Measuring Voltage, Current & Resistance Building: Resistive Networks, V and I Dividers Design and Build a Resistance Indicator

Transcription

1 ECE 3300 Lab 2 ECE 1250 Lab 2 Measuring Voltage, Current & Resistance Building: Resistive Networks, V and I Dividers Design and Build a Resistance Indicator Overview: In Lab 2 you will: Measure voltage and current Calculate, Simulate, Build, Test (and compare these) for o Serial/Parallel resistive network o voltage divider o current dividers. Design and build a resistance indicator (a light that will go on/off depending on the resistance). This lab will build on Lab 1 by using the series potentiometers as the variable resistance for your resistance indicator. This lab will also help you think through some beginning debug skills. (Check out the 'Sherlock Ohms' Extra Credit.) Equipment List: mydaq board with cables. (You can hook them to the lab computers if you don t want to bring your laptop.) Multisim software. From Lab 1: o Breadboard & wire kit o Resistor (1 kω, 4.7 kω) o Potentiometers (10kΩ and 100Ω) Additional parts: o Red LED o Resistors: 160 Ω, 470 Ω, 510 Ω, 1.5 kω, 2.2 kω, 3.3 kω, 10 kω Safety Precautions: 1) Blown Fuse: If you use the mydaq as an ammeter (as you are told to do in the lab manual) and accidentally try to read the current across a short circuit (which is a very easy mistake to make), you may blow out the fuse in your mydaq. A better way to measure current (to prevent this problem) is to measure voltage across a shunt resistor and calculate the current using Ohm s Law. Please do it this way in our labs, it will save you and the TAs a lot of grief: 2) Short Circuiting the Power Supply: When you are using the power supplies on the mydaq, you will have several wires screwed to the black holder on the side of the mydaq, all hanging loose together. If their ends touch, they will short-circuit. If you short-circuit the power (+/- 15V or 5V) together or to ground, you may blow the fuse in the mydaq. Take care to prevent this. Keep your bench and wiring neat, always hook the power and ground to the same points on your circuit board, always use the same colors so you recognize them (red, black, white are typically used), etc. 1

2 3) If you do blow a fuse: Instructions for changing it are on the class website (on the Canvas Home page: Resource pages by topic: Labs then click on mydaq Resources then click on mydaq Manual 3rd item on first line: then go to p. 14), and the ECE stockroom has them in stock. 4) General Electrical Care: It is pretty hard to actually hurt yourself with this equipment and circuits. Throughout our labs, it is possible you may miswire something and create a short circuit, which can make parts get hot, or even pop. We call this letting the magic smoke out of the box, after which these parts don t work any more, and you can get new ones in the stockroom. If you smell something hot, ok, unplug your circuit and try to figure it out. Try to be aware and prevent short circuits. For instance, it isn t really a great idea to probe around in your circuit with a metal screwdriver, which can easily create short circuits. Mistakes happen, and the mydaq has a fuse, which should protect it from any circuit mistakes you might make in this class. 5) A few hints I ve used for wiring circuits: Keep your circuits neat. Label the nodes on your diagram, and keep track of where they are on your board (label them with tape, if necessary). Don t hook up the power until you are ready to use it. Measure your voltage before you hook it up. Disconnect between circuit revisions. Build your circuit in stages, testing as you go. Measure your resistors before you put them on the board (colors can be easy to mistake). Instructions & Reference Material: mydaq Quick Start Guide mydaq as voltmeter mydaq measuring current through shunt resistor mydaq measuring resistance Multisim demos : See DVD in back of your book. Data Sheets: Light Emitting Diodes (LED) Sheets/Fairchild PDFs/MV5x64x, HLMP-15x3,130x.pdf Prelab: Run Multisim Simulations 1. Review the videos (on website) and written material. 2. (Optional) You will be faster if you do the circuit calculations and Multisim simulations before you come to lab. WRITEUP: Take notes during the videos and information from written information so you don t have to go back and watch or review them again. 2

