ELECTRONIC MEASURMENTS

Transcription

1 ELECTRONIC MEASURMENTS LAB 1 INTRO: SETTING UP A DC POWER SUPPLY AND MAKING DC AND AC VOLTAGE MEASUREMENTS USING A SCOPE AND DIGITAL MULTIMETER. GOALS In this lab, yu will use basic test and measurement equipment that are useful fr building and testing simple circuits. In particular, yu will make measurements f vltage using an scillscpe and digital multimeter. Yu will als determine an accurate measurement technique fr measuring small resistances. Prficiency with new equipment DC Pwer Supply: Set up the cnnectins t supply bth (+) and (- ) vltage Operate in vltage cntrlled and current cntrlled mdes Oscillscpe: Measure DC vltage levels Determine the effects f DC and AC input cupling Determine the effects f changing the input impedance Trigger the scpe n different wavefrms Functin generatr: Create varius shaped wavefrms, and mdify amplitude and frequency Change the utput impedance t match the rest f yur system Digital Multimeter (DMM): Measure DC vltages and DC currents Determine the frequency and impedance limitatins f the DMM. Experimental design Develp familiarity with the design- based apprach t experiment Measuring small resistances using a 4- terminal apprach LAB NOTEBOOK GUIDELINES The lab ntebk will play an essential rle in this curse. Yu will use yur ntebk fr keeping recrds f many things including Answering lab- prep questins frm the lab guide. Answering in- lab questins. Recrding data. Including plts f data. Analysis and results. 1

2 Diagrams and pictures. Prcedures f experiments that yu design. The lab ntebk will be an imprtant part f yur grade because learning t keep a gd lab ntebk is an imprtant part f yur prfessinal develpment. Yu may find it helpful t write up many f yur ntes n the cmputer, fr example, within Mathematica r anther prgram. This is fine. Hwever, befre yur ntebk is turned in, the ntes, plts, and analysis shuld be transferred t the lab ntebk by printing and taping the pages r keeping them in a three ring binder. This is standard practice in research labs. Yur lab ntebk is the main mechanism fr cmmunicating yur prcess and results f the lab experiments. Each week, yu will be respnsible fr turning in bth yur pre- labratry wrk and yur lab bk / analysis via D2L in scanned frmat. See syllabus fr mre infrmatin. DEFINITIONS Pwer rail (r rail) this refers t the ± V pwer supply utputs that pwers the circuit. Electrical Lad (r lad) this refers t the circuit r impedance cnnected t the utput f a circuit. RMS (Rt Mean Square) is the square rt f the average f a peridic functin squared ver ne perid. Example: Fr the functin Y = A sin(ωt), the RMS value is Y! = A/ 2. USEFUL READINGS 1. Fischer- Cripps (FC) chapters 1 (electricity), 2 (dc circuits), and 3 (ac circuits) prvide an intrductin t imprtant tpics relevant t the first tw weeks f lecture. Nte: we will revisit varius sectins in these chapters in mre detail later fr example filters and scillatrs. Als lk at p. 274 fr the resistr clr cde (r Ggle it). 2. The fllwing dcuments n the curse website will be useful: a) AGILENT 33220A functin generatr manual pp b) Tektrnix 3014 Scpe Users Manual, pp , c) Math review (cmplex numbers) 3. Chapter 1, Sectins f Hrwitz and Hill (H&H). 2

