Download Experiment 2 - Department of Electrical and Electronics Engineering

HACETTEPE UNIVERSITY
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
ELE 271 MEASUREMENT LABORATORY
EXPERIMENT #2
USE OF BASIC METER AS AN AVO METER IN DC CIRCUITS
Object :
To utilize the basic measuring instrument in DC circuits, to measure its internal resistance
and to investigate the loading effect in the circuit.
1. THEORY :
Basic meter is the main construction element of the AVO meters that we are using in the
laboratory. Essentially it is an ammeter, but by some modifications one is able to measure
voltage and resistance values. The use of basic meter as DC ammeter, DC voltmeter and
ohmmeter is explained in examples below.
1.1 DC Ammeter :
Assume we are given a basic meter having an internal resistance of 100  and a full
scale deflection of 1 mA. In order to measure current values greater than 1 mA, say 3A, a
resistor should be connected in parallel. The typical connection of a DC ammeter is
given in figure.1.
R
I
I
+V
m
0-1mA
sh
t
sh
I
m
R
m
=100 
Figure.1
The calculation of the shunt ( parallel ) resistor Rsh is as follows:
The voltage drop across the basic meter, Vm is given by
Vm  I m * Rm
Vm = 1mA *100  100mV
Since the full scale value of the total current to be measured is 3A. and the current
sensitivity of the basic meter is 1mA full scale, the current through the shunt resistor is
I sh  I t  I m  3000  1  2999mA
Hence the value of the Rsh ,
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Rsh  Vm / I sh  100mV / 2999mA  0. 03334
If a shunt resistor of 0.03334 W is used, then the basic meter can measure currents up to 3
A. Note that the measuring range of the ammeter is extended by 3000.
1.2 DC Voltmeter :
Without any modification to the previous basic meter, it is able to measure a full scale
voltage of 100 mV. In order to measure greater values of voltages, a series resistor should
be connected to the meter. The connection diagram is given in figure.2.
R
0-1mA
s
R =100
m
+ DC Voltage to be measured -
Figure.2
In order to measure a full scale voltage of 10 V., the value of the series resistor Rs is
calculated below.
Vt  I m * Rs  I m * Rm
Rs  (Vt  Vm ) / I m  (10V  100mV ) / 1mA  9. 9 k
If a series resistor of 9.9k  is used, then the basic meter can measure voltages up to 10V.
1.3 Ohmmeter :
In order to use the basic meter as an ohmmeter, a variable resistor and a DC voltage
source, for example a battery, should be connected in series to the meter. The connection
diagram is given in figure .3. The resistor to be measured is connected between terminals
a and b.
R
0-1mA
v
R =100
m
+
a
R
b
-
V
dc
x
Figure.3
The variable resistor Rv is used for the zero ohm adjustment procedure. The meaning is,
when terminals a and b are short circuited, the basic meter has to deflect full scale. By use
of previous basic meter, if a-b are short circuited, the total resistance in figure .3 is
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Rt  Rv  Rm  3V / 1mA  3000
then, for a full scale deflection, Rv should be adjusted to 2900 .
When a resistor between a-b is connected, the total resistance in effect is changed and it
also changes the value of the current in the network. By measuring the new value of the
current, the resistance can be calculated by use of Ohm's law :
Rx  (V / I m )  Rt
For example, if I m is measured as 0.2mA , the value of the unknown resistance Rx is
calculated as 12000 .
1.4 Measurement of Internal Resistance :
The internal resistance of the basic meter can be measured by use of the circuit in figure.
4. The procedure is given below:
* When the switch is open, R1 is adjusted in such a way that the meter deflects full
scale.
* Then, the switch is closed, and R2 is adjusted so that the meter deflects half scale.
* The adjusted value of R2 is measured, and this value is equal to the internal resistance
of the meter.
R
1
DC
Pow er
R
Supply
m
R
2
Figure. 4
Also, by using the circuit given in figure.5, internal resistance can be calculated from the
formula Rm  V / I , where V and I are the voltage across and the current through the
meter.
R
1
+
DC
R
Pow er
Supply
m
V