3 Experiment 1: Measure Voltage (10 points) The mydaq puts out two voltages (+15V and 15V relative to the ground, which is labeled AGND, and 5V relative to digital ground, labeled DGND). It also provides a variable DC voltage using the function generator. Find the +15V, -15V, and AGND pins on the long side of the mydaq, and screw wires in to them. Be careful their ends do not touch each other and short out. Use the mydaq as a Voltmeter to measure the voltages to see how close they are to what you are expecting. You may need to use alligator clips to connect the wires on the voltage pins to the Voltmeter probes on the bottom side of the mydaq. Repeat for meas V from +15V to AGND = This is the power used for the rest of this lab. meas V from -15V to AGND = meas V from +15V to -15V = meas V from +5V to DGND = Repeat for variable voltage source. See MyDAQ quick start guide section F.7 for information on how to use the function generator as a variable DC voltage source. What is the largest and smallest voltage you can measure on the MyDAQ? WRITEUP: Explain your procedure, including a diagram of your connections. What voltages are available on the MyDAQ? How accurate is the expected voltage compared to your measured voltage? Note any abnormalities or unexpected information that happens. Explain why, if you can. Experiment 2: Resistive Networks, Voltage & Current Dividers (30 points) Calculate, simulate, build and test the circuit for problem m2.3 on page 93 of your text (Fig. 1, below). 1. Extra Credit: Calculate the total resistance using the methods in section of your text. See Additional file for this extra credit, with hints, etc. 2. Calculate the voltages using voltage dividers, described on page 55 of your textbook. 3. Calculate the currents using current dividers, described on page 57 of your textbook. 3

4 Fig. 1. Circuit for problem m2.3, page 93 of the Ulaby textbook. Value Calculated (Extra Credit 20 pts) TABLE I RESISTIVE NETWORK VALUES Total Resistance connected to V1 voltage across R1 U1= voltage across R2 U2= voltage across R4 U3= voltage across R6 U4= Current through R1 Current through R2 Current through R4 Current through R6 Simulated (Multisim) 1 Measured WRITEUP: Explain your procedure in your own words. Sketch how the voltmeter and current meter are connected to the circuit (for at least one measurement). Provide the solution for the circuit above. Explain any anomalous results. Extra Credit (10 points): Sherlock Ohms Debugs a Circuit Have another student or the TA change any one of your resistors for another resistor with the WRONG value. Using your mydaq as a voltmeter, find which resistor it is, and determine if the resistance value is too large or too small. WRITEUP: Record information in your notes and turn in: (a) Indicate which resistor was changed on Fig. 1, (b) describe how you tested, and the (c) reasoning behind your testing method, and (d) anything you found that complicated your testing. 1 Multisim files are available for download from the lab site, or you can create your own. 4

5 Experiment 3: Resistance Indicator (30 points) Now let s build a resistance indicator to turn on a light when the resistance is below a certain value. Fig. 2 shows the circuit schematic. The user will connect a resistor they want to test, (called R 1 in Fig. 2), and the LED will light up if the resistance is less than 1 kω. We will use a standard red Light Emitting Diode D1 2 (LED) as the light. The other resistors in the circuit, R 2 and R 3, have been calculated to accomplish two goals: 1) Limit the LED current to at most 10 ma to prevent the LED from burning out when the user chooses a wire for R 1, and 2) Start turning on the LED when the user selects 1 kω for R 1. The circuit uses the +15V source from the mydaq for power. Fig. 2: Resistance Indicator circuit to turn on an LED when R 1 < 1 kω. 1. Model the LED: Determine V F (Forward Turn on voltage) and R LED (equivalent resistance) The LED is a nonlinear device that turns on when the voltage across it reaches a certain value called the forward voltage. If the voltage is increased further, the current through the LED rises very rapidly. Consequently, we have to control the CURRENT through the LED. To do so, we need a model of the LED. You have already seen the LED I-V curves in Fig. 3 from the application section of some of our early lectures. LEDs are diodes that turn ON at currents of a few ma. They have maximum current ratings of typically 20 ma or 30 ma. If we look at the I-V curve for a "standard RED" LED in the graph on the right in Fig. 3, we can approximate it as two intersecting straight lines. (The actual curve has a small elbow that we, as engineers, will ignore.) One line is on the x-axis where the LED is off, and the other line rises steeply where the current in the LED rises rapidly. WRITEUP:Put a ruler on the "standard red" LED I-V line and record the value where it intersects the bottom axis. This voltage is called the forward voltage of the LED and is the voltage where the LED starts to turn on. Record this value of V F. V F = volts. 2 Light Emitting Diode (LED) 5

