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l03acml - AC Voltmeter Loading A A When an AC voltmeter is...

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Unformatted text preview: AC Voltmeter Loading A A When an AC voltmeter is connected across nodes AA’ to measure the voltage both the magnitude and phase of the AC voltage change. Prior to connecting the volt— meter the voltage is V and after it is connected the voltage is V’. The type of voltmeter that will be con— sidered measures only the rms magnitude of the AC voltage; phase changes cannot be measured. Therefore, the percentage error due to AC voltmeter loading in the measurement of the rms magnitude of an AC voltage is defined V —— V %er'r0r= lOOXL—i—z—L—i | V | iffV¢ 0 andis zeroifV= 0. Example. 12k 10nF A e(t)=5 2 cos(mt) e(t) 6 F 47k f=1kHz " . A' 12k -j15.91k A 540115 %-j26.5k %47k <- 2th Al _.. —'26.5 -—'15.91 47 Eth = 510°x———————————————————‘7 _“['3, + 1 12 + (—326.5|| [—315.91 + 47]) 3.541 — 4.62OV X________—_ 47 — 315.91 7th 2 47|| [—315.91 + 12“ (—j26.5)] : 17.651 —— 44.27OkQ _ _ Z- Vl / I Ethz—fl——-— AA Z7177. + Zth For the measurement of AC voltages the input impedances of the three voltmeter used in lab may be modeled as simply a resistor. The input impedance of the Fluke 73 HHDlVflVI is 10 M Q; the input impedance of the Hewlett-Packard 34401A DMlVI is only 1 M Q; the in— put impedance of the Simpson Model 260-7 is Zin = S st Where S is the AC sensitivity Which is 5 k0 / Volt and st is the range voltage. So, if a Simpson meter is being used and is set to the 10V range the input impedance is Zz-n = .3st = 5M2 x 10 = 50.0129. The voltage measured by the AC voltmeter is then _ _ z. VIAA’ = Ech—ZL-r = Zin + Zth 3.541 —4.620 ><—————L—— = 2.7716.510V 50 + 17.651 — 44.270 175m: WI 0/0 error = 100 X 2.77— . 4 100 X ————§i = —21.67% 3.54 Note: % error 75 _100§-—th—_—— because of the phase of —Z—th ...
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