Names and Student Numbers of the lab partners:______________________________________
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EXERCISE : THE OUTPUT RESISTANCE OF A POWER SUPPLY
Goal: to investigate properties of power supplies; to review basic electrical measurement techniques.
Introduction
Any source of electrical energy (generator, battery, thermocouple, etc.) with no load attached to it
produces voltage potential across the terminals called an electromotive force (emf), or an open-circuit
voltage, V∞.
This number does not completely specify the power supply. In a closed circuit a current I will be
drawn from the power supply, and the voltage at the terminals, V, called the terminal voltage, will
typically fall below V∞:
A plot of the terminal voltage V vs. current I may look like in Fig. 1.
Linear
Nonlinear
Imax
I
FIG.1: Terminal voltage vs.
Many power sources will exhibit a linear variation of V for small current values, followed by a
nonlinear behaviour at higher currents. The linear part of the curve can be described by
V = V∞ - RI ,
(1)
where R is the output resistance of the power source. In this linear regime, according to Thevenin’s
theorem, the power source is completely represented by this equivalent circuit as in Fig. 2.
Fig. 2. Equivalent circuit of an electric power source
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The output resistance, R, can be determined by seeing how the output voltage, V, varies with current,
by attaching external resistances, Rl, to the power source, and measuring the current and voltage with
a multimeter.
Figure 3 shows two possible ways of doing this. Both would be equivalent if the multimeter were
ideal (e.g., if no current flows through the voltmeter, and if the resistance of the current meter is 0).
However, a real multimeter is not ideal.
Option 1
Power
source
Option 2
Power
source
Rl
Rl
Fig. 3. Possible circuits for determining the output resistance of a power source.
Question 1: Which one of these two arrangement options would be better given a non-ideal
multimeter? Explain.
Question 2: Could the details of how you make the connections matter? Explain.
Experiment:
For (i) the battery and (ii) the DC power supply set to around 6.5V do the following:
A) Choosing 4 different values of Rl from the box provided, take measurements of the terminal
voltage V and the current I. Include an estimate of the uncertainty in these measurements.
Organize all data in a table. Provide your notes.
B) Using Excel (or any other graph software), plot V vs I to determine the output resistance of
the power source, R. Estimate the uncertainties in R.
Sketch: Provide a sketch of your setup or clearly explain the circuit you have used. This is more
detailed than Figure 3, and should be sufficient for another student to exactly reproduce your setup,
given only your sketch or description.
Question 3: Justify your choice of the resistors, Rl.
Question 4: State the requirements to the resistance of the voltmeter and the resistance of the
ammeter.
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