ES 3: Introduction to Electrical Systems
Laboratory 1: DC Measurements
I. GOALS:
In this lab you will learn how to measure DC voltages, DC currents, and electrical resistance. You will create an Excel spreadsheet to graph data and use the “trend line” feature to find the best fit line to experimental data. We will investigate the statistical variation of resistance values and use Excel to quantify this variation.
Many sensors use DC resistance to translate a physical measurement (for example, temperature) into an electrical signal. You will investigate light sensors (photoresistors), temperature sensors (thermistors), and angular position sensors (potentiometers).
Finally, we will learn that wire has a small but measurable resistance. An audio speaker is one application in which the resistance of wire is important. You will measure the resistance of very thin magnet wire, and then determine the correct length of wire needed to produce standard 8and 16speaker voice coils.
II. PRE-LAB QUESTIONS:
Before coming to lab, you must complete the Pre-Lab exercises. The Pre-Lab work is due at the beginning of the lab period.
First, read the entire lab manual for Laboratory #1. It is important that you understand the instruments and measurement methods before coming to lab so that you can complete the experiments efficiently.
After reading this manual, answer the following questions:
- What is the color code used to denote a 47,000Ω (±10%) resistor?
- An air conditioner requires 12 amperes (rms) of current. A non-engineer decides to connect the a/c to an electrical outlet in another room using a 50-meter long extension cord. The extension cord is made using AWG-16 wire (see Section V). What is the totalresistance of the extension cord (all 100 meters)? What is the power dissipated within the extension cord when the a/c is running? Remember: P=I2R.
III.EXPERIMENTS:
Before starting to measure DC voltages, you need to get familiar with DC voltage sources. In this lab, you will use the Regulated DC Power Supply, Model 3010, by R.S.R. Electronics that is shown in Figure 1.
Figure 1. The DC power supply
This power supply contains three independent voltage sources. The leftmost voltage source produces +5 volts. This voltage appears across the red connector (+5V 3A) and the black connector (COMMON). The current from this source depends on the resistance of the device being powered, but the maximum current is electronically limited to be less than 3 amperes.
The power supply also contains two adjustable DC voltage sources. One source produces positive voltages (+1.5 to +15V, red banana plug) and the other source produces negative voltages (-1.5 to -15V, yellow banana plug). The output voltage of each source can be adjusted with one of the two knobs. The current depends on the resistance and the voltage setting, but the current may not exceed ±1 ampere.
The schematic diagram of the 3010 power supply (Figure 2) shows the three ideal voltage sources. The COMMON terminalsof the variable voltage sources are connected together. Note that the symbol for an ideal voltage source that can be varied includes a diagonal arrow.
Figure 2. The schematic of the DC power supply.
A. Measurement of DC Voltage
A voltmeter measures the difference in voltage between two nodes in a circuit. In this lab you will use a digital multimeter (Figure 3). This instrument contains several different meters in a single instrument: voltmeter, ammeter, ohmmeter, and a frequency counter.
Figure 3. The multimeter (voltmeter, ammeter, ohmmeter, and frequency counter)
To measure DC voltages, press the button labeled . Insert red and black test wires into the “V Hz” and “COM” jacks on the front panel of the voltmeter. Attach the test wires to the +5V and COMMON terminals of the power supply. Now, you may need to select the correct RANGE for the voltage measurement. Press the RANGE button for 20; this means that the largest voltage that you can measure is 20 volts. (Note that pressing 2 would not work because you are measuring 5V!). The “V Hz” input is always considered to be the positive input: a positive voltage reading means that the node voltage read by the “V Hz” input is greater than the node voltage read by the “COM” input. The meter should tell you that the voltage is 5.0 volts; if you switch the two input wires, the meter reads -5 volts. To avoid confusion, most people use red wires and red connectors to indicate positive DC voltages. Black is often used to designate COMMON or negative voltage.
IMPORTANT TIP: To make the most accurate measurement, you should always select the lowest possible RANGE. To demonstrate this principle, hit the 200 button and observe that the measurement is less accurate. (Now return the RANGE to 20!).
Use the voltmeter to adjust the two variable voltage sources to +6 volts and -12 volts (within ±0.1 volt). Record these two voltages. Now measure the voltage produced by both voltage sources in series. (Measure from the RED to the YELLOW connector). Record this voltage and explain your result using Kirchhoff’s voltage law.
B. Measurement of DC Current
The measurement of an electric current is performed using an ammeter and is quite different than the measurement of voltage. When we speak of current, we are referring to the flow of charge through a circuit element. This means that we must place the ammeter in series with that circuit element. Usually placing an ammeter in a circuit involves breaking a connection and inserting the meter, as shown below:
Figure 4. To measure current (left), it is necessary to open-circuit the branch to be measured and insert an ammeter (right)
To use the multimeter as a DC ammeter, press the Function button marked . The RED test lead must be inserted into either
(1) the jack marked “mA” for low current measurements, or
(2) the jack marked “10A” for high current measurements.
