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DC VOLTMETER
Done By:Ala’ Al-Hiary
Nidal Al-Hamad
The Volt
• Volt: is the SI derived unit for electric potential
electric potential difference
electromotive force.
History of the Volt
• The volt is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery
voltaic pile
is a set of individual Galvanic cells placed in series.
the first electric battery invented by Alessandro Volta in 1800
What is voltage???
• Volta characterized the emf of a pair of metals in terms of the difference in their voltages, which he could measure
VOLTMETER
• A voltmeter is an instrument used for measuring electrical potential difference (voltage) between two points in an electric circuit.
How does is work???
• When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil.
The connection
• The voltmeter must be connected in parallel with the element we want to measure the voltage across.
Types of Voltmeters:• Analog Voltmeter• Digital Voltmeter
Analog Voltmeters
• Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit .
Analog Voltmeters
Analog Voltmeters
Digital Voltmeters
• Digital voltmeters give a numerical display of voltage by use of an analog to digital converter.
ADC
Digital Voltmeters
Digital Voltmeters
DC Voltmeter Equivalent Circuit
Rs
Rm
Im
V+
-
DC Voltmeter Equivalent Circuit
Rs
Rm
Im
V+
-
>> A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with it .>> Rs limits the current through the coil not to exceed the full scale deflection current .
DC Voltmeter Equivalent Circuit
Rs
Rm
Im
V+
-
>> Rs is relatively large so that the voltmeter does not affect the current to be measured.
DC Voltmeter Equivalent Circuit
Rs
Rm
Im
V+
-
Im : deflection current of the movement.
Rm : internal resistance of the movement.
V : the full range voltage of the voltmeter.
DC Voltmeter Equivalent Circuit
Rs
Rm
Im
V
+
-
V = RI
V = (Rs + Rm)Im
Rs = (V – ImRm)/Im
Rs = V/Im – Rm
Multi-Range Voltmeter
>> Here, we use a number of resistors in order to control the voltage range.
Rm
Im
Multi-Range Voltmeter
Multi-Range Voltmeter
Multi-Range Voltmeter
Multi-Range VoltmeterR1
Rm
Im
V1
+-
V2
V3
V4
R2
R3
R4
0 to V1 Range
R1
Rm
Im
+-
R2
R3
R4
R1
Rm
Im
+
-
0 to V2 Range
Rm
Im
+-
R2
R1
R3
R4
0 to V2 Range
R2
Rm
Im
+
-
0 to V3 Range
Rm
Im
+
-
R3
R1
R2
R4
0 to V3 Range
R3
Rm
Im
+
-
Multi-Range Voltmeter R1
Rm
+
-
R2 R3 R4
V1V2 V3 V4
0 to V4 Range
Rm
+
-
R4R2 R3R1
V1V2 V3 V4
0 to V3 Range
Rm
+
-
R3 R4
V3
R1 R2
V1V2 V4
0 to V2 Range
Rm
+
-
R2 R3 R4
V2 V3
R1
V1V4
0 to V1 RangeR1
Rm
+
-
R2 R3 R4
V1V2 V3 V4
Sensitivity of Voltmeters
>> The sensitivity of a voltmeter is defined as the inverse of the full scale deflection of the element.
S = 1 / (I full scale)
Sensitivity of Voltmeters
Rs
Rm
Im
+
-
>> Using “S” we can determine the value of Rs, as follows :
R total = S*VR total = Rs + Rm
Rs = S*V - Rm
Example
A galvanometer with internal resistance of (200 ohm) and maximum current (500 uA) to be converted to a multi-range voltmeter with the following voltage ranges : 1)0 to 100 mV2)0 to 1 V3)0 to 10 V4)0 to 100 V
Solution
R1
Rm+-
R2 R3 R4
S = 1/(I full scale) = 1/500 uA = 2000 ohm/V
>>Range (0 to 100 mV) :R4 = Rs = S*V – Rm = 2000*0.1 – 200 = 0
R4 is S.C.
Solution
R1
Rm+-
R2 R3 R4
>> Range (0 to 1 V) :Rs = R3 + R4
R3 + R4 = S*V – Rm = 2000*1 – 200 = 1800 ohm R4 = 0 , so : R3 = 1800 ohm = 1.8k
Solution
R1
Rm+-
R2 R3 R4
>> Range (0 to 10 V) :Rs = R2 + R3 + R4So:R2 + R3 + R4 = S*V – Rm = 2000 * 10 – 200 = 19800 ohmR2 = 19800 – 1800R2 = 18000 ohm = 18k
Solution
R1
Rm+-
R2 R3 R4
>> Range (0 to 100 V) :
Rs = R1 + R2 + R3 + R4So:
R1+ R2 + R3 + R4 = S*V – Rm = 2000 * 100 – 200 = 199800 ohm
R1 = 199800 – 18000 - 1800
R1 = 180000 ohm = 180k
Solution
1.8 k
Rm+-
18 k180 k
R1 = 180 k R2 = 18 k R3 = 1.8 kR4 = 0 (i.e. S.C.)
THANK YOU
