ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren ELECTRONIC VOLTMETERS AND AMMETERS Alessandr Ferrer Dpartment d Elettrtecnca, Pltecnc d Mlan, Italy Halt Eren Curtn Unversty f Technlgy, Perth, Western Australa Keywrds: currents, vltages, measurements, standards, analg vltmeters, dgtal vltmeters, mcrvltmeters, scllscpes Cntents 1. Intrductn. 2. Analg Meters 2.1. DC Analg Vltmeters and Ammeters 2.2. AC Analg Vltmeters and Ammeters 2.3. True rms Analg Vltmeters 3. Dgtal Meters 3.1. Dual-Slpe DVMs 3.2. Successve-Apprxmatn ADCs 3.3. AC Dgtal Vltmeters and Ammeters 3.4. Frequency Respnse f AC Meters 4. Rad-Frequency Mcrvltmeters 5. Vacuum-Tube Vltmeters and Oscllscpes 5.1. Analg Oscllscpes 5.2. Dgtal Strage Oscllscpes (DSOs) 5.3. Prtable Oscllscpes 5.4. Hgh-Vltage Oscllscpes Appendx Glssary Bblgraphy Bgraphcal Sketches Summary Vltage and current measurements are essental parts f engneerng and scence. Instruments that measure vltages and currents are called vltmeters and ammeters, respectvely. There are tw dstnct types f vltmeter and ammeter, whch dffer frm each ther by the peratng prncple that they are based n: electrmechancal nstruments and electrnc nstruments, whch als nclude scllscpes. Electrmechancal vltmeters and ammeters, ncludng thermal-type nstruments, represent early technlgy, but stll are used n many applcatns. Basc elements f vltages and currents frm the basc physcal prncples have been ntrduced n the electrmechancal vltage and current measurements sectn. Als, vltage and currents standards have been dealt wth n detal n ther artcles. Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren In ths artcle, mdern electrnc vltmeters and ammeters are dscussed. Sme nfrmatn s gven n scllscpes as ne f the mprtant vltage measurng devces, althugh they are generally nt classfed as vltmeters r ammeters. The analg electrnc vltmeters and ammeters n the frm f DC, rectfer-based AC, and true rms devces are explaned n detal. Dgtal meters are ntrduced, as are types f vltmeters and ammeters peratng n dfferent technques f analg-t-dgtal sgnal cnverters. A lst f sme manufacturers f electrnc vltmeters, ammeters, and scllscpes s gven n Appendx 1. 1. Intrductn The develpment f semcnductr devces, ncludng peratnal amplfers, had an mpressve mpact n measurement nstruments, ncludng vltmeters and ammeters. Vltmeters and ammeters, n partcular, have mved steadly frm cnventnal electrmechancal archtectures t electrnc nes. In general, electrnc meters and ndeed many electrnc devces can be represented as three-prt crcuts, as n Fgure 1. Fgure 1. Three-prt representatn f electrnc meters In the three-prt mdel, the nput sgnal enters the nstrument thrugh the nput prt, whch s usually characterzed by a hgh mpedance. The majrty f electrnc meters requre hgh-nput mpedance; they can, therefre, be regarded as vltmeters that present a neglgble lad fr the sgnal surce. In addtn, electrnc ammeters can be realzed frm the basc prncples f vltmeters by means f varatns n the nput stages descrbed n the chapter dedcated n the electrmechancal nstruments f ths encyclpeda. Once the nput sgnals are sutably prcessed, the measurement result s prvded by the utput prt n ether analg r dgtal frm. In ths artcle, dscussn wll cncentrate n the nature f the nput sgnals, sgnal prcessng, and the utput requrements t dsplay the measurement results, alng wth the pwer needed t energze the devce. The electrc pwer requred t energze the meter s nternal crcuts and the utput dsplay cmpnents s suppled t the meter tself thrugh the pwer-supply prt. In cntrast t electrmechancal types, the need fr an external pwer supply s ne f the man characterstcs f electrnc meters. Thugh ths may appear as a drawback especally when prtable meters are cncerned t shuld be cnsdered that the external pwer supply allws the devce nt t draw, r draw mnmal, energy frm the sgnal surce fr the measurements. Ths explans why electrnc vltmeters d nt lad Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren the sgnal surce as much as ther electrmechancal cunterparts. It als leads t many ther mprtant advantages f mdern electrnc meters ver tradtnal electrmechancal nes, such as the accurate measurement f very small ampltude and hgh-frequency currents and