POLYTECHNIC UNIERSITY Electrcal Engneerng Department EE SOPHOMORE LABORATORY Experment 1 Laboratory Energy Sources Modfed for Physcs 18, Brooklyn College I. Oerew of the Experment Ths experment has three man objectes. The purpose of the frst part of the experment s to acquant the student wth the major equpment that he/she wll be usng throughout the semester. In the second part, a current lmted D-C Power Supply wll be studed. Lastly, the source resstance of a Functon Generator wll be measured. II. Equpment Requred 1 DC Power Supply, Constant oltage - Current Lmted 0-20, 0-0.6 A 1 Functon Generator 1 Dgtal Multmeter, 3-1/2 dgts, preferably wth current range. 1 arable Resstor (Rheostat) 0-250 ma, 0-400 ohms. 1 0-250 ma/0-300 ma, full scale dc meter. 1 100 ohm decade - 10 ohms/step. IIIa. Equpment Orentaton Many electrcal engneers spend a good part of ther tme workng n a laboratory. For ths reason, t s essental for the electrcal engneerng student to feel at home n the laboratory enronment. The purpose of the frst part of the experment s to orent the student to ths new enronment. More specfcally, the student wll become acquanted wth the followng nstruments found n the laboratory: 1
1. Dgtal Multmeter (DMM) As opposed to an analog meter, whch ndcates a measured quantty by means of a mong ponter, a dgtal meter ndcates a measured quantty by a numercal dsplay. The DMM s capable of measurng oltage and current (both D-C and A-C) as well as resstance. 2. D-C Power Supply Ths nstrument conerts 117, 60 Hz, a-c lne oltage to a regulated d-c oltage. The power supply prodes a nearly constant output oltage despte changes n the lne oltage or n the load. Essentally, t behaes lke an deal power supply oer a lmted range of output current. 3. Functon Generator Ths dece produces tme-aryng sgnals. It s capable of generatng sne, square, and trangle waeforms oer a wde range of frequences. Some applcatons of ths nstrument nclude the testng of electronc amplfers, and the ealuaton of the transent response of arous crcuts. IIIb. Current-Lmted D-C Power Supply The power supply, or battery whch we use n crcut analyss s smply an dealzed erson of an actual laboratory power supply. The deal oltage source shown n Fgure 1-1(a) has the property that the oltage across ts termnals s ndependent of the current through the supply. If we were to decrease the resstance RL, the current, I, would ncrease, but the oltage,, would reman constant. Fgure 1-1(b) shows graphcally the relatonshp between and I. I o o (a) (b) I Fg. 1-1 (a) Crcut contanng an deal (b) -I characterstc of deal power supply and a arable resstor. power supply 2
Because of our assumpton that s ndependent of I, we can arre at the unusual concluson that an deal supply can deler an nfnte amount of power. The power suppled to RL s smply, P = I = = o o R L 2 o. (1-1) R L Clearly, as RL s decreased to zero, P approaches nfnty. Unfortunately, there aren't any deces known to man whch can supply nfnte power. In an actual laboratory power supply, the amount of current, and hence power, whch can be delered s lmted. Sometmes we delberately want to lmt the current n a crcut so as to not damage certan electronc components. In the laboratory power supply, the maxmum output current can be preset to a desred alue wthn an allowed range. We wll see n the second part of the experment that an attempt to ncrease the current through the Power Supply beyond the current lmted alue wll result n a correspondng reducton of output oltage. IIIc. Source Resstance of Functon Generator In the thrd part of the experment, we wll be tryng to fnd the equalent source resstance of the Functon Generator. As we stated preously, the Functon Generator prodes a tme-aryng sgnal at ts output termnals, but the nstrument cannot be modeled smply by a tme-aryng oltage source. A more realstc model for the Generator would be a tme-aryng oltage source n seres wth an nternal resstance, RT, as shown n Fgure 1-2. Functon Generator T R T Fg. 1-2. A Functon Generator and ts Equalent Crcut Model By applyng Krchoff's oltage law to the crcut of Fgure 1-2, we fnd that, = T - R T (1-2) Note that T s the oltage that you would measure across the Functon Generator termnals wth RL dsconnected from the crcut, that s, T s the open crcut oltage of the Generator. Once a 3
connecton s made to the Functon Generator, the oltage across ts output termnals would no longer be equal to T snce a oltage drop would now appear across the nternal resstance, RT. RT cannot be measured drectly snce t s an nternal resstance. To measure RT, you wll expermentally obtan a graph of s. by aryng RL. The graph you obtan should be a straght lne and, accordng to equaton (1-2), the slope of ths lne s equal to RT. I. Expermental Procedure Ia. Equpment Orentaton 1. The followng exercse wll help you become famlar wth the arous controls and tems on the front panel of the DMM, Power Supply and Functon Generator: draw three large rectangles on your data sheet to represent the front panels of your nstruments. Locate all of the controls, buttons, termnals, etc., that are on the front panels and draw them on your data sheet. ery brefly, wrte down the functon of each of these controls. If you are not sure what ther functon s, ask your nstructor. 