3 SETTING UP YOUR DC POWER SUPPLY Yur DC pwer supply will prvide all f yur circuits with the required pwer. Setting up the pwer supply will always be the first thing yu d when yu begin wrking with a circuit. Step 1 Electrnic Lab Set up the DC pwer supply t pwer yur circuits. a. Cnnect the pwer supply s yu have ne variable (+) and ne variable (- ) utput with respect t grund, ne frm each side f the supply. Each side has utputs labeled (+), (- ), and grund. The vltage displayed n the LED display shws nly the ptential difference between the (+) and (- ) utput. Yu must cnnect the (- ) t grund t get a (+) vltage and the (+) t grund t get a (- ) vltage. (See Figure 1.) Als, make sure the tw utputs, (+) and (1), are independently cntrlled (rather than in series r parallel). b. Cnfirm the sign and magnitude f the vltages displayed n the pwer supply using the DMM. In yur lab bk, dcument the setup, the utput vltage frm the pwer supply, and the measured utput n yur DMM (Hking up the pwer t yur circuit with reversed plarity r t t high a vltage can cause yur cmpnents t be destryed.) Step 2 Electrnic Lab Use the pwer supply as vltage r current surce. A vltage surce maintains a cnstant ptential difference with the current utput dependent n the lad resistance. A current surce maintains a cnstant current with the vltage utput dependent n the lad resistance. The supply will tell yu which mde it is in by an illuminated LED n the frnt panel. (C.V. fr cnstant vltage and C.C. fr cnstant current.) NOTE: If yu are wrking with the Agilent pwer supplies, the cntrls are slightly different and mre intuitive. a. With n lad attached, recrd the utput vltage and current fr bth pwer rails (see definitins). Yu will have t mve the switch t see the vltage and then the current. b. Try adjusting the value f the vltage and current using the knbs. Why is the current utput always zer? c. Nw cnnect a lad frm the (+) rail t grund. Yu can use a small resistance such as a wire with banana plugs n the end as the lad. d. Vary the value f the vltage and current using the knbs. Describe the behavir f the vltage and current readings and the mde (C.V./ C.C.) f the pwer supply. What happens when yu shrt the utput t grund (have t small a lad)? What is the maximum utput f current and vltage yur supply can prduce? 3

4 Figure 1 DC pwer supply cnnectins MEASURING VOLTAGE WITH YOUR OSSCILOSCOPE The gal f this part f the lab is t be able t use a scpe t make measurements f DC vltages and AC wavefrms. Yu will als learn hw t prduce varius wavefrms using a functin generatr. There are a few precautins t bserve when perating the scillscpe: Avid verheating the instrument. D nt blck ventilatin f the interir by laying bks n the case. D nt apply mre than 400 V t any input terminal. Avid serius r fatal injury frm electrical shck. D nt remve the cver t expse the 120 V mains. Otherwise, the instruments are rbust and cannt be damaged by wrng settings. S try whatever yu re curius abut and measure what happens. Step 3 Electrnic Lab Mdeling the measurement system. Measuring a DC vltage n a Scpe. The Tektrnix 3014 has fur independent vertical channels s that fur separate signals can be displayed at nce. Each trace is clr cded with the buttns n the panel. a. Cnnect a +5V signal frm yur pwer supply t the scpe using the supplied cnnectrs. Measure the vltage n scpe using the cursrs. Try explring the different knbs and menus n the scpe t make the measurement. There are a few hints listed at the end f this dcument if yu get stuck. b. In yur lab bk, describe the setup f the electric circuits (diagrams are useful) and the utcmes measured. NOTE: Oscillscpes can nly distinguish abut 100 different values n the vertical axes f the screen. S befre yu use the scillscpe t measure anything make sure that the trace cvers at least 50% f the vertical screen withut clipping at the tp r bttm. This way yu get a reslutin/accuracy f apprximately ±2%. c. Vary the vltage using the knb n the pwer supply. What is the maximum vltage yu can measure with the scpe? 4

