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Exp_2_fa09

# Exp_2_fa09 - Physics 3330 Experiment#2 Fall 2009 DC...

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Physics 3330 Experiment #2 Fall 2009 DC Measurements, Voltage Dividers, and Bridges Purpose You will gain familiarity with the circuit board and work with a variety of DC techniques, including the Wheatstone bridge and the 4-terminal resistance measurement techniques. Reading: H&H Section 1.03, 1.04, 1.05. Theory 1. The Basic Wheatstone Bridge Bridge circuits are used to precisely compare an unknown impedance with a standard. The simplest example is the Wheatstone bridge (Fig. 2.1), a four-arm bridge with a resistor in each arm, which is usually used at DC or low frequencies. It has many applications in measurement circuits, where often the unknown resistance R x is a resistive sensor, such as a platinum thermometer or a mechanical strain gauge (see H&H 15.03). There are other types of bridges, including AC bridges with capacitors or inductors in one or more arms, radio-frequency bridges, and bridges that use precision transformers to generate voltage ratios. In our the basic bridge R x , R s , R 1 , & R 2 are each >>0.1 so that all contact resistances (typically of order .1 ) can be ignored. The bridge is made from two voltage dividers, each connected to the same source voltage ε . When the division of the two dividers is adjusted to the same value the null meter reads zero voltage ( V=0). This occurs when R x /R s = R 1 /R 2 , a formula which can solved for the unknown resistor R x in terms of the standard R s and the ratio R 1 /R 2 . Fig. 2.1 a) Basic Wheatstone bridge. b) Thevinin equivalent circuit. (b) A B ε _ + Τ R T V } 2 =10k =SR P =(1-S)R P R x Null-meter R S ε + _ Voltage Source } R 10-turn potentiometer R P R 1 Standard Unknown B V (a) A Experiment #2 2.1 Fall 2009

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