The Oscilloscope and Ohm s Laws

Transcription

1 Purpose: In this lab, you will become familiar with the oscilloscope (CRO) as a measuring and detecting device. In the second section, you will verify Ohm's Law with resistors in parallel and series. Equipment: Digital Oscilloscope Function Generator DIGI 35A Power Supply Patch Cords, BNC Cords, Alligator Clips 3 Resistors (1 kω 27 kω) Bread Board and Wire Jumper Kit 2 DMMs Theory: The primary purpose of an oscilloscope is to plot in real time an electric signal. Before the advent of cheap solid state electronics, the main component of an oscilloscope was a Cathode Ray Tube or CRT. You are already familiar with a CRT because it is what creates the image on an ordinary TV set. Since solid state electronics are affordable now, we are able to use digital oscilloscopes with Liquid Crystal Displays or LCDs. The Oscilloscope and Theory To get started you need to understand some concepts about waves and how to measure tem on an oscilloscope. The y-axis represents the voltage and the x-axis is time. The period, T, of the signal is given by the product of the horizontal sweep speed, s.s, and the distance x, that is, T = (s.s)(x). The voltage at any point in time can be found by taking the product of the vertical deflection, y, and the amplification in volts/div, a. You thus obtain V = ay and δv = aδy + yδa. The accuracy of the vertical deflection on the oscilloscope is +3%, and the horizontal sweep is +4%; for instance: δ s.s = (.04)(s.s). To find the frequency of a sinusoidal signal, you can use the relation: f 1 T = Eq. 1 1 of 7

2 The corresponding uncertainty in the frequency is given by: δt δ f = Eq. 2 T where δt = (s.s.)(δx) + (δs.s.)(x). The Bread Board, or Terminal Board The two features of the terminal board you need to understand are the binding posts and the buses. The binding posts facilitate power, signal and ground connections to and from external sources on the board. These posts can accept banana plugs, pin jacks and spade lugs as well as alligator clips, solid and stranded wire. Horizontal and vertical buses are provided on each board. In the figure below, the solid lines represent the configuration of holes on the board which give access to solid conductors under the holes. These lines show the equipotentials, meaning the holes in each line are connected to one another. 2 of 7

3 Experiment: Part A: Getting to know the Scope LCD Screen Menu of tools that will measure aspects of the signal. Cursor controls to determine the height and width of the signal manually. Side control buttons These control the trigger properties. Controls the unit measurements on the y-axis. Controls the unit measurements on the x-axis. 3 of 7

4 1. Connect a BNC cable to CH 1 of the oscilloscope to the output connector on the function generator. Set the frequency to about 1kHz by using the course and fine frequency knobs on the generator. 2. Test the following functions of the oscilloscope: When you hit a menu button, the menu appears on the right side of the screen and can be interacted with via the buttons to the right of the screen. CH 1 Menu We are mostly concerned with Coupling, Probe, and Invert. Coupling means should the scope be expecting an AC, DC, or ground signal. When using the signal generator, set it to AC, or with the DC power supply set it to DC. Probe should be set to 1X, and Invert should be off. TRIG Menu Proper settings: o Type Edge o Source CH 1 o Mode Auto o Coupling AC o Slope Falling or Rising (change this and notice how the wave changes at time equals zero) o Experiment with the Trigger knob and see what happens when it is and isn t within the voltage range of the wave CURSOR Menu This menu is to be used in conjunction with the vertical position knobs. Adjust the cursors to take manual time and voltage measurements. MEASURE Menu There are 5 places to display measurements. Use the side buttons to select one and then change to the different types of measurements for CH 1. Experiment with the Volts/Div and Sec/Div knobs. The Sec/Div controls the sweep speed. 3. When you are familiar with the scopes functions, set the take the following measurements for 1000Hz and 5200Hz: Using the cursor, measure the Peak-to-Peak voltage, mean voltage, sweep speed, and period. Determine your error (δv or δt). Redo these measurements, but this time use the Measure feature. 4. Disconnect the oscilloscope leads from the signal generator and connect them to DC power supply and set it to 1V. Adjust the oscilloscope to read DC voltages and determine the voltage, V, of the power supply and its uncertainty, δv. FYI If your probes are from the oscilloscope probes box, make sure they are 1X probes or are set to 1X if they have that feature. Otherwise, your measurements will be off by a multiple of 10 or 100. NOTE: When measurements are made with any measuring device, there is an uncertainty in the reading of the measurements and also an inherent uncertainty in the measuring device itself. These uncertainties should be noted and included as uncertainties in the final values. 4 of 7

5 Part B: 1. Measure the resistance of three resistors using the Ω setting on the DMM. 2. Measure the resistance of each resistor using one DMM as an ammeter and the other DMM as a voltmeter. (See circuit diagram below.) For each reading on the ammeter be sure to start with the 10 A scale and then try the 300 ma scale if need be. NOTE: The ma setting on the DMM goes up to 300 ma. A current above 300 ma will blow the fuse. Please start on the 10 A setting and then switch to the ma setting if needed. 3. Using the terminal board, connect the three resistors in series (see Figure a, next page). Using the method in either Step 1 or 2 above, measure their resistance and compare (by percent difference) with the resistance from the addition formula: R s = R 1 + R 2 + R Repeat Step 3 with the same three resistors in parallel (1/R p = 1/R 1 + 1/R 2 + 1/R 3 ). (See Figure b, next page.) 5. Repeat Step 3 with the same three resistors connected as shown in Figure c on the next page. V R A + 5V 5 of 7

6 (a) R 1 R 2 R 3 DMM DMM V R 1 A R 1 R 2 R 3 (b) R 2 R 3 R 2 R 1 (c) 5V R 3 Analysis: Part A: 1. For each sinusoidal signal applied in Part A, indicate the peak-to-peak voltage, V p-p, and its uncertainty δv p-p, and calculate the frequency, f, of each signal and its uncertainty δf. Do the signal generator settings and the oscilloscope readings agree? If not, why not. Can this problem be corrected? 2. Indicate the voltage, V, of the power supply and its uncertainty δv. Do the values on the power supply and the oscilloscope readings agree? Results: Write at least one paragraph describing the following: What you expected to learn about the lab (i.e. what was the reason for conducting the experiment?), your results, and what you learned from them. Think of at least one other experiment might you perform to verify these results Think of at least one new question or problem that could be answered with the physics you have learned in this laboratory, or be extrapolated from the ideas in this laboratory. 6 of 7

7 Clean-Up: Before you can leave the classroom, you must clean up your equipment, and have your instructor sign below. How you divide clean-up duties between lab members is up to you. Clean-up involves: Completely dismantling the experimental setup Removing tape from anything you put tape on Drying-off any wet equipment Putting away equipment in proper boxes (if applicable) Returning equipment to proper cabinets, or to the cart at the front of the room Throwing away pieces of string, paper, and other detritus (i.e. your water bottles) Shutting down the computer Anything else that needs to be done to return the room to its pristine, pre lab form. I certify that the equipment used by has been cleaned up. (student s name),. (instructor s name) (date) 7 of 7