Download Experiment 1: Voltage, Current and Resistance

FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
EXPERIMENT 1
VOLTAGE, CURRENT & RESISTANCE
1.0 PRELIMINARIES
1.1 INTRODUCTION
The voltage (V), current (I) and resistance (R) are fundamental elements of any
electrical or electronic circuit. The voltage, measured in volts, will cause a
current, measured in amperes, to flow around a circuit. A resistance, measured
in ohms, is required to limit the current to safe levels. The three are related by
Ohm’s Law, which can be expressed in three ways:
V= IR
I = V/R
R = V/I
Relationships for electric power (P), measured in watts, can be determined
indirectly using Ohm’s Law:
P=VI
P = V2/R
P = I2R
1.2 PURPOSE OF THE EXPERIMENT
This experiment is intended to:
- Provide experience of conducting electrical experiments and
presenting results
- Provide experience in the use of basic laboratory equipment
- Develop familiarity with the basic units of measurement of voltage,
current and resistance.
1.3 PROCEDURE
There are several sections to this experiment. You should follow the
instructions given so that you obtain the experimental data for each part. This
must obviously be done in the lab. Outside the lab you may have to plot
graphs and write the results up in your lab book as a report, adding
conclusions and comments as suggested.
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
2.0 TEST 1: RESISTANCE MEASUREMENTS
2.1 PREREQUISITE
You will be using a Digital Multi Meter (DMM) on many occasions. The
DMM in the lab is part of a multifunction workstation, which includes three
other built in features: a function generator, a frequency counter, and a DC
power supply. The DMM is located in the top left quadrant of the workstation.
There are two types of workstation in the lab, both made by S.J Electronics,
the Mark II and the newer Mark IV. Please identify which workstation you
will be using, and make a note of this in your lab book.
2.2 PART 1 – SET UP AND RESISTANCE MEASUREMENTS
a) The on/off switch for the whole workstation is independent of the on/off
switch for the DMM. Locate the on/off switch for the DMM only, and
press to activate it and hence its display panel. Set the DMM to read
OHMs/ (using the dial selector on the Mark II, or the button selector on
the Mark IV).
b) Find two test leads (one red and one black) with 4mm plugs at both ends,
and on one end of each connect a croc clip. The other end is to be plugged
into the appropriate terminals on the DMM for resistance measurement,
i.e. black to COM and red to V/.
c) Fetch
1) A 680 (in the drawer marked ‘680R’) or 820
2) A 2.7k (in the drawer marked ‘2k7’) or 3.0k
3) A 1.5M (in the drawer marked 1M5) or 2.2M resistor.
d) Connect the 680 (or 820) resistor between the two croc clips and
record the DMM display. Repeat for the 2.7k (or 3.0k) and the 1.5M
(or 2.2M) resistors, each time recording your results in your lab book.
Note how the DMM automatically sets the range for the different resistors
measured.
The resistors in the lab have a tolerance of ±5%, i.e. a resistor whose stated
(nominal) value is 1k (1000) may in fact have an actual value anywhere in
between 1050 (+5%) and 950 (-5%). With this in mind, complete the table
below in your lab books for the resistors you have chosen in part c):
Nominal
value 
Max. possible
value (+5%) 
Min possible
value (-5%) 
Measured
value 
Actual % error
Table 1
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
2.3 PART 2 – COMMENTS AND CONCLUSIONS
This part is to be written up outside lab time. Comment on the consequences
of percentage errors and tolerances, especially in relation to Ohm’s Law.
3.0 TEST 2: BREADBOARD CONNECTIONS
3.1 PREREQUISITE
Breadboards can be purchased from the lab technician for a moderate fee.
Most of the circuits you will be using will be constructed on breadboards
(Figure 1), so it is essential to know how they function electrically.
Figure 1
3.2 PART 1 – CENTRAL BREADBOARD SOCKETS
a) Set the DMM to read OHM/.
b) Find two test leads, one red and the other black, with 4mm plugs at both
ends. On one end of each lead, attach a croc clip. The other ends are to be
connected to the correct terminals on the DMM.
c) Find two short lengths of wire (about 2 inches/5cm long) and strip the
insulation from each end (about 5mm) and fit an end into each croc clip.
d) Push the free ends of the two short wires into the pairs of breadboard
sockets as indicated in Table 2, and thus measure the resistances between
the various sockets
Connection 1 to socket
D3
D3
D3
D3
D3
D3
D3
D3
Connection 2 to socket
A3
B3
C3
E3
F3
H3
D2
D4
Resistance value 
Table 2
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
e) Based on your observations, which of the following connections are
correct practice when connecting the ends of a resistor into the breadboard,
and explain why?
i. A3 and D3
ii. D3 and B3
iii. C3 and D3
iv. D3 and E3
v. H3 and D3
vi. D3 and D2
vii. D3 and D4
3.3 PART 2 – OUTER EDGE BREADBOARD CONNECTIONS
Use a similar procedure to that above to find out where (a) there are, and (b)
there are not connections between the lines of sockets along the long outside
edges of the breadboard.