DC
Voltmeter
-
Figure.5
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1.5 Loading Effect Of The Voltmeter & Ammeter :
Although the internal resistance of a voltmeter is ideally assumed to be infinite, in
practice this condition is not satisfied. Voltmeters with finite input resistance always load
the circuit, and this causes erroneous measurements. For example, in the given circuit in
figure.6, if a voltmeter with internal resistance of 90K is used to measure the voltage of
R1 , the equivalent resistance between the terminals a and b will be
Rab  (45K / /90 K)  30 K
R = 
1
180V
+
-
a
R

V
m
b

ab

Figure.6
Hence, the voltmeter will read the voltage drop across the 30K resistance as
Vab  (30 K / (45K  30 K)) *180V  72V
The measured value of the voltage is 18V lower than the correct value. That's why, the
internal resistance of the voltmeter should be at least 20 times greater than the resistance
of the element under consideration.
For ammeters, this time, the inner resistance should be 20 times smaller than the
resistance of the element.
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2. PRELIMINARY WORK :
2.1 Explain why the adjusted value of R2 is equal to the internal resistance of the basic
meter in figure 4.
2.2 Assume we are given a basic meter having an internal resistance of 50  and a full
scale deflection of 1 mA.
a) Design an ammeter having a full scale deflection of 10mA.
b) Design a voltmeter to measure a full scale voltage of 30V.
2.3 Consider the circuit diagram in figure 3, and V =3V.
dc
a) When a-b are short circuited, determine the value of Rv for full scale deflection.
b) Find the value of the unknown resistor Rx , if the meter current is 0.33mA.
2.4 For the given circuit in figure.7, the voltage across the element Rl is required to be
measured using two voltmeters whose internal resistances are 300K and 6M.
R = 150K
1
300V
+
-
a
R

V
m

b
R
L

ab
Figure.7
a) Calculate the correct value.
b) Calculate the values measured by each voltmeter and corresponding errors
3. Experiment :
3.1 a)Set up the circuit in figure 4 ( R1 =10K ) and measure the internal resistance of
the basic meter.
b)Set up the circuit in figure 5 and adjust R1 to obtain 0.6mA meter current. Then, by
measuring the voltage across the meter, calculate the internal resistance.
3.2 Set up the circuit in figure.8.
R =  
1
I

25V
+
-
V

R
L
2.2K 
Figure.8
a) By using 10mA ammeter designed in preliminary work ,measure the current I.
b) By using 30V voltmeter designed in preliminary work measure the voltage V.
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c) Connect a resistor of 6.8K,instead of 30K ,in series with the basic meter and
measure the voltage V again.
d) By using AVO meter, measure the current and voltage again.
3.3 Set up the circuit in figure 9.
R = 
v
0-1mA

3V
a
R
+
-
b
x
Figure.9
a) At first, short circuit the terminals a-b and adjust Rv such that the meter deflects full
scale.
b) Then, connect the resistors 2.2K, 5.6K, 6.8K, 15K, and 30K between the
points a and b separately and record the current values.
4. RESULTS AND CONCLUSION :
4.1 Calculate the current and voltage values in figure 8, and compare them with the
measured values.
4.2 By using your results, comment on which voltmeter is the best one , and explain why.
EQUIPMENT AND COMPONENTS :
DC power supply, AVO meter, 0-1mA Basic Meter
10K pot, 1K pot, 100 pot
Resistors : 2.2K (#2), 5.6K (#1), 6.8K (#1), 15K (#2)
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HACETTEPE UNIVERSITY
DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING
ELE 271 MEASUREMENT LABORATORY
EXPERIMENT #2
Experiment Date:
Group No
:
Group Members :
RESULTS:
4.1 a) Rin=
4.2 a) I=
b) Rin=
b) V=
c) V=
d) I=
V=
4.3
R
2.2K
5.6K
6.8K
15K
30K
I
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