ASSUMPTION UNIVERSITY
Vincent Mary School of Engineering
EE 2202: ELECTRIC CIRCUIT LABORATORY
EXPERIMENT NO.1.
NAME
:
SECTION
:
ID NO.
:
GROUP
:
TOPIC
1.
Resistance, Measuring Techniques, Simple Circuits and Bridge
Circuits, Internal Resistance Measurements
2.
OBJECTIVE
2.1
To learn the measuring techniques of resistance values
2.1.1 By color code
2.1.2 Using Ohm-meter
2.1.3 Using Ohm’s law
2.1.4 Using Bridge circuit
2.1.5 Using Voltmeter and Ammeter
APPARATUS
3.
No.
Description
1
Function Generator
2
Oscilloscope
4
Multimeter
5
7
DC Milli-ammeter
Decade Resistance Box
8
Regulated DC Power Supply
Range
Maker
Maker’s No.
4.
THEORY/BACKGROUND
4.1
Resistance Color Code
The resistance values of the carbon resistors can be obtained from their color
codes as follows:
RESISTANCE COLOR CODE
Tolerance band
Silver
± 10%
Gold
± 5%
No band ± 20%
Color code Legend
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Gray
White
0
1
2
3
4
5
6
7
8
9
Example
Red = 2, (1st digit )
Violet = 7,( 2nd digit)
Orange = 3,( power
of multiplier )
Tolerance
Value of resistance = 27x1000=27 kΩ
4.2
Ohm’s Law
The current flowing through a conductor is directly proportional to the potential
difference across the conductor.
For resistor
4.3
V =IR
Bridge Circuit
A bridge circuit is a topology of electrical circuitry in which two circuit branches
(usually in parallel with each other) are "bridged" by a third branch connected
between the first two branches at some intermediate point along them. The
bridge was originally developed for laboratory measurement purposes and
one of the intermediate bridging points is often adjustable when so used.
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Bridge circuits now find many applications, both linear and non-linear,
including in instrumentation, filtering, and power conversion.
The best-known bridge circuit, the Wheatstone Bridge, was invented by
Samuel Hunter Christie and popularized by Charles Wheatstone, and is used
for measuring resistance. It is constructed from four resistors – two of known
values, one whose resistance is to be determined, and one which is variable
and calibrated. The Wheatstone Bridge circuit is nothing more than two simple
series-parallel arrangements of resistances connected between a voltage
supply terminal and a ground producing zero voltage difference between the
two parallel branches when balanced. Although today digital multimeters
provide the simplest way to measure a resistance, the Wheatstone Bridge can
still be used to measure very low values of resistances down in the milli-Ohms
range.
b
i2
R1
R2
i1
a
A
c
i4
R3
R4
R4
i3
d
DC
For balanced condition ( when ammeter shows zero reading )
nodes b and d are at the same potential and
Thus, i1 R1 = i3 R3 and i2 R2 = i4 R4
From above
R1
R2
=
R3
R4
3
i1 = i2
;
i3 = i4
5.
PROCEDURE
5.1
Resistance Measurements
(a)
Take any carbon resistor of 5 kΩ from the bench. Check the color
code of the resistor and write down the resistance value in the table
below.
(b)
Set the multi-meter to Ohm-meter setting and measure the
resistance and note down the value.
(c)
Connect the circuit on the bread-board as follows with the carbon
resistor in series with the ammeter and DC source supply.
10 mA range
mA
Carbon
resistor
30V DC
( ii ) Set the ammeter 10 mA range.
( iii ) Increase the DC voltage until the ammeter shows 5 mA.
( iv ) Measure the voltage across the resistance.
( v ) Write down the data in the table below and calculate the
resistance value using Ohm’s law.
( vi) Repeat all the above procedures with a 1 kΩ carbon
resistor
(d)
Set up a bridge circuit on the bread-board with the above resistor
R = 5 kΩ, 100 k, 2.2 k and decade resistance box ( Rd ).
