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
Ω
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.
I
m
R
m
=100
Ω
I
I
sh
t
R
sh
+ V

m
01mA
Figure.1
The calculation of the shunt ( parallel ) resistor R
sh
is as follows:
The voltage drop across the basic meter,
V
m
is given by
V
I
R
m
m
m
=
*
V
m
=
1
100
100
mA
mV
*
Ω =
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
I
I
sh
t
m
=

=

=
3000
1
2999
mA
Hence the value of the
R
sh
,
R
V
I
sh
m
sh
=
/
=
=
100
2999
0 03334
mV
mA
/
.
Ω
8
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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.
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 Spring '11
 UgurBaysal
 Electronics Engineering, Resistor, Electrical resistance, DC voltage, basic meter

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