Lab 1: Basic Lab Equipment and Measurements

Abstract: Lab 1: Basic Lab Equipment and Measurements This lab exercise introduces the basic measurement instruments that will be used throughout the course. These instruments include multimeters, oscilloscopes, DC power supplies, and function generators. Although the particular measurement devices used in this lab are not highly sophisticated, the basic operation and measurement concepts presented here are extremely important: this basic knowledge is assumed of electrical engineers regardless of specialization. Introduction: Electrical laboratory work depends upon various devices to supply power to a circuit, to generate controlled input signals, and for circuit measurements. The basic operation of these instruments may seem somewhat complicated at first, but you will gain confidence as your experience grows. Eventually the operation of an oscilloscope or multimeter should seem as natural to you as punching an expression into your scientific calculator. Electrical measurements, like all physical measurements, are subject to uncertainty. The sources of uncertainty include so-called human errors, like misreading a dial setting, systematic errors due to incorrectly calibrated instruments, and random errors due to electrical noise and interference, environmental changes, instrument resolution, or uncertainties in the measurement process itself. An effective engineer needs to keep measurement uncertainty in mind: nothing is gained by performing mathematical operations to 8 significant digits if the original lab data contains, say, only 3 significant digits with +/- 5% error. Thus, it is very important to consider measurement errors when performing laboratory work. Frequently you may find that several possible approaches are available to make a particular measurement. By considering the shortcomings of each measurement technique you may discover that one of the possible methods is better (less prone to error) than the others. Moreover, you may be able to verify a questionable result by choosing alternate measurement methods. For example, a resistance can be measured by supplying a known current and measuring the voltage, or by supplying a known voltage and measuring the current, or by using a voltage divider or bridge containing a known resistance. Multimeter The multimeter is an instrument that can be used to measure voltage (voltmeter), current (ammeter), or resistance (ohmmeter). An ideal meter should not disturb the circuit when taking measurements. For example, a voltmeter should act like an open circuit and an ammeter like a short circuit. However, the meter does have some finite internal resistance and may actually load the circuit being measured by changing the resistance between the test points. The problem of loading a circuit is particularly important when measuring circuits with resistances comparable to the meter resistance. Depending on the type of measurement, for most meters today (especially the digital type) these effects are often negligible. You will learn about loading effects in several labs. It is extremely important to connect the meter correctly depending upon how it is being used. Always ask you lab instructor if you have any questions about how to make a measurement. Refer to Figure 1-1 for correct meter connections. The voltmeter measures the voltage difference between two nodes in a circuit, i.e., the voltmeter is connected in parallel with a circuit branch. The ammeter measures the current through a circuit branch. Therefore, the circuit must temporarily be broken at the point where the current measurement is desired and the ammeter inserted there, i.e., the ammeter must be connected in series with a circuit branch. When inserting and removing the ammeter from the circuit, the power to the circuit must be turned off. Also, be especially careful that the ammeter is connected properly before energizing the circuit. If the ammeter (which has a very low internal resistance) is placed across an element as if to measure voltage (essentially short circuiting the circuit), the current through the meter would be extremely large. This could permanently damage the meter, the circuit, or the student! The ohmmeter is essentially an ammeter and an internal voltage source. It measures the current that the internal voltage source causes to flow in the resistance, then calculates the resistance using Ohm s law. The ohmmeter generally can only be used to measure resistances that have been disconnected from the circuit. It is essential that there are no external power supplies connected to the resistor when measuring the resistance with the ohmmeter. If there is any external power supply connected, this also could permanently damage the meter or the student! file:///h:/dung/research/lab/training/elec233/rlab1.htm 1/5