3.4 PART 3 – COMMENTS AND CONCLUSIONS
These comments and conclusions should be written up outside lab time.
a) What is the resistance value, in Ohms, of a ‘short circuit’ and of an
‘open circuit’?
b) How many sockets are there in total on your breadboard?
c) State which groups of sockets have a short circuit between them,
and where there are open circuits between sockets.
d) Sketch two diagrams on how you could connect three resistors in
series (adjacent to each other) using i) the coloured sets of
connectors at the sides of the breadboard and ii) the lettered sets of
connectors at the centre of the breadboard.
4.0 TEST 3: VOLTAGE, CURRENT AND POWER IN RESISTORS
4.1 INTRODUCTION
A basic electrical circuit will be connected up and the effects due to various
changes in the circuit parameters investigated with the results presented in
graphical form.
4.2 PART 1 – TESTS WITH CONSTANT RESISTANCE
a) Fetch a 680 or 820 resistor (record which one you use) and connect it
to sockets G2 and D2 (or equivalent) of a breadboard.
b) Use the DC power supply located in the bottom right quadrant of the S.J
Electronics (MK II and MK IV) workstation, as shown below in Figure 2.
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
Figure 2
c) Using appropriate sockets and test leads (one red and one black), connect a
resistor into the breadboard with croc clips placed on each leg. Do not
connect anything to the DC power supply at this stage.
d) The DC power supply, as shown in Figure 2, has three sets of terminals,
each with a pair of red (+) and black (-) sockets. The top red and black
terminal provides a constant 5V DC output that cannot be adjusted. The
second pair of terminals provide a 15V DC output that also cannot be
adjusted. The lower pair of terminals are capable of producing a maximum
DC output voltage of 30V, which can be varied to anywhere between 0V
and 30V by using the dial marked ‘VOLTAGE’, situated to the left of the
fixed 5V DC output terminals. Ensure that the ‘CURRENT’ dial is in the
12 ‘o’ clock position, i.e. pointing ‘north’.
e) Set the DMM to measure voltage. On the MK II, this requires that the dial
is turned to the ‘V’ position, whereas on the MK IV, select the ‘DC V’
button so that ‘V’ is shown on the right of the display panel.
f) Plug the 4mm red test lead plug into the V/ socket of the DMM, and the
black plug into the ‘COM’ socket of the DMM. Set up the DMM and DC
power supply to measure voltage across the resistor, as shown in Figure 3.
Turn the ‘VOLTAGE’ dial to minimum (several turns anticlockwise), and
have your circuit checked by a member of staff before proceeding.
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
BLACK COM
R
RED +
V/
DMM
DC POWER SUPPLY
0 ~ 30V
Figure 3
g) Switch on the DC power supply. Slowly increase the voltage (by turning
the ‘VOLTAGE’ dial clockwise) to the first required value (2V). The
power supply has its own voltmeter display, but use the DMM display
panel to record your voltage in a table like Table 3 in your lab book.
h) Without changing the ‘VOLTAGE’ dial, switch off the DC power supply
and unplug the DMM from the circuit. You must now set the DMM to
measure current. On the MK II, the DMM dial should be turned to the mA
position (to read milliamps,  10-3A), whereas on the MK IV, the ‘DC A’
button should be pressed until the green mA indicator appears on the right
of the display panel. Connect your circuit as shown in Figure 4 below in
order to be able to measure current
mA
DMM
COM
R
RED +
BLACK -
DC POWER SUPPLY
0 ~ 30V
Figure 4
i) Note that measuring current involves a different circuit configuration to
that of measuring voltage. Here, the DMM in ammeter (device for
measuring current) mode is situated in series with the resistor. This set up
first requires ‘breaking’ the circuit and then ‘remaking’ it so the ammeter
is in the correct position. To measure voltages, the DMM in voltmeter
(used to measure voltages) mode is placed in parallel with the resistor.
Once you have constructed the circuit as shown in Figure 4, have it
checked by a member of staff. Then switch on the DC power supply and
record the current as required in Table 3, in your lab book.
j) Repeat the voltage and current measurements to complete Table 3. Take
readings for power supply voltages of 2V, 4V, 6V, 8V, 10V and 12V. You
should plot a ‘control graph’ of V versus I in the lab so that any dubious
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
points can be immediately checked out (e.g. the voltage control might have
been accidentally varied – note comment in Appendix 4). This needs to be
done now as points cannot be checked after you have left the lab. Have
your control graph verified by a member of staff. Do not forget to note
down the value of the resistor you used for this part of the test.
