trical Engineering I Laboratory
Experiment I
Power Supplies, Multimeters,
and Simple Resistive Circuits
1
Introduction
Objectives
To get acquainted with typical equipment used in
experiments involving circuits.
To establish basic rules for data acquisition and
manipulation
To begin using PSpice for simulations
1
.
Overview
The main objective of this laboratory experiment is to get acquainted with various
typical equipment that is needed in order to perform experiments involving circuits.
Such equipment include Power Supplies, Multimeters, and Oscilloscopes. In this
laboratory experiment Power Supplies and Multimeters are covered
2
. These will
suffice to perform some simple experiments dealing with Ohms law, voltage
division and current division.

1
For your laboratory report, you will be asked to simulate in PSpice the behavior of the circuits you
worked with. If you need more time for such simulations, obtain permission to first submit the report
except of the PSpice simulations, and then at a later time specified by the instructor submit the PSpice
simulations.
2
The study of Oscilloscopes is reserved for next experiment (II). PEEI-I-2/13
1.1-The Power Supply
The power supply is a device that provides a DC or AC
3
voltage or current source
whose level can be adjusted by a knob on the device.
1.2-The Multimeter
The Multimeter is the basic tool in any circuits laboratory, and it is used to measure
the primary signal variables, namely current (in amperes, A) and voltage (in volts,
V). It is further used to measure what is probably the most important parameter of an
element namely its resistance (in ohms, ).
There are two common types of multimeters, the digital multimeter (DMM) and the
analog multimeter (AMM). The value indicated by the analog multimeter is
determined by comparing the position of the pointer, or needle, to one of the scales
printed on the face of the meter. Selecting the particular scale for the particular
measurement is achieved by utilizing one or more of the controls on the front of the
multimeter. These controls provide for combinations of three measurement
parameters: function, scale, and zero ohms.
The function variable selector determines the variable that will be measured by the
multimeter from among volts, amperes or ohms. This manual uses the widely
accepted vernacular to describe the three standard functions on AMMs and DMMs:
Ohmmeter identifies an instrument used to measure resistance. Ammeter and
Voltmeter denote instruments used to measure current and voltage respectively.
The scale switch is used to set the sensitivity (or range) of the multimeter. Typically,
multimeters can measure voltage and current on four or more different scales. For
voltage and current functions, the number selected by the scale switch indicates the
maximum value that can be measured on that particular scale. Therefore if the scale
switch is set to 50 V, the scale that ends with 50 should be used to determine the
measured value.
Resistance measurement is usually an iterative process. Unlike the voltage and
current functions, the ohmmeter needs to be zeroed on the particular resistance scale
before it is used for measurement. Zeroing a resistance scale can be achieved as
follows: a scale is selected and a simple copper wire is connected between the two
terminals of the meter. Since the resistance of a simple copper wire is practically zero
(a short circuit), the meter should indicate a zero value. If it is not so, the knob on the
meter is adjusted until it does. A good practice is to start at the highest scale and go
down to lower scales as necessary zeroing on each scale before switching to the next
lower one.

3
The notation of DC (Direct Current) and AC (Alternating Current) may be confusing when applied to
voltage but, unfortunately, this notation is too well established; think of DC voltage as direct voltage PEEI-I-3/13
2
Theory
2.1
Ohm's Law
Ohm's Law states that the voltage across a resistor is directly proportional to the
current flowing through that resistor. The constant of proportionality is the resistance
value of the resistor in ohms (
!). The circuit symbol for the resistor is shown in fig.
1. For the current and the voltage shown, Ohm's Law is
v
= Ri
(1)
where R 0 is the resistance in ohms (
!),
v is the voltage in volts (V), and
i is the current in amperes (A).
+
-
V
R
!
i
Fig. 1 Ohm's Law
The instantaneous power consumed by a resistor can be calculated in several ways:
P
= iv = i
2
R
= v
2
R = v
2
G
(2)
where
G = 1/R
is the conductance (in
-1
or mhos or siemens). PEEI-I-4/13
2.2
Voltage Division
There are times, especially in electronic circuits, when it is necessary to develop
more than one voltage level from a single voltage supply. This can be achieved by
the circuit shown in Fig. 2 which is known as a voltage divider circuit.
+
-
V
s
R
1
R
2
+
-
V
0
Fig. 2 The Voltage Divider Circuit
Direct application
4
of Kirchhoff's Laws and Ohm's Law, results in:
V
0
= V
S
R
2
R
1
+ R
2

