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on April 28, 2020
GALVANOMETER TO AMMETER AND VOLTMETER CONVERSION
Galvanometer to Ammeter conversion
Galvanometer can only measure a smaller current such as 0.02A . For a galvanometer to measure a higher current such as 2A or 5A, a resistance needs to be connected in series.
From figure 1 above
Voltage across the galvanometer =Gr where G = Galvanometer current and r= galvanometer internal resistance.
The voltage across the resistance R =( A-G)R where A-G = current across R
The two voltages are equal since they are connected in parallel.
Voltage across the galvanometer = voltage across the resistance R
Gr=(A-G)R therefore
Where A is the current to be measured by the galvanometer
Example
What is the value of resistance needed to be used in fig 1 above to measure current of value A= 2A?
Solution
G=0.02A
r=10Ω
A=2A
In general
To convert a Galvanometer to an Ammeter , you need an External parallel Resistance of value
Where R = the external parallel resistance needed
A = Total current to be measured
r= the resistance of the Galvanometer
G= galvanometer current rating
Galvanometer to Voltmeter Conversion
Galvanometer can be converted to a voltmeter when a series resistance is connected to the galvanometer. For a galvanometer to be able to measure a voltage of about 0-5V a series resistance is needed.
From figure 2 above
Voltage across the galvanometer =Gr =V1 where G = Galvanometer current and r= galvanometer internal resistance.
The voltage across the resistance R =GR , where G = current across R = Galvanometer current
The two voltages are equal to the total voltage V in the circuit since they are in series
Voltage across the galvanometer + voltage across the resistance R = Total voltage V
V= V1 + V2= Gr + GR , V-Gr=GR
Where V is the Voltage to be measured by the galvanometer
Example
What is the value of resistance needed to be used in fig 2 above to measure Voltage value V=5v ?
Solution
G=0.02A
r=10Ω
V=5v
In general
To convert a Galvanometer to a Voltmeter , you need an External series Resistance of value
Where R = the external Series resistance needed
V = Total voltage to be measured
r= the resistance of the Galvanometer
G= galvanometer current rating
Posted in: Education
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