vltages. The hgh-level, lw-nse perfrmance f present-day electrnc devces allws fr realzng meters that are as accurate as, and smetmes even mre accurate than, the mst accurate electrmechancal meters. Snce they d nt requre the extensve use f precsn mechancs, they are presently less expensve than electrmechancal meters, and are slwly, but cnstantly, replacng them n almst all applcatns. In accrdance wth the prcessng f nput sgnals, measurement, and dsplay requrements, electrnc meters are dvded nt analg meters and dgtal meters. Analg meters attan the requred measurements by cntnuus-tme prcessng f the nput sgnals usng analg crcuts. The measurement results are dsplayed ether n analg frm, fr nstance, as an electrmechancal meter, r n dgtal frm, by cnvertng the utput sgnal nt a dgtal frmat. On the ther hand, dgtal meters attan the requred measurements by cnvertng the analg nput sgnal nt a sequence f dgtal samples unfrmly spaced n tme n the early stages f the sgnal prcessng. The nput sgnals are, therefre, prcessed n the dscrete-tme dman, and the measurement results are usually dsplayed n a dgtal frm. It s wrthwhle t nte that the dstnctn between analg and dgtal meters des nt refer t the way the measurement results are dsplayed, but t the dman cntnuus-tme r dscrete-tme dman n whch the nput sgnals are prcessed n the man bdy f the devces. The basc cmpnent n dgtal meters s the analg-t-dgtal cnverter (ADC) that takes care f cnvertng the analg samples f the nput sgnals nt prprtnal dgtal values. Ths artcle cncentrates n analg electrnc meters, dgtal vltmeters (DVMs), ADCs, and dgtal measurement systems. Vacuum-tube vltmeters and scllscpes are als dscussed snce they are amng the mst wdely emplyed electrnc meters. 2. Analg Meters An electrnc analg vltmeter s prmarly based n an electrnc amplfer fr nput sgnal prcessng and a DC ammeter usually an electrmechancal meter t dsplay the utput sgnal f the amplfer. The peratng prncple cnssts f the generatn f the utput DC current f the amplfer prprtnal t the nput sgnal t be measured. Ths DC current flws nt the DC ammeter, thus frmng a DC mvng-cl mllammeter. Analg meters are usually vltage measurement systems, wth a hgh-nput mpedance as hgh as a few megahms. Current measurements can be als realzed by prperly mdfyng the nput stages f the vltage-measurng devces. The nternal resstance f the realzed ammeters s n the range f a few hms. Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren When a vltmeter s cnfgured, dfferent full-scale values can be btaned by usng selectable-rat vltage dvders f the nput vltage s hgher than the amplfer dynamc range. The same bjectves can als be acheved by selectng the apprprate amplfer gans f the nput vltage stays wthn the amplfer s dynamc range. Fr the ammeters, dfferent full-scale values can agan be btaned by usng selectablenput shunt resstrs n rder t cnvert the nput currents nt vltage sgnals cverng the dynamc range f the peratnal amplfer. The man features f analg meters are hgh gan and wde bandwdth fr AC measurements. The relatve measurement uncertanty f the nstrument can be as lw as the 0.1% f the full-scale value. Hwever, the readng errrs may be much hgher, snce human judgment and nterpretatn are nvlved. Because f these features, electrnc analg vltmeters may have better perfrmances than electrmechancal nes wthn the same prce range f devces. 2.1. DC Analg Vltmeters and Ammeters Fgure 2 shws a typcal basc structure f a DC analg vltmeter. Assumng that the peratnal amplfer has an deal behavr, t can be readly checked that current I m, flwng n the mllammeter A, s gven by: U U Im = I + I2 = + R R 2 = R R + R U = R + 2 2 2 U 1 R 1 R 2 R R 1 R If R 1 = R 2 s taken and the same resstances are far greater than R Eq. (1) becmes: I U m = (2) R (1) Fgure 2. Electrnc DC analg vltmeter structure Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren Eq. (2) shws that the mllammeter readng s drectly prprtnal t the nput vltage thrugh resstance R nly. Ths means that nce the mllammeter full-scale value s set the vltmeter full-scale value can be changed wthn the dynamc range f the amplfer by changng the value f R. Ths way, the full-scale value f the meter can be changed wthut changng the nput mpedance. The structure n Fgure 2 