lab2_u11_dc_measurements - University of Florida Department...

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EEL 3111 — Summer 2011 Drs. E. M. Schwartz & R. Srivastava Department of Electrical & Computer Engineering Ode Ojowu, TA Page 1/6 Revision 0 25-May-111 Lab 2: DC Measurements Figure 1 D’Arsonval galvanometer (from Wikipedia). Figure 2 Ammeter made from galvanometer and shunt resistor. Im I Ish OBJECTIVES Understand the galvanometer and its limitations. Use circuit laws to build a suitable ammeter and voltmeter from the galvanometer. Understand the loading effect meters have on a circuit. MATERIALS The lab assignment (this document). Your lab parts. Pre-lab questions including Multisim. Decade resistor box. Read section 3.5 in the textbook. INTRODUCTION The moving coil galvanometer (also known as a d’Arsonval meter) shown in Fig. 1, contains a pivoted coil of fine wire through which a current passes. The magnetic interaction between the magnetic field set up by this current and the magnetic field of a permanent magnet housed in the meter casing results in movement of the coil against a hair-spring (torsion spring). The resulting displacement is shown by the meter needle against a scale. The current that causes full-scale deflection (needle deflects to the maximum extent of the meter) is labeled on the galvanometer. Meter movements are usually rated by current and resistance. For our meters, this full-scale current is 1.0 mA and the meter resistance is between 2.3k and 2.4 k . The current through the meter is proportional to the deflection. If the meter deflects 40% of the full- scale (or to 0.4 on the meter), the measured current is value of 40% of 1.0 mA, i.e., 0.4 mA. The galvanometer also has an internal resistance. If the resistance is known, the galvanometer can be used to measure voltage by utilizing Ohm’s law. The full-scale voltage is found by multiplying the full-scale current by the meter resistance. If the full-scale voltage is 10 mV and the needle deflects to 0.2, the measured voltage is 2 mV. Ammeter The galvanometer, by itself, is not a very effective ammeter because its range is small (the full-scale current is only 1.0 mA). It also has a relatively high meter resistance, which changes the circuit it is measuring. Both problems can be mitigated with a shunt resistor . The shunt is a small resistor placed in parallel with the galvanometer. Because its resistance is much lower than that of the galvanometer, most current passes through the shunt and we don’t overshoot the full-scale current of the galvanometer. (Going beyond the full- scale value of the galvanometer may damage the device.) An improved ammeter that can be made with the galvanometer and a shunt resistor is shown in Fig. 2. The current to be measured is
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lab2_u11_dc_measurements - University of Florida Department...

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