5 Step 4 Electrnic Lab Triggering an AC wavefrm n the scpe. The scpe prduces a 0-5V square wave n the prbe cmp pins t use fr testing. We will use this t get familiar with triggering the scpe. a. Cnnect the Prbe Cmp utput t the scpe using mini grabbers. (See figure 3) b. Display the wavefrm n the scpe. Yu will need t trigger the scpe ff f the wavefrm. Cnfirm with yur instructr that yu have the scpe triggered crrectly befre cntinuing. Again, describe the setup and utcmes in yur lab bk. The trigger level cntrls the vltage at which the trace starts. Stability is lst when the trigger level lies utside range f the displayed vltage. Change TRIGGER MODE t NORMAL. Nte that the trace nw freezes when the trigger level is misadjusted. Yu can see whether r nt the scpe is actually being triggered by lking fr the small writing TRIG D r TRIG? at the tp f the display. See Hints fr the trigger menu. c. What is the frequency f the wavefrm? Yu can measure the perid and then calculate the frequency using that measurement. Include yur measurements, prcedure, and calculatins in yur lab bk. Step 4 Electrnic Lab Creating an AC wavefrm using a functin generatr and measuring it n a scpe. The Agilent 33120A functin generatr can prduce sine, square, triangle, pulse and ramp wavefrms ver the frequencies frm Hz t 15 MHz. The utput amplitude can be varied between 50 mv and 10 V peak- t- peak with an utput impedance f 50 Ω. There is ne main precautin t keep in mind: D nt cnnect any utput f the 33120A directly t dc pwer r t the utput f any ther instrument r circuit. Ding s will burn ut the utput amplifier! a. Create a 1 V peak- t- peak (p- p) sine wave at 1 khz with n dc ffset (yu may want t use this standard setup in the future if yu have truble). Yu will need t set the utput terminatin f the functin generatr t HIGH Z (see Hints Sectin r page 40 f manual). b. Display the wavefrm n the scpe. Yu can use a BNC cable t make the cnnectin. See Figure 2. Yu will need t trigger the scpe ff f the wavefrm. c. Reduce the amplitude t 100 mv p- p. Yu may ntice the scpe stpped triggering. This is because the wavefrm never crsses the trigger level nw. d. The functin generatr has a trigger utput that can be used t trigger the scpe (called SYNC). Using the trigger utput is mre cnvenient than triggering the scpe ff f the wavefrm itself because yu avid having t readjust the scpe trigger every time yu change the wavefrm. Cnnect the SYNC utput f the functin generatr t Channel 4 f the scpe and trigger ff f that wavefrm. Nw change the amplitude and frequency f the sine wave, and ntice hw the scpe remains nicely triggered. e. Cme up with a wavefrm f yur wn that yu want t create (e.g., triangle wave at 1 MHz with a 200 mv p- p amplitude) and measure its prperties with the scpe. Reprt n the ne yu created and recrd this in yur lab bk. Yu can either print a screensht directly frm the scpe r take a picture and print it. 5

6 Grund cnductr Signal cnductr (gld) Figure 2: BNC cnnectr Figure 3: Mini grabber t BNC MEASURING QUANTITES WITH THE DIGITAL MULTIMETER The multimeter is a useful device t measure cnstant vltages, currents, and resistances. Step 6 T measure varius quantities with the DMM, yu have t have the dial set crrectly and the tw leads cnnected t the crrect input prts. a. Cnnect the leads and set the dial t measure resistance. Get five 5k (5%) resistrs frm the stck drawers. Measure the resistance f each resistr. D all the resistrs meet the 5% specificatin? b. Cnnect the leads and set the dial t measure vltage. Measure the vltage f bth sides f the DC pwer supply again. Des it agree with the LED display? c. Cnnect the leads and set the dial t measure current. Measure the current prduced by the pwer supply. Make sure the current is limited t a value belw the fuse f the DMM. Hw d yu experimentally set the current limit? If yu blw the fuse, just replace it. Des the measured current agree with the LED display? Using the current and vltage readings, what is the impedance f the ammeter? HINT: The DMM leads must be in different places t measure current vs. vltage. Als, it is recmmended t use the high current input until yu are certain that the current is lw enugh t be read by the lw current input. This avids blwing the fuse. d. The DMM als has an AC vltage setting. Over what frequency range is the reading accurate t 2%? Yu will need t use the functin generatr t prduce an AC wavefrm. Nte: the DMM displays the RMS amplitude f the wavefrm. See Definitins at the beginning f the guide fr an explanatin f RMS. e. Remember in the future that this is the usable frequency range fr the AC setting f the multimeter. Recrd this frequency in yur lab bks fr future reference. 6