can be used as a DC ammeter f the current I t be measured s cnverted nt vltage U by means f the shunt resstr R s. Prvded that R s << R 1, the mllammeter readng s gven by: I m U R s = = I (3) R R The ammeter full-scale value can agan be set wthn the dynamc range f the amplfer by changng R nly, wthut changng the ammeter external resstance R s. 2.2. AC Analg Vltmeters and Ammeters The prncple f peratn f mst AC vltmeters s based n measurng vltage amplfcatn r attenuatn and ts cnversn nt anther physcal quantty. The basc structures cnssts f vltage dvders and AC vltage amplfers. The analg methds f AC vltage cnversn are used n bth analg AC vltmeters and dgtal nes. Usually, such analg methds prvde AC vltage cnversn nt DC vltage by usng AC DC cnverters. Dependng n the type f AC DC cnverter, the DC vltage can be made prprtnal t average, rms, r peak value f the AC vltage. The AC DC cnverters can be: average AC DC cnverters (half-wave r full-wave rectfers), rms AC DC cnverters, r peak AC DC cnverters (peak detectrs). Recently, AC vltmeters have been desgned usng the methd f samplng cnversn synchrnus r randm samplng. Hwever, such methds f cnversn are used bth n analg AC vltmeters and n dgtal nes. Fr example, the True RMS Analg Mllvltmeter 9301A and the RF Dgtal Mllvltmeter 9303 Racal-Data Instruments use a randm samplng methd. Anther example s n the Hewlett Packard 3458A, n whch multmeter three methds f cnversn are used: the analg rms methd, the synchrnus samplng, and the randm samplng methd. In dgtal AC vltmeters based n analg methds f AC vltage cnversn, the DC vltage s cnverted nt dgtal frm by means f ADCs. The methds f cnversn nclude dual-slpe, successve apprxmatn technques, and ntegratng types, as well as sgma delta ADCs. In many applcatns, AC analg vltages and currents are measured by means f DC analg vltmeters and ammeters wth an addtn f a rectfyng crcuts n the nput stage. A typcal basc structure fr an AC analg vltmeter can be derved drectly frm that f Fgure 2, mdfyng t as shwn n Fgure 3. Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren Fgure 3. Rectfer-based AC analg vltmeter Because f the hgh nput mpedance f the electrnc amplfer, 2 (t) = 0, and hence current m (t) flwng thrugh the mllammeter, A als flws thrugh the lad resstance, and, therefre, m (t) = (t). Snce the amplfer s cnnected n a vltage-fllwer cnfguratn, the utput vltage s gven by: u ( t) = u ( t) (4) Due t the presence f the nput dde, current m (t) s gven by: () t u () t m = (5) R0 when u (t) > 0, and m (t) = 0 when u (t) 0. The DC mvng-cl mllammeter measures the average value I m f m (t), whch under the assumptn f snusdal sgnals s related t the rms value U f u (t) by: I m 0 2 2 = U (6) πr The realzed meter s actually an average detectr, whch can be used as an rms detectr by labelng the DC mllammeter scale accrdng t Eq. (6), prvded that the nput sgnal s a snewave. Therefre, the realzed nstrument s nt a true rms meter. The structure n Fgure 3 can readly be changed nt an ammeter by cnvertng the current t be measured (t) nt vltage u (t) by means f a shunt resstr R s. In ths stuatn, the average value I m f m (t) s related t the rms value I f the snusdal nput current (t) by: Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren I m 2 2 Rs = I (7) π R The same cnsderatns as thse reprted fr the AC vltmeter apply. The structure f Fgure 3 can be mprved t the structure shwn n Fgure 4, where the nput rectfyng stage realzes a full-wave rectfer. The utput sgnal cmng frm amplfer A 1 s gven by: () t fr u () t fr u () t u 0 u 1() t = (8) 0 < 0 where u (t) s the nstrument nput vltage. If capactr C s suppsed t be nt cnnected, the utput vltage f amplfer A 2 s gven by: () t [ u () t + u ( t) ] u 2 1 = (9) Takng nt accunt Eq. (8), ths utput vltage can be als wrtten as: () t fr u () t () t fr u () t u 0 u () t = (10) u < 0 thus prvng that the crcut n Fgure 4a realzes a full-wave rectfer. In the case f a snusdal nput sgnal, the sgnal wavefrms are shwn n Fgure 4b. When capactr C s cnnected n the feedback lp f amplfer A 2, the last stage f the crcut n Fgure 4a s changed nt a frst-rder, lw-pass flter. The crcut utput vltage s then gven by the average value f u (t): () t U = u (11) In the case f a snusdal nput vltage, ts rms value U s related t U by: U 2 2 = U (12) π and U can be measured by a DC vltmeter. Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren Fgure 4. Average vltage AC DC cnverter vltmeter: a. Structure; b. Sgnal wavefrms Once agan the realzed meter s actually an average vltage detectr, that can be used as an rms detectr nly f the nput sgnal s a sne wave. Therefre, the realzed nstrument s nt a true rms meter. The structure n Fgure 4a can readly be changed nt an ammeter by cnvertng the current t be measured (t) nt vltage u (t) by means f a shunt resstr R s. In ths stuatn, the average value nput current (t) by: U s related t the rms value I f the snusdal U 2 2 = Rs I (13) π The same cnsderatns as thse reprted fr the AC vltmeter apply. Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren - - - TO ACCESS ALL THE 28 PAGES OF THIS CHAPTER, Vst: http://www.elss.net/elss-sampleallchapter.aspx Bblgraphy Bentley J.P. (1988). Prncples f Measurement Systems, 2nd Edn. Burnt Mlls, UK: Lngman Scentfc and Techncal. [A cmprehensve bk n measurements systems.] Bracc M. (1978). Basc Electrcal and Electrnc Tests and Measurements. Restn, VA: Restn Publshng. [A useful bk fr basc nfrmatn.) Hlman J.P. (1989). Expermental Methds fr Engneers, 5th Edn. New Yrk: McGraw-Hll. [A cmprehensve bk n measurement systems.) IEEE (Insttute f Electrcal and Electrncs Engneers). Transactns n Instrumentatn and Measurement. New Yrk: IEEE. [Cntans many artcles n vltage and current measurements.] Rathre T.S. (1996). Dgtal Measurement Technques. Lndn: Narsa Publshng. [Dgtal measurements are dealt n detal.] Schnell L. (1993). Technlgy f Electrcal Measurements. Chchester, UK, and New Yrk: Wley. [Gves an up-t-date nfrmatn n technlgcal aspects.] Taylr D.M. (1994). Industral Electrstatcs: Fundamentals and Measurements. Electrnc and Electrcal Engneerng Research Studes. Tauntn, UK: Research Studes Press, and New Yrk: J. Wley. [Cntans useful nfrmatn n electrstatcs.] Bgraphcal Sketches Alessandr Ferrer was brn n Mlan (Mlan), Italy, n 1954. He receved hs M.S. degree n Electrcal Engneerng frm the Pltecnc d Mlan n 1978. In 1983 he jned the Dpartment d Elettrtecnca f the Pltecnc d Mlan as an Assstant Prfessr n Electrcal Measurements. Frm 1987 t 1991 he was Asscate Prfessr f Measurements n Electrcal Machnes and Plants at the Unversty f Catana, Italy. Frm 1991 t 1994 he was Asscate Prfessr f Electrcal Measurements at the Dpartment d Elettrtecnca f the Pltecnc d Mlan Unversty. He s presently Full Prfessr f Electrcal and Electrnc Measurements at the same department. Hs current research nterests are cncerned wth the applcatn f dgtal methds t electrcal measurements and measurements n electrc pwer systems. He s a fellw member f IEEE, a member f AEI (the Italan Asscatn f Electrcal and Electrnc Engneers), a member f ANIPLA (the Italan Asscatn n Industral Autmatn), and was als Charman f the Mlan Sectn fr the tw-year term 1997 1998, a member f the Italan Infrmal C.N.R. Grup n Electrcal and Electrnc Measurements, and a member f the Nrth Italy Chapter f the IEEE IM Scety. Dr. Halt Eren has receved the degrees f B.Eng. (1973), M.Eng. n Electrcal Engneerng (1975), and Ph.D. n Cntrl Engneerng all frm the Unversty f Sheffeld, UK. Recently he has btaned an MBA frm Curtn Unversty f Technlgy, Perth, Western Australa. Dr. Eren has been lecturng at the Curtn Unversty f Technlgy snce 1983, frst at the Kalgrle Schl f Mnes and then at the Schl f Electrcal and Cmputer Engneerng, Perth, Western Australa. He has served as the Head f Department f Electrnc and Cmmuncatn fr sme tme. Hs expertse areas are cntrl systems; nstruments, nstrumentatn, and netwrkng; mneral prcessng; sgnal prcessng; and engneerng mathematcs. Hs prncpal areas f research are ultrasnc and nfrared technques, densty and flw measurements, msture measurements, feldbus, telemetry, telecntrlers, mble rbts, hydrcyclnes, Encyclpeda f Lfe Supprt Systems (EOLSS)
ELECTRICAL ENGINEERING Vl. II - Electrnc Vltmeters and Ammeters - Alessandr Ferrer, Halt Eren and applcatns f artfcal ntellgence. He serves as a cnsultant t a number f ndustral establshments. He has wrtten numerus artcles n bks publshed by CRC Press and Wley and Sns. Encyclpeda f Lfe Supprt Systems (EOLSS)