Kirchhoff lows
Investigate some properties of real voltage sources, as opposed to idealised
ones. DC voltage sources always have some output resistance which affects how the circuit behaves.
The output resistance of a laboratory DC power supply is small, as it should be but in this experiment, we’re
going to simulate a voltage source with a rather larger resistance by placing a fixed resistor in series with
the DC power supply.
Instructions
Figure 1: Circuit for maximum power transfer theorem experiment
a) Connect a laboratory power supply in series with a fixed 100 ohm resistor. Across this simulated
voltage source connect a decade resistance box to act as a variable load resistor, RL.
b) Temporarily remove RL and connect the meter to read the voltage across the power supply and
100 ohm resistor.
c) Adjust the power supply so that the voltmeter reads exactly 1V. Do not adjust the voltage setting
from now on. The open circuit voltage of your simulated voltage source is thus 1V.
d) Reconnect RL. Measure both the current and voltage, by connecting an ammeter and voltmeter as
shown above in Figure 1, for a range of values of load resistance, RL, from zero to 200 ohms.
e) Record your readings in the form of the table shown overleaf.
20
40
60
80
100
120
140
160
180
200
f) On a single graph, plot the following three functions:
i. VL
as a function of RL
ii. I
L
as a function of RL
iii. The product V
L
I
L
as a function of RL
In each case, RL is plotted on the x-axis. You will need to define a different vertical scale for each
function.
g) From your graphs, determine the following:
i. At what value of load resistance is the load voltage half of its open circuit value
(1V)?
ii. Is this what you would expect? If not, why not?
iii. At what value of load resistance is the load current half of its short circuit value (i.e.
when RL = 0)?
iv. Is this what you expect? If not, why not?
v. What value of load resistance is the power that is dissipated in the load (VL
I
L
) at its
maximum?
vi. Does your result agree with the maximum power transfer theorem, which states
that the power dissipated in a load is a maximum when the load resistance is equal
to the source resistance?
4110ENG Engineering Practice 1 Lab Sheet
Jan 2015 3 MMS
Experiment 2 – Kirchhoff’s Laws
Introduction
Kirchhoff’s first law states that the algebraic sum of the currents at a node is zero.
Kirchhoff’s second law states that the algebraic sum of the potential differences and voltage sources in any
closed loop in a network is zero.
In this experiment, you will attempt to verify these laws.
Instructions
a) Set up the circuit shown in Figure 2 below.
Figure 2: Circuit for Kirchhoff’s laws experiment
b) Measure the current at points A, B & C and record your readings and the direction of the current
( i.e. moving towards or away from node D). Calculate the algebraic sum of the currents at node D.
Is this what you expect?
c) Measure the voltage dropped across each resistor, again recording your readings. Make sure you
note the polarity of each voltage.
i. voltage across 220 ohm = ?
ii. voltage across 330 ohm = ?
iii. voltage across 470 ohm = ?
d) Demonstrate that Kirchhoff’s voltage law holds true around loops ADBE, DCEB and ADCE.
e) Comment on your results.
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