7 APPLICATION EXPERIMENT: MEASURE SMALL RESISTANCES In the previus sectin, yu measured resistances using the hmmeter in the DMM. Yu will nw use all f the measurement techniques and devices yu have learned t determine mst accurate way t measure a small resistance. Step 7 Step 8 Measure a small resistance using a DMM hmmeter. a. Use a ~ 2 m length f magnet wire (26 r 28 gauge) as yur small resistr. Magnet wire has a very thin amber- clred insulating cating. Make sure yu remve the insulatin frm the ends f the wire t make a gd electrical cnnectin and measurement f the diameter f the wire. Yu can burn ff the insulatin with a flame r carefully scrape it ff with a razr blade. b. What is the resistance based n the diameter, length, and resistivity? (The resistivity, ρ, f cpper at rm temperature is 1.68 μω- cm) Reminder R = ρl/a (l = length; a is area) It is hard t measure the diameter f the wire with the insulating cating n and the stripping prcess usually defrms the cpper. Hwever, yu can just lk up the diameter f yur particular gauge f wire nline. c. Use the hmmeter in the DMM t measure the resistance f the wire. Dcument yur setup, measurements, and calculatins in yur lab bk. Measure a small resistance using a 4- terminal apprach a. Use yur DC pwer supply t run a current thrugh the wire. Yu can then measure the vltage drp acrss the wire t determine its resistance based n Ohm s law. b. Yu can use the display n the pwer supply t measure the current r use yur DMM as an ammeter. (Cnsider the reslutin f bth devices when making yur chice.) Yu can use yur DMM as a vltmeter t measure the ptential difference acrss the wire. Can yu just use the vltage reading n the pwer supply? Explain. c. Draw a diagram f yur experimental set up. d. Cnsider hw the amunt f current flwing thrugh yur resistr affects the sensitivity f yur measurement. e. Calculate the resistance f the wire frm yur measurements using Ohm s Law. Step 9 Cmpare the tw measurement techniques. a. The hmmeter in the DMM wrks by supplying a current and measuring the ptential difference between the utputs. Hw is this measurement different than the 4- terminal apprach yu built? Hint: cnsider where the current is flwing and what resistances are invlved with each measurement technique. b. Which methd is mre accurate fr measuring small resistances based n yur explanatin fr part a? c. D yur tw measurements cnfirm yur scientific argument made in part b? Defend yur assertin using yur data. 7

8 HINTS 1. Oscillscpe Cntrls T change the hrizntal (time base) scale: 1) Scale knb changes the time per divisin (sec/div are clr cded and displayed n the screen) 2) Psitin knb changes lcatin f the trigger (labeled as a range arrw n the tp f the screen) T change the vertical (vltage base) scale: 1) Scale knb changes the vltage per divisin (V/div are clr cded and displayed n the screen) 2) Psitin knb changes lcatin f the grund (labeled as a clred- cded arrw n the left f the screen) T access the parameters fr each channel: Ch. 1 (r 2,3,4) buttn è Vertical MENU buttn (under Vertical Scale knb) 1) Input Cupling (AC/DC/Grund) 2) Invert Signal 3) Prbe Setup 4) Input Impedance (1MΩ r 50 Ω) Ch. 1 (r 2,3,4) buttn è Vertical Off buttn (abve Vertical Scale knb) 1) turns ff that channel s trace T adjust the trigger: 1) Trigger knb changes the vltage level f the trigger 2) Trigger MENU buttn a. Select which channel t trigger ff f (trigger level arrw changes clr t shw which channel is being triggered) b. Slpe: can trigger ff a rising r falling edge. c. Trigger Mde: Aut, Nrmal, ther T select the AQUIRE mde: 1) Run/Stp: sets the scpe t cntinuusly quire r freeze after last trigger 2) Single sequence: draws nce after it triggers 3) Autset: scpe tries t chse verall best scpe settings (smetimes useful when yu can t see anything, but generally used as a last resrt because it can als mislead yu ) Measure Menu (tp f panel): Yu can select varius measurements and which channel t measure. Be very careful with autmatic measurements (again these can be deceptive). Generally use cursrs t make accurate measurements. Nte that the autmatic mde searches fr the very tp and bttm f the signals. Fr example, in the schematic f a nisy sine wave belw, autmatic mde will return the upper and lwer mst lines which will include the nise cntributin. A careful manual measurement indicated by the inner (shrter) lines can remve the nise frm the amplitude measurement. With small r nisy signals, the autmatic mde will give very pr results. 8

9 Cursr Menu (tp f panel): 1) Chse time (vertical bars) r vltage (hrizntal bars) measurement. 2) Chse which channel t measure (measurement cursrs are the same clr as the channel they are measuring) 3) Big tp knb mves the cursrs 4) SELECT buttn changes which cursr t mve. 5) Psitin relative t grund (trigger zer time) is displayed n the screen with symbl. 6) Relative distance between cursrs is displayed n the screen with the Δ symbl. 2. Changing the Output Terminatin n the functin generatr (Agilent 33120A). 9