Download Laboratory Exercise Basic Electrical Calculations and

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Team Members: ______________________________
______________________________
______________________________
ENGR 199 – Introduction to Engineering Principles and Practices I – Fall 2009
Laboratory Exercise
Basic Electrical Calculations and Measurements
Objective:
Students successfully completing this lab exercise will accomplish the following
objectives:
1. Gain familiarity reading resistor color codes.
2. Learn how to construct simple resistive circuits.
3. Gain familiarity with basic electrical calculations.
4. Learn how to measure resistance, voltage and current using a Digital
Multimeter (DMM)
Equipment:
Digital Multimeter, various fixed resistors, breadboard, power supply
Procedure:
1.
Resistance Identification and Measurement
Obtain three resistors and sort them as R1, R2, and R3. Using the table of resistor color
codes shown at the end of this document, determine the nominal resistance of each
resistor from its color bands. Measure the actual resistance using the DMM as an
ohmmeter.
Using the formula below, calculate the percent error between the measured value and the
nominal value of each resistor. From the percent error calculation, determine whether or
not the resistor is within tolerance. Record all resistor information in table 1.
%
100 %
Table 1: Resistor identification and measurements
#
Color Code
Color Code
Resistance
Color Code
Tolerance
Measured
Resistance
% error
Within
Tolerance?
(Y or N)
R1
R2
R3
2.
Circuit Assembly and Measurement
Construct the voltage divider circuit shown in the schematic diagram of Figure 1 below.
Measure the source voltage (VIN) to be approximately 10 V before applying it to the circuit.
Record your measured source voltage.
VIN (measured) = _______________
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Figure 1: Series Voltage Divider Circuit
Using the measured values of R1, R2 and R3 from step 1, calculate the total resistance of
the three resistors in series.
RTotal = R1 + R2 + R3 = _______________
Using the value of total resistance calculated in the previous step and Ohm’s Law, calculate
the expected current flowing through the circuit.
I (calculated) = VIN / RTotal = _______________
Connect the DMM in series with the three resistors as an ammeter and measure the
current. Record your results.
I (measured) = _______________
Does your measured result match your calculated result? __________
Using the measured value of current from the previous step and Ohm’s Law, calculate the
expected voltage drops across each resistor.
VR1 (calculated) = I x R1 = _______________
VR2 (calculated) = I x R2 = _______________
VR2 (calculated) = I x R3 = _______________
Connect the DMM in parallel with each of the three resistors as a voltmeter and measure
the voltage drops. Record your results.
VR1 (measured) = _______________
VR2 (measured) = _______________
VR2 (measured) = _______________
Do the measured voltages closely match your calculated values? ________
Calculate the total voltage drop across the three resistors.
VTotal (calculated) = VR1 + VR2 + VR3 = _______________
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Does the calculated value of VTOTAL closely match your value for VIN? ________
Assume that the previous circuit is altered to match the schematic diagram of Figure 2
below with a pair of 4700 Ω resistors in parallel. Calculate the equivalent resistance of
these two resistors in parallel.
RParallel = 1 / (1/R3 + 1/R4) = _______________
R1
1500
R2
3000
VIN
10 V
R3
4700
R4
4700
Figure 2: Series-Parallel Circuit
Is the equivalent resistance of R3 and R4 greater or less than the resistance of R3 alone?
__________ .
What effect will the parallel combination have on total circuit current? (will the circuit current
be greater or less than the previous circuit current?)
________________
Why?
Table of Resistor Color Codes
Band 3
(Multiplier)
Band 4
(Tolerance)
Band 5
(Reliability)
(if present)
Color
Abbr.
Band 1
Band 2
Black
Bk
0
0
0=1
Brown
Br
1
1
1 = 10
1%
Red
R
2
2
2 = 100
0.1%
Orange
O
3
3
3 = 1000
0.01%
Yellow
Y
4
4
4 = 10000
0.001%
Green
Gn
5
5
5 = 100000
Blue
Bl
6
6
6 = 1000000
Violet
V
7
7
7 = 10000000
Gray
Gy
8
8
White
W
9
9
Gold
Go
5%
Silver
S
10%
No Color
20%
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Example. Suppose a resistor has colors Green – Blue – Red – Gold, and the measured
resistance is 5775 Ω.
(a) Calculate the nominal resistance,
(b) Calculate the percent error between the measured value and the nominal value,
(c) Determine if the resistor is within tolerance.
Answer:
(a)
Colors:
Green – Blue – Red – Gold
Numerical codes: 5
6
2
5%
Nominal Resistance = 56 × 100 = 5600 Ω.
(b)
%
100 %
175
5600
5775 5600
5600
100 %
0.03125
100 %
100 %
3.125 %
(c) The fourth color is Gold, which indicates a tolerance of 5%. This means that the
manufacturer has specified that the percent error between the measured value and the
nominal value should be within ± 5%.
Since the percent error, 3.125 %, is less than the tolerance of 5 %, the resistor is within
tolerance.