ESP1104 Introduction to Electronic Systems A. Khursheed
Tutorial 1
1. The following circuit is used to experimentally obtain the I-V characteristics of a certain device whose resistance is given by RL in Fig. 1. The DC variable voltage source has a voltage VS, the ammeter measures a current I, and the voltmeter measures a voltage V.
Assume that the internal resistances of the DC voltage source, ammeter and voltmeter are given by RS , Rma and Rmv respectively.
(a) Derive an expression for the measured voltage V. (b) Derive an expression for the measured current I
(c) How does the presence of the ammeter affect the voltage measurement V? (d) How does the presence of the voltmeter affect the current measurement I?
(e) The I-V graph is plot by varying VS and monitoring I (y-axis) and V (x-axis), how does the presence of RS , Rma and Rmv affect the I-V graph?
A
R V V L
S
Figure 1
2) The circuit in Fig. 1 is used to measure the I-V characteristics of a light bulb, and a schematic of the type I-V curve obtained is given in Fig. 2. The ac resistance can be found from the slope at each point of the graph, dV/dI, while the dc resistance is defined by the ratio, V/I.
Figure 2
(a) What is the bulb’s dc resistance at 0.5V?
(b) How does the bulb’s ac and dc resistance change with voltage?
(c) Suggest an explanation why the bulb’s resistance is non-linear and why it takes the form it does
3) Describe how you would go about measuring the internal resistance of a 1.5V D-cell. 0.5
V
2 V
1 m A 2 mA I
4) The Wheatstone bridge circuit shown in Fig. 4 is used to measure the value of the unknown resistance Ru of a strain gauge. The output voltage across the points DB in the bridge is measured by an ideal voltmeter, and the internal resistance of a current
measuring instrument across the output is Rm.
a) Derive an expression for the equivalent Thevenin resistance and Theveinin voltage for the bridge circuit as seen from DB.
b) If Vi = 10 V, R1 = R2 = R3 = 500 Ω, Rm = 10 kΩ, and the nominal value of Ru is 500 Ω, calculate the measurement sensitivity in volts per ohm change in Ru if a) the resistance Rm of the measurement instrument is neglected, and b) Rm is taken into account.
A
D B
C
Rm Ru
R3
R1
R2
Vi
Vm
Figure 4