2. The followng questons wll further assst you n locatng the releant controls and tems on the front panel of your nstruments: a. Power Supply How many oltage and current controls are there? How many meters? (These meters are usually accurate to ±5%.) What s the rated output oltage and current? Are the output termnals color coded? Are they marked? Is there a meter swtch? What s ts purpose? b. Functon Generator What s the frequency range? Is there a erner control? Is the source resstance shown on the front panel? Is there a dc offset? What s ts functon? What are the aalable waeshapes? c. Dgtal Multmeter What quanttes can be measured? Specfy ranges and color codng f any. 3. Further exercses to be performed wth DMM. a. Set the DMM controls to the 2000 Klohms range. Wth nothng connected to the termnals, the measured resstance s nfnte. How does the DMM dsplay a readng when the range s too low? b. Connect the meter leads together. Do you read 0, 00 or...? c. A rheostat s a arable resstor. (The symbol for a arable resstor s shown n Fgure 1-3.) Usng the DMM, measure and record the total resstance. 4
R Fg. 1-3. Symbol for a arable resstor, (can be a rheostat, or a resstance decade box). Ib. Determnaton of - Characterstc of Current-Lmted DC Power Supply 1. Prelmnary Exercse on DC Power Supply. Wth the power supply dsconnected from the power lne, and the on-off swtch n the on poston, determne f there s any connecton between pars of the sx termnals of the power supply. (There are three termnals n the power lne plug and three output termnals.) A drect connecton would hae less than one ohm as measured wth the DMM. Record the pars of termnals whch are connected. Is there a connecton between the round termnals of the power lne plug and the bare metal surfaces of the case, such as the metal screw heads? 2. Prelmnary Exercse on DMM. (a) Connect the DMM to the (+) and (-) termnals of the power supply. turn on the power supply and set t for 2 usng the power supply panel meter. Set the current adjustment to md range. Begn wth the maxmum dc oltage range of your DMM and note the DMM readng. Reduce the range, one step at a tme, and note how the readng changes. Does the DMM readng agree wth the panel meter readng of the power supply? Set the output oltage to 20. Usng the DMM, measure the d-c oltage between the (+) and ( gnd ) termnals, and between the (-) and ( gnd ). Record the readngs. Explan your readngs. (b) Do not change the oltage settng from part (a) (should be at 20 ), but set the current control to a small alue. Turn off the power supply and assemble the crcut of Fgure1-4. Power Supply a Rheostat 0-300 ma meter g 5
Fg. 1-4: Crcut to obtan the -I characterstc of the power supply. (c) We now want to set the maxmum output current of the power supply to 200 ma. Set the rheostat to ts maxmum alue. Set the current lmt knob to the mddle (~1 A). Turn on the power supply, and turn ts oltage to 20. Turn down the current lmt untl the oltage starts to reduce. Increase the lmt a bt; then decrease the resstance of the rheostat untl the oltage starts to reduce. Keep repeatng the preous step untl 200 ma s the current lmt. 3. -I Characterstc We are now ready to obtan the -I characterstc of the power supply. At ths pont, the power supply output should be at 20, the rheostat should be at ts maxmum resstance alue, and the power supply s preset to prode a maxmum current of 200 ma. Slowly decrease the resstance of the rheostat and record approxmately 10 pars of alues of ag and I. Measure ag wth the DMM. Do not moe the current lmt knob durng ths process. If the power supply cuts off, reset the rheostat to ts maxmum resstance, and press the reset button on the power supply. Before dsconnectng the crcut, roughly plot the cure of ag (ertcal axs) s. I (horzontal axs) on a sheet of graph paper and show t to your nstructor. After your graph has been approed, you may dsmantle the setup. Ic. Measurement of Source Resstance of Functon Generator 1. Prelmnary Exercse Before connectng the Generator to the power lne, measure the resstance across the output termnals wth the DMM. Check to see f one of the output termnals of the Generator s connected to ground. (Note that the case of the nstrument s connected to ground.) 2. -I Characterstc of Functon Generator (a) Before assemblng the crcut of Fgure 1-5, set the Functon Generator to 100Hz, 7 a-c sne wae (use DMM on ac mode to measure oltage). (b) Assemble the crcut shown n Fgure 1-5. The arable resstor n the fgure s a decade box. If the Generator resstance s 50Ω, you wll use 10 steps of 10 ohms each on the decade box. If the generator resstance s near 600Ω, then you wll use 10 steps of 100 ohms each on the decade box. (c) Decrease the decade box resstance from 100 ohms to 10 ohms (do not decrease to zero) and record the output oltage and current ( = / ). From the plot of s., you wll be able to determne the source resstance of the generator. 6
Functon Generator DMM Fg. 1-5: Crcut for -I characterstc of Functon Generator. The Report Include n your report: 1. Two graphs showng the - characterstcs of the dc power supply and functon generator, fully labeled wth ttle, fgure number, scales, unts and dates. 2. Dscuss the meanng of these graphs, and compare the measured alue of the source resstance of the Functon Generator wth the alue gen by manufacturer. 3. Dscuss any nterestng obseratons you hae made durng ths experment. 7