current, it also should have a by-pass which provides the shortest practical closed loop. The capacitor should be large enough that it presents negligible impedance for the frequency range it must handle. In lab, a 0.1uf capacitor should be connected from the voltage distribution lines (red and green) to common. Usually one capacitor at the post edge of sockets is sufficient for each voltage line. (For the red line, connect the capacitor to a white socket and then continue to the common with a direct wire.) However, if the circuit oscillates, you should connect a capacitor to ground from a point on the power line very close to the transistor. By-pass capacitors usually will not be shown on lab circuit diagrams to help you develop a habit of including them in circuits. 13.2* BY-PASS CAPACITORS WITH RESISTORS. Another illustration of short AC loops is shown below. R1 is included with the C1 by-pass to direct the high-frequency current from circuit-A. The capacitor provides the low impedance path and the resistor inhibits the current from going onto the voltage line where it could interfere with the other circuits. R1 and C1 also act as an AC voltage divider to prevent noise on the voltage line from affecting circuit-A. R1 can be replaced by an inductor if low DC resistance is needed between circuit A and