Connect DC source across the node at which R and 100 k are
connected and the node at which 2.2 k and Rd are connected.
Connect ammeter across R with 2.2 k node and 100 k with Rd node.
Draw the resulting circuit diagram in the space below:
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Set Rd = 0 and increase V until ammeter reads full scale.
Increase Rd until ammeter shows zero. Note down Rd value in the
table.
At the balanced condition of the bridge circuit, calculate R.
(e)
Calculate the percent errors of the results using the formula
Percent error =
Measured − Calculated
 100 % .
Calculated
Resistance measurements experimental Data
Resistance Measurements
a
Color code [………………………]
b
Ohm-meter measurement
c
d
V
=
I
=
Resistance value
% Error
Rd =
5.2
Internal Resistance Measurements for Voltmeter and Ammeter
Voltmeter
An ideal voltmeter has infinite resistance: It is an open circuit. Although it is
impossible to make a physical voltmeter with infinite resistance, a well-designed
voltmeter exhibits a very large internal input resistance. In some experiments,
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it is important to take into account the finite, non-ideal, internal resistance. To
determine the internal resistance of the voltmeter, set up the circuit shown in
figure 5-1. The voltmeter reads the voltage across itself, which includes its
internal resistance. Since the circuit has only a single branch, the current
flowing through the resistor also flows through the voltmeter. The current is
given by the equation:
I = (VS – VM) /R
From Ohm's Law, if we know the current (I) and the voltage (VM) we can
compute RM .
RM = VM/I = VM/[(VS - VM)/R] = RVM / (VS – VM)
(5-1)
Figure 5-1: Circuit for measuring the resistance of the voltmeter.
a) Select a 1MΩ resistor.
b) Measure its value using the multimeter.
c) Set the power supply to provide 10 V (Remember, always measure the
voltage provided by the power supply with either the oscilloscope or the
voltmeter. Do not rely on the digital display on the front panel of the
power supply
d) Assemble the circuit in Figure 5-1. Record the voltage measured by the
voltmeter Compute the internal resistance of the voltmeter using
Equation (5-1)
e) Present your data for VS, VM, and RM in a table.
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Ammeter
An ideal ammeter has zero resistance so that the circuit in which it has been
placed is not disturbed. An ideal ammeter is a short circuit. However, as with
the voltmeter, no ammeter can ever be ideal, and therefore all ammeters have
some (hopefully) small internal resistance. To determine the resistance of the
ammeter, we will use the circuit in Figure 5-2. According to Ohm's Law, the
current in this circuit will be I = V/R where R = R + RM so the current can be
found using the equation:
I = VS/ (R + RM)
(5-2)
By using the known quantities I, VS, and R, we can solve for the unknown
quantity RM.
Figure 5-2: Circuit for measuring the resistance of the
ammeter.
In the procedure that follows it is extremely important that you take precise and
accurate measurements. Record each measurement as precisely as the
instrument will allow.
a) Select a 100 Ω resistor. Measure and record its actual value.
b) Assemble the circuit in Figure 5-2. Set the multimeter to the ammeter
mode for dc current measurement.
c) Use the oscilloscope or another multimeter to measure the voltage
across the DC power supply (DPS).
d) Measure the value of the current using the ammeter.
e) Determine the value of RM of the ammeter from Equation (5-2)
f) Present your data for VS, I, and RM in a table.
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6.
DISCUSSION
Answer the following questions.
(1)
What is resistance?
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(2)
How the resistance value related to the length and cross-sectional
area of the conductor?
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(3)
To measure the current, an ammeter is connected in series with
the element. Does the resistance of the ammeter effect the circuit or not?
Why?
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(4)
To measure the voltage a voltmeter is connected in parallel with
the element. Does the resistance of voltmeter effect the circuit or not?
Why?
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(5) What will be the causes of errors in performing this experiment?
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