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.

0
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.