6 Also, determine the slope (rise over run) of the "standard red" I-V line where it is going up, and use Ohm's law to determine the equivalent resistance of the LED. R LED = ohms. Using the forward voltage and the equivalent resistance, the LED may now be modeled by the circuit shown in Fig. 4 when it is on. (The LED may be modeled as an open circuit when it is off.) Fig. 4 also shows the values of R 2 and R 3. Fig. 3. Light Emitting Diode (LED) I-V curves. The curve on the right is slightly more linearized (idealized) and easier to read. Fig. 4. Resistance Indicator circuit model when LED is turned on. 2. Evaluate the circuit. WRITEUP: Verify that the Resistance Indicator circuit satisfies two conditions: 1) The voltage across the LED is close to its forward voltage, V F, when R 1 = 1 kω. Since this is the point where the LED is just about to turn on, you may treat the LED as an open circuit and use the circuit model in Fig. 5(a). 2) The LED carries approximately I LED = 10 ma when R 1 = 0 Ω (a wire). Since R LED found earlier is much smaller than R 2 and R 3, we may treat it as a wire and use the circuit model in Fig. 5(b). 6

7 (a) Fig. 5. Resistance Indicator circuit models: (a) with R1 = 1 kω, (b) with R 1 = wire. Experiment 4: Simulate the circuit with Multisim 3 (10 points) Simulate the circuit in Multixim, as shown in Fig. 6, (but use the values of R 2 and R 3 from Fig. 4.) Use Multisim to evaluate the current (U1) and voltages (U2-U4) as shown. 4 Compare the ON and OFF cases, and experiment with the value of R 1. The LED will NOT actually turn OFF, because Multisim allows dim LEDs to continue to show as being ON in the simulation. (b) OFF Fig. 6. Multisim circuits. ON What could go wrong in this circuit (and check to make sure it won t)? Several potential problems occur when you build a theoretical circuit in real life. These gremlins include (but unfortunately are not limited to): a) Exceeding the current or voltage limits of the components. b) Exceeding the power rating of the mydaq. c) Components not being exactly as designed. What is the expected range of R2 and R3? Approximately how much will this affect your circuit? 3 Multisim files are available for download from the lab assignment on Canvas, or you can create your own. 4 Find parts in multisim from Select All Groups and typing the various component names. U1 is an ammeter. U2, U3, U4 are voltmeters. Note their connection for measuring voltage differences across components. LED1 is an LED, choose a red one. V1 is DC_POWER, and you can change its voltage once you have it set down. Don t forget the GROUND. 7

8 WRITEUP: a) Calculate the maximum power for the resistors, and verify that you will not exceed the 1/8W power rating in any configuration (on,off). If the power does exceed 1/8 W, use parallel resistors to get more power dissipation capability. b) Look up the maximum current that can be sourced by the +/-15V and 5V power supplies on the mydaq. Will you be exceeding that rating? c) What is the expected range of R2 and R3 for 5% tolerance in the value? Approximately how much will this affect your circuit? Experiment 5: Build and Test the Circuit (10 points) Build the resistance indicator circuit on your breadboard, using your mydaq's +15 V power supply for V s. The long lead on the LED is the plus side. The short lead is connected to reference (AGND on mydaq). Insert different R 1 's or a potentiometer into your breadboard to test your indicator. It should start to light up when R 1 is about 1 kω, and it will get brighter as R 1 gets smaller. Using your mydaq voltage and current meters, measure the currents and voltages shown in the Multisim simulation in Fig. 6 for two cases: R 1 = 1 kω, and R 1 = 0 Ω (wire). WRITEUP: Indicate your measured values corresponding to those shown in Fig. 5. Comment on how they compare to the expected values. At approximately what value of resistance does your LED turn on ON? (You may use your resistors from Fig. 1 in various combinations, or you may use the 10 kω pot from Lab1. If you put the 10 kω and 100 Ω pots in series as you did in lab 1, it is easier to tune your resistance. ) Ron min = Discussion and Conclusions: How to Debug a Circuit (10 points) WRITEUP: You have now calculated, simulated, and built a few simple circuits. You have measured resistance, voltage, and (using voltage and Ohm's law) current. 1) List what you have found to be best practices for building circuits. 2) Whether or not you actually made wiring mistakes as you built these circuits, list at least three different ways you could figure out what is wrong in a circuit. 8