Figure 1-1: Diagram showing basic meter connections. The particular multimeter used in the lab is autoranging, which means that it will automatically choose the proper display range for the measurement being made. You may encounter other meters that require you to manually set the proper range using a panel switch. To measure an unknown signal with such a meter, you should always start with the highest range setting, then reduce the range setting until you reach the optimum level. Oscilloscope The oscilloscope (or scope for short) is arguably the most fundamental measurement device in electrical engineering. The usefulness of the oscilloscope is due primarily to its ability to display electrical signal information directly in visual form. The most important use of oscilloscopes is in the observation of periodic signals. Repetitive waveforms (oscillations) can be viewed by synchronizing the sweep generator, which controls the speed of the horizontal sweep, with the repetition rate of the input signal. This is accomplished using a trigger generator which starts the horizontal sweep when the input signal exceeds an adjustable voltage threshold. The trigger can be selected to occur for either a positive or negative slope at the threshold voltage. Thus, by adjusting the vertical gain and the sweep speed, time varying input signals can be viewed directly as a voltage versus time display. Oscilloscopes have many other features for producing displays of particular types of signals. For example, most scopes have two or more independent input amplifiers for displaying two or more input signals simultaneously. Some special features will depend upon the instrument manufacturer and the sophistication of the scope itself. Some of these advanced features will be considered later in the course. You are always welcome to look at the manuals or to ask the lab instructor for help with operating the oscilloscope. DC Power Supply Active electronic circuits require a power supply providing electrical energy to operate the circuit. The power supply can be a battery, a DC supply operating from the AC power line, or some other source such as a solar cell, fuel cell, generator, or thermoelectric element. The line-powered DC power supply is used in many of the experiments in this course. It has a floating ground, meaning that it is not internally connected to any of the other instruments. Hence, the supply can be used like a battery. Some DC supplies have current limit to prevent too high currents from damaging the circuit. This current limit is adjusted by means of a current knob. Normally, this knob should be turned completely clockwise which maximizes the current output of the supply. Also, some DC supplies have internal meters which are not very accurate and should not be used. Instead, use the multimeter when adjusting the DC supply. Function Generator The function generator, or signal generator, is another standard piece of laboratory equipment. The function generator produces a number of various periodic voltage waveforms with adjustable frequency and amplitude. Typically, these include sinusoidal, triangular, sawtooth, and pulse wave forms. Resistors Resistors are labeled with a nominal resistance value and a tolerance value. The tolerance means that the actual value of the resistor is guaranteed to be within this amount of its nominal value. For example, a resistor labeled as 1kΩ +/- 5% means that the manufacturer guarantees that the actual resistance will be between 950Ω and 1050Ω. The nominal resistance value and its tolerance are either printed numerically on the resistor body or indicated with color bands on the resistor body. There are certain standard nominal values of resistors that are produced by manufacturers. The range in significant figures from 1.0 to 10 have been divided into 24 steps, each differing from the next by. The standard values for the +/- 5% tolerance resistors are: 1.0 1.5 2.2 3.3 4.7 6.8 1.1 1.6 2.4 3.6 5.1 7.5 1.2 1.8 2.7 3.9 5.6 8.2 file:///h:/dung/research/lab/training/elec233/rlab1.htm 2/5

1.3 2.0 3.0 4.3 6.2 9.1 Figure 1-2: Color bands on a resistor. Figure 1-2 shows how to read the color bands on a resistor. The numerical values corresponding to the colors are: 0 Black 5 Green Tolerance 1 Brown 6 Blue 20% No band 2 Red 7 Violet 10% Silver 3 Orange 8 Gray 5% Gold 4 Yellow 9 White -1 Gold For example, a 1kΩ +/- resistor (10 x 10 2 ) is labeled brown-black-red-gold. Resistors are also available with 1% tolerance. The 1% resistors have five color bands: the first three color bands represent the resistor s value, the fourth is the power of 10, and the fifth band is colored brown. Resistors also have a maximum power dissipation rating. Most resistors that are used in electronic circuits and that you will be using have a 0.25 watt maximum power rating. Resistors with other power ratings are also available. The power rating of the resistor is generally a function of the physical size of the resistor. For specific questions regarding the use of lab instruments, consult the operating manuals for each device. The manuals should be available in the lab, or you can consult your lab instructor. Root Mean Square (rms) Value When we measure constant or DC voltages or currents when we use a multimeter it is clear what we are measuring. But, what are we measuring when we measure a time-varying voltage or current? Consider the sinusoidal voltage shown in Figure 1-3. Figure 1-3: A Sinusoidal Signal The average value for this signal is clearly zero regardless of how large the peaks. Therefore, there is nothing to be gained for the multimeter to display the average value. Instead the multimeter displays the root mean squared or rms value which is defined as: Where T is the period of the periodic signal. As an example let s evaluate this integral for a sinusoidal signal. file:///h:/dung/research/lab/training/elec233/rlab1.htm 3/5