In this lab, all of our currents are less than 1 ampere, so use the “mA” input. The other ammeter connection is the black COM jack, just like the voltmeter. Next, select an appropriate range, choosing the smallest range possible to maximize the accuracy of your measurement.
Positive and negative currents: Current that enters the “mA” jack flows out the “COM” jack and provides a positive reading on the meter. If the current flows from “COM” to the “mA” jack, then the meter displays a negative current.
Turn the DC power supply OFF. Create the circuit shown on the right hand side of Figure 4. The ammeter has a fuse to protect against accidental overloads. If you blow the fuse, you will be substantially delayed in completing the lab! Ask one of the TAs to check your circuit before turning the DC power supply ON to avoid this delay. Then proceed to the following questions:
Measure the current that flows through a 1k resistor (Figure 5) when the resistor is connected toa voltage source of 5V. Use the 5V output of the 3010 power supply. Refer to the circuit diagram above for guidance when you set up the circuit. Record the following data: The measured voltage produced by the power supply and the measured current that flows through the 1k resistor. Verify that this result obeys Ohm’s Law: V=IR
Figure 5. A 1000Ω resistor, ±5% (code: brown-black-red-gold[1])
How is current measured by an ammeter? Actually, it is not difficult. A simple digital ammeter consists of a small resistor (Rs) and a digital voltmeter in parallel. As current (I) flows through the small resistor (Rs), the voltage that forms across Rs is measured by the voltmeter (see the figure below). The current is displayed as I = V/RS.
An ammeter in a simple circuit The actual measurement uses a resistor and a voltmeter
Figure 6. A digital ammeter measures the voltage that forms across a small resistance RSto determine the current that flows through the meter.
If your ammeter is set to the 20mA range, then Rs = 10. If a current of 5 mA is flowing through the resistor, then the voltage measured across RS is I*RS = 50 mV. The meter then divides the measured voltage by Rs = 10Ω, and displays 5 mA.
If you accidentally attempted to measure 10 amperes of current using the “mA” input of the ammeter with the RANGE set to 20, what would be the power dissipated in RS? If RS is designed to withstand 1 W of power dissipation, what would happen to RS in the accidental situation above? Note: Most current meters have a fuse to protect the ammeter from such large currents.
C. Measurement of DC Resistance
According to Ohm’s Law, we can determine the resistance of a circuit element if we know the voltage across the element and the current through the element: R = V/I.
Use the DC power supply, a 1k resistor, and the multimeter to measure the actual resistance of the 1k(±5%) resistor. The circuit diagram for this measurement is shown in Figure 7. Record the voltage measured across the resistor, the current flowing through the resistor, and calculate its actual resistance. Please don’t forget to include units in your lab report (V, mA, k).
If you are having difficulty making the proper connections, you can refer to the photographs in the Supplementary Materials section. Try to hook-up the circuit without looking at the photo first. This is good practice, and in the future the lab manual won’t provide photographs!
Figure 7. Schematic for the Measurement of the DC Resistance of a Resistor using a Voltmeter and an Ammeter
Often a single measurement may suffer from errors, so it is arecommended engineering practice to make multiple measurements. Using the circuit above (Fig. 7), measure the current that flows through the 1k resistor for at least five (5) different voltages. Spread out the measurements between voltages 1.5 volts to 15 volts.
Using the lab computer, start the spreadsheet software called Excel. Enter the data into the spreadsheet in two columns, one for the measured voltage and one for the measured current, as shown in Figure 8.
Next, make a graph of the data by selecting Insert>Chart… from the top menu bar. Choose “XY(Scatter)” as the Chart Type and hit Next.
Figure 8. Excel spreadsheet with voltage and current measurements.
Next, select the data that you wish to plotby clicking and dragging the mouse to form a box around the data as shown in Figure 9.
Figure 9. Graphing data requires the selection of the data cells.
Finally, click Next> and then add a title and label the axes of the plot (including units). Click Finish to add the plot to your spreadsheet.
Figure 10. Adding a title and labels to the axes (including units!)
The experimental measurements should fall in a straight line. To find the equation for this line, you can add a “trend line” by choosing Chart>Add trendline… from the top menu bar. Choose a Linear trendline as seen in Figure 11.
Figure 11. Adding a “best fit” trendline to your data. Technically, this is called linear regression.
Finally, add the equation to the graph by clicking on the Options tab (Fig. 11) and using the checkbox for Display equation on chart (Fig. 12).
Figure 12. Displaying the equation for the trendline on the chart.
Include the spreadsheet and a description of the experiment in your lab report. You can save the entire spreadsheet, or you can take a screen shot (hit the Prnt Scrn key), then paste the screen shot into a Word file (ctrl-v). Send this file to yourself by email or save it to your memory stick.
The Ohmmeter
You may have thought that the last circuit was unnecessarily complicated. You are correct! All of the necessary circuitry is already included in the digital multimeter: a DC voltage source and an ammeter. You can quickly measure the DC resistance of a circuit element by connecting the element to the “V Hz” jack and the “COM” jack. Switch the FUNCTION button to and select the proper RANGE (i.e., the smallest possible range). In this configuration, the multimeter applies a DC voltage to the circuit element and measures the current that flows through the element. From this data the DC resistance (R=V/I) is found and displayed.