(3)
If a load R
L
is connected to the voltage divider, as shown in Fig. 3, the expression for
the output voltage becomes:
V
0
=
V
S
R
2
R
1
1
+ R
2
R
L
!
"
#
$
%
& + R
2

(4)
In this case R
L
loads the voltage divider
5
, and consequently V
0
is smaller than it
would be for the ideal case in which R
L
was infinity (eq.3). Note in eq. 4, that the
output voltage V
0
depends explicitly on the load resistance R
L
.
+
-
V
s
R
1
R
2
R
L
+
-
V
0
Fig. 3 A Voltage Divider Connected to a Load R
L

4
For more detail, see section 3.3 of your textbook
5
i.e. draws current from the voltage divider PEEI-I-5/13
2.3
Current Division
The current divider circuit, shown in Fig. 4, consists of two resistors connected in
parallel across a current source. The current divider is designed to divide the current i
between R
1
and R
2
.
Direct application
6
of Ohm's Law and Kirchhoff's Current Law, gives:
i
1
= iR
2
R
1
+ R
2
(4A)
and
i
2
= iR
1
R
1
+ R
2

(4B)
!
i
R
1
R
2
"
"
i
1
i
2
+
-
Fig. 4 A Current Divider Circuit
3
Prelab Exercises
There are no pre-lab exercises for this, the very first lab
7
.

6
For more detail, see section 3.3 of your textbook
7
Pre-lab exercises are included in most subsequent lab experiments, and they form an integral and crucial
part of preparation for each lab experiment. They are designed in a way that enhances understanding and,
more often than not, makes it impossible to execute the lab experiments without having first completed the
pre-lab exercises. Completing them and coming prepared to the lab is an integral part of students
responsibility (and grade). PEEI-I-6/13
4
Experiments
Suggested Equipment:
TEKTRONIX PS 503 Power Supply
Keithly 179A TRMS Multimeter
0-1000 Simpson Milliammeter or Suitable DMM as available
10 K
! Resistor
Protoboard
4.1
Ohm's Law
This experiment is designed to verify Ohm's Law
4.1.1 Wiring
+
-
DVM
A
+
-
V
s
+
-
+
-
V
R = 10K
Power
Supply
!
"
!
I
s
I
A
I
V
Fig. 5 Schematic Diagram of circuit for V-I measurement
Arrange the circuit of fig. 5 on the lab bench where each circle indicates a device
and the circled A with the arrow through it is an ammeter.
As a matter of safe practice and convenience, get in the habit of following these
rules:
1- Wire always from the load toward the source, not vice-versa.
2- Never make the final connection to the power supply without the
instructor's approval. Too many milliammeters are lost and a lot of fuses
are blown unnecessarily the other way.
3- Ammeters must be hardwired while building the circuit. It is usually
better to add voltmeters last to the complete functional circuit
8
.

8
This makes it easy to change a voltmeter connection or to remove it temporarily for a resistance check or
some other use PEEI-I-7/13
4- Ammeters are practically short circuits: make sure that ammeters are
always placed in series with a circuit element through which
measurement of the current is required.
5- Voltmeters are practically open circuits: make sure that voltmeters are
always placed in parallel with the terminals across which measurement
of the voltage is required
9
.
4.1.2 Data Acquisition:
After the instructor has approved your connections, connect the source with an
initial output voltage of 0 volts.
Slowly, increase the output voltage of the power supply from 0V to 10V in
increments of 1V and read the current I
A
on the ammeter. Complete the table
10
with the data V vs. I
A
.
V
nominal
V
I
A
0 V
1 V
2 V
3 V
4 V
5 V
6 V
7 V
8 V
9 V
10 V
Table 1.

9
Adding a voltmeter in series instead of in parallel will open the branch with the only ramification of a
non-working circuit. Adding an ammeter in parallel, however, will at best result in a blown fuse and at
worst in a ruined ammeter and/or power supply.
10
Notice that there are two separate columns for the ordinate (voltage): the first labeled