Therefore if we use a multimeter to measure a sinusoidal voltage we won t measure the peak value, rather the value we see is a fraction of the peak value. Note that the expression of the rms value will be different if the signal is a square wave or a triangular wave. Figure 1-4: Voltage divider circuit. Experiment: (1) Select two 1kΩ, two 2.2kΩ, and two 10kΩ resistors. Measure these resistors using the ohmmeter function of the multimeter. Keep track of which resistor is which: you will use the resistors later in the experiment. Make a table listing (i) the nominal value (as given by the color code), (ii) the value measured using the ohmmeter, and (iii) the calculated percentage error given by: Percentage error is an effective quantitative measure that you should use in discussing the results of experiments. (2) Turn on the DC power supply. If there is a current limit knob, turn it to its maximum position. Adjust the voltage knob to give as close to 10.0V as possible. Use the DC volts function on the multimeter to measure the voltage so the supply can be adjusted accurately. Record the obtained voltage. (3) With the supply off, connect the circuit shown in Figure 1-5. Carefully check your connections before turning on the DC supply. An incorrectly connected circuit could damage the multimeter! Turn the supply on and measure the current in the circuit with the DC amps function of the multimeter for each of the resistors from part (1) of the experiment. Make a table listing (i) the DC supply voltage, (ii) the measured resistor values, (iii) the measured current values, (iv) the analytical value of the current if the DC supply was 10.0V and the resistors were their nominal values, and (v) the percentage error between the measured current and the analytical value determined in (iv). Figure 1-5: Circuit diagram for part (3) of the experiment. file:///h:/dung/research/lab/training/elec233/rlab1.htm 4/5

(4) With the supply off, connect the circuit shown in Figure 1-4 using the resistors measured in part (1), where and. For and 10.0 volts, measure and record the voltages and and the current I. Carefully check your circuit connections before turning on the DC supply. An incorrectly connected circuit could damage the miltimeter! (5) Connect the circuit shown in Figure 1-4 with R A =10kΩ and R B =1kΩ and the signal generator as the voltage supply. Generate a square wave, a triangular wave, and a sinusoidal wave. Examine the voltage across the 1kΩ resistor using the oscilloscope with the AC setting. Sketch one cycle of each waveform noting the peak values and the period. (6) Now practice using the oscilloscope and signal generator. Set the signal generator to output a sinusoidal wave. Connect the signal generator to the oscilloscope. Using the scope to measure the amplitude and frequency of the sinusoid, adjust the signal generator so that the amplitude of the sinusoid is 5V and its frequency is 1kHz. Notice how the different controls of the oscilloscope affect the waveform display. Record the vertical and horizontal settings that provide approximately two complete signal periods filling the entire screen. Try different waveforms with different frequencies and amplitudes. Spend some time investigating the different controls of the scope and becoming comfortable adjusting the scope in order to view a waveform. Refer to the manuals in the lab or your lab instructor for help with different controls and features of the oscilloscope. (7) Set the signal generator to output a sinusoid. Record the peak value of the sinusoid from the oscilloscope. Now use the multimeter to measure the voltage across the 1kΩ resistor. How does this voltage compare with the peak value you saw on the oscilloscope? Repeat with a triangular wave and a square wave. Results: (a) From the results in part (1) of the experiment, did all of the resistors measured fall within their specified tolerance? (b) (c) (d) It is important to know the precision and accuracy of any lab instrument. For example, say that when measuring a 10V DC source a multimeter has an accuracy specification of +/- (0.1% + 1 digit) and a resolution of 10mV per digit. This means that the actual voltage lies within the range: {(reading) x (99.9%) 10mV} to {(reading) x (100.1%) + 10mV}. If the meter reading was 10.00 in part (2), what could be the minimum and maximum actual voltages due to meter uncertainty? The range between minimum and maximum represents what percentage of the 10V DC indicated voltage? Repeat these calculations for a meter reading of 3.50V DC assuming the same accuracy and resolution specifications. Discuss the results. From the results in part (3) of the experiment, discuss the difference between the measured currents and the expected analytical currents. Is the percentage error significant enough that a circuit expecting the nominal values would not operate correctly due to the tolerances of the supply and resistors? Explain. Discuss the resolution of the voltage measurements made with the oscilloscope, i.e., how many significant figures can you obtain from the scope display? Is it better to use the multimeter or oscilloscope to measure voltages? Explain. (e) Using the measurements made in part (4) of the experiment, make a plot showing and versus I. (Plot voltages on the vertical axis and current on the horizontal axis). Is the plot linear as expected by the relationship? What is the slope of each line? Did the voltage measurements across the resistors give the expected results? What is the ratio between and? Discuss the results. Remember to use percentage error as a quantitative argument in your discussion. (f) (g) Discuss your experience with the oscilloscope in parts (5) and (6) of the experiment. Briefly discuss how some of the oscilloscope s controls affect the waveform display. Discuss how you used the scope s settings and the waveform display to determine the amplitude and frequency of the sinusoid. Discuss the results of part (7) of the experiment. How close are the measured rms values to what you would calculate from the measured peak values. file:///h:/dung/research/lab/training/elec233/rlab1.htm 5/5