Use the ohmmeter to find the resistance of the same 1k resistor used in your previous measurement. Compare this result with your measurement of resistance using I and V. Why do the two methods produce slightly different results? Hint: Think about RS.
It is impossible to manufacture resistors that are identical. Therefore, there is always some variation of the actual resistance value among a random sampling of resistors. The last color band on the resistor indicates the uncertainty in the resistance (silver ±10%, gold ±5%). This lab uses ±5% resistors, so every 1k resistor should have an actual resistance between 950 and 1050.
D. Sensors
1. Thermistor
A thermistor is temperature-sensitive resistor. Many thermistors are made by connecting wires to a small piece of silicon. Silicon is a semiconductor, meaning that its electrical resistance is somewhere between an insulator (like glass) and a metal. Semiconductors have far fewer free charges (electrons) than metals, which is why a semiconductor does not conduct current as freely as metal. As the temperature of a semiconductor increases, however, the heat causes electrons to break free of the silicon atoms. The larger number of free electrons allows the semiconductor to conduct electricity more freely. In other words, the resistance decreases as temperature increases.
The NTC-102 thermistor (Figure 13) has a temperature-dependent resistance of
R(T) = 1000 + K*(T-25oC)
…where the temperature coefficient is K = -43 /oC, and T is the temperature in Celsius.
Figure 13. NTC-102 Thermistor
Measure the resistance of the thermistor at room temperature, RRT. Next, pinch the thermistor between your fingers to increase its temperature and measure Rwarm. (Try to avoid touching the bare wires; your fingers will reduce the measured resistance!)
Record these data for your lab report, and calculate the temperatures of the room and your fingers using the equation for R(T).
2. Photoresistor
A photoresistor is a resistor that is sensitive to light. Photoresistors work in a manner that is similar to a thermistor. Light enters a transparent window and falls on a thin layer of semiconductor (cadmium sulfide, CdS)that has a serpentine shape (Fig. 14). The photons striking the semiconductor release free electrons and increase the electrical conduction through the device. That is, more light will produce more electrons, and this decreases the resistance of the photoresistor.
Measure the resistance of a photoresistor when it is exposed to normal room light.
Repeat the measurement in darkness (you may cup your hands around the sensor, but don’t touch the wires or you will also measure the resistance of your body!)
Include this data in your lab report. Then write a description of how this device might be used to automatically control streetlights by sensing day and night.
Figure 14. A photoresistor (also called a photocell)
3. Potentiometer
A potentiometer can be used as a rotational position sensor. A small metal wiper is connected to the shaft of the potentiometer. As the shaft rotates, the position where the wiper contacts a semicircular resistor changes (Fig. 15). This changes the resistance between the nodes A and W, as well as between nodes B and W.
Figure 15. A sketch and a photograph of a potentiometer. (Sketch source:
Measure the resistance between terminals A and B. Then measure and record the resistance between terminals A-W and B-W every 90oas you rotate the shaft of the potentiometer in the clockwise direction. In your lab report write an explanation of how this device works based on your observations and describe how one might use a potentiometer to electrically determine the position of the gas pedal of an automobile.
E. Audio Speakers
An audio speaker produces sound by displacing air with a rigid cone (see Figure 16). The cone is connected to a coil of wire called a “voice coil”. This coil is placed between the north and south poles of a permanent magnet. If current flows in the coil, the current creates a magnetic field that either attracts or repels the voice coil from the permanent magnets. If the current is varying in time such that the waveform represents music, the speaker then reproduces that music.
Figure 16. Front and side views of an audio speaker.
Later in ES-3 you will build your own speaker, including the voice coil. One important design parameter for a voice coil is its DC resistance. Typically, the DC resistance of a voice coil is 4, 8 or 16.
In class, we usually assume that wires have zero resistance, that is, wires are perfect short circuits. In reality, all wires have a small, nonzero resistance. You will design the voice coil of your speaker to have one of these standard resistances (4, 8, 16).
The resistance of a wire can be determined from
R = (L/r2)
…where L is the length of the wire, r is the radius of the wire, and is a property of the metal wire called resistivity. Copper has one of the lowest resistivities of all metals, and it is a commonmetal used for wires -- including the microscopic wires within an integrated circuit or chip.
From the equation above, you should notice that long wires have greater resistance. Very thin wires (small radius, r) also have a high resistance. If you look around your room, you will notice that appliances that require a large amount of power (that is, a high current) also have thick, short power cords. This minimizes the resistance of the wires that connect the appliance to the wall outlet, and it prevents the wires from overheating due to large power dissipation: Pwire = I2Rwire.
You should have two 1-meter long pieces of thin magnet wire labeled A and B. Measure the resistance of these wires (take care to subtract the measuredresistance of the red and black test wires!). Record the resistance values and then use the tabulated data in Section V to determine the wire gauge (AWG) of each 1-meter sample. Include these calculations and results in your lab report.