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HACETTEPE UNIVERSITY ELE 271 MEASUREMENT LABORATORY EXPERIMENT #2 USE OF BASIC METER AS AN AVO METER IN DC CIRCUITS Object : To utilize the basic measuring instrument in DC circuits, to measure its internal resistance and to investigate the loading effect in the circuit. 1. THEORY : Basic meter is the main construction element of the AVO meters that we are using in the laboratory. Essentially it is an ammeter, but by some modifications one is able to measure voltage and resistance values. The use of basic meter as DC ammeter, DC voltmeter and ohmmeter is explained in examples below. 1.1 DC Ammeter : Assume we are given a basic meter having an internal resistance of 100 and a full scale deflection of 1 mA. In order to measure current values greater than 1 mA, say 3A, a resistor should be connected in parallel. The typical connection of a DC ammeter is given in figure.1. I m R m =100 I I sh t R sh + V - m 0-1mA Figure.1 The calculation of the shunt ( parallel ) resistor R sh is as follows: The voltage drop across the basic meter, V m is given by V I R m m m = * V m = 1 100 100 mA mV * Ω = Since the full scale value of the total current to be measured is 3A. and the current sensitivity of the basic meter is 1mA full scale, the current through the shunt resistor is I I I sh t m = - = - = 3000 1 2999 mA Hence the value of the R sh , R V I sh m sh = / = = 100 2999 0 03334 mV mA / . 8
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If a shunt resistor of 0.03334 W is used, then the basic meter can measure currents up to 3 A. Note that the measuring range of the ammeter is extended by 3000. 1.2 DC Voltmeter : Without any modification to the previous basic meter, it is able to measure a full scale voltage of 100 mV. In order to measure greater values of voltages, a series resistor should be connected to the meter. The connection diagram is given in figure.2. + DC Voltage to be measured -
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This note was uploaded on 05/25/2011 for the course ELECTRONIC 271 taught by Professor Ugurbaysal during the Spring '11 term at Hacettepe Üniversitesi.

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