Nominal
voltage V
Accurate Voltage
V
Measured current
mA
V  I ratio
V  I product
Table3
4.3 PART 2 – TESTS WITH CONSTANT VOLTAGE
a) Fetch the following six resistors: 2.2k, 1.8k, 1.5k, 1k, 820 and
680.
b) Draw Table 4 in your lab books.
Nominal
Resistance 
Measured
Resistance 
Measured
Current mA
V / I ratio
V  I product
Table 4
c) Use the DMM to measure the resistor values, and record your results in the
table.
d) Set the variable DC supply, VS, to be 5.0V  VS  6.0V. Use the DMM to
measure the actual value, and record it in your lab books. It is important
that this stays constant for the entire test.
e) Reset the DMM accordingly and use it to measure the current for each
resistor value and for the same VS. Record all the values in tabular form
(Table 4) in your lab books, and plot a control graph of I versus R in the
lab so that any dubious points can be immediately checked out.
4.4 PART 3 – COMMENTS AND CONCLUSIONS
Note the instructions in Appendix 1 about presenting graphs. Plot the
following five graphs:
a) I versus V and power versus V for Test 3, Part 1
b) I versus R and power versus R for Test 3, Part 2
c) Power versus 1/R for Test 3, Part 2
Comment on the shape of the graphs, relating those to Ohm’s Law and the
equations for power. Also explain the ‘first break circuit then remake’
technique needed to make a measurement of current (Test 3, Part 1).
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
5.0 TEST 4: RESISTOR COLOUR CODE SYSTEM
This part of the experiment can be done outside lab time. No experiments are required
here. Draw these tables in your lab book, and complete them. Use the colour code
system outlined in Appendix 3.
1st Digit
Band 1
Black
Orange
Brown
Yellow
Orange
Brown
Brown
2nd Digit
Band 2
Brown
White
Black
Violet
Orange
Black
Black
Multiplier
Band 3
Black
Brown
Red
Green
Orange
Gold
Yellow
Tolerance
Band 4
Gold
Brown
Gold
Red
Silver
Gold
Silver
Resistor Value
1st Digit
Band 1
2nd Digit
Band 2
Multiplier
Band 3
Tolerance
Band 4
Resistor Value
680k, 5%
1.2k, 10%
1.2, 10%
820, 1%
1.5M, 10%
1, 5%
3.9k, 20%
Nominal Value
680k
820
3.9k
± Tolerance
5%
1%
?
Minimum value
?
?
3.822k
Maximum value
?
?
?
APPENDIX 1: GRAPH PRESENTATION PRINCIPLES
Graphical presentation of results is very important, and the standard presentation
guidelines should be followed:
a) Plot the quantity being directly controlled by the experiment along the
horizontal (x) axis, and the quantity that responds along the vertical (y).
b) Axes must be marked up with
1. The numerical value of the main marker lines on both axes
2. The quantity being measured
3. Its unit in brackets
c) Identify the measured points by drawing a small circle round each point.
d) Provide a brief heading for each graph (e.g. Plot of x against y for z held
constantly).
Dr. Daniel Nankoo
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FOUNDATION EXP 1 – VOLTAGE, CURRENT AND RESISTANCE
APPENDIX 2: COMMON SYMBOLS FOR MULTIPLIERS
Upper case
Upper case
Lower case
Lower case
Greek letter mu
Lower case
Lower Case
G
M
k
m
µ
n
p
Giga
Mega
kilo
milli
micro
nano
pico
1,000,000,000
1,000,000
1,000
0.001
0.000 001
0.000 000 001
0.000 000 000 001
1109
1106
1103
110-3
110-6
110-9
110-12
APPENDIX 3: RESISTOR COLOUR CODE
1st
2nd
3rd
Tolerance
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
0
1
2
3
4
5
6
7
8
9
Tolerances:
Brown = 1%
Red = 2%
Gold = 5%
Silver = 10%
None = 20%
APPENDIX 4: TROUBLE-SHOOTING OR FAULT-FINDING
Because of the nature of things, faults and errors will arise in experiments from many
possible cases, e.g. a wire may be invisibly broken (i.e. under the insulation) or
pushed into the wrong socket on a breadboard, a DMM fuse may be broken, it may
also be incorrectly connected and it may be internally faulty, etc. Thus engineers need
to obtain experience of, and hence develop skills in trouble-shooting or fault-finding.
If a problem arises with your circuit, have a go at seeing if you can at least make a
start at finding the fault (but do not spend too long) before asking staff for assistance.
Dr. Daniel Nankoo
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