A galvanometer is a sensitive instrument used to detect small electric currents, while an ammeter measures larger currents in a circuit. Since a galvanometer alone cannot measure high currents, it must be converted into an ammeter using a shunt resistor.
Understanding this conversion process is important for students studying electrical circuits and instrumentation. This topic covers key concepts, important questions, and solved examples related to the conversion of a galvanometer into an ammeter.
What is a Galvanometer?
A galvanometer is an instrument used to detect and measure small currents in a circuit. It works on the principle of electromagnetic deflection, where a coil in a magnetic field rotates when current passes through it.
However, a galvanometer:
✔ Is very sensitive and can only measure microamperes (µA) or milliamperes (mA).
✔ Has high resistance, which limits the amount of current it can handle.
✔ Cannot be used directly as an ammeter because it may burn out if excess current flows.
Why Convert a Galvanometer into an Ammeter?
An ammeter is designed to measure large currents in a circuit without significantly affecting the current flow. Since a galvanometer is too sensitive, we must modify it by adding a shunt resistor in parallel to convert it into an ammeter.
Key Reasons for Conversion:
✔ To extend the range of current measurement.
✔ To reduce the resistance so that the ammeter does not interfere with circuit operation.
✔ To protect the galvanometer from damage due to high current flow.
How to Convert a Galvanometer into an Ammeter?
The conversion process involves connecting a low-resistance ‘shunt’ resistor (Rₛ) in parallel with the galvanometer coil. This allows most of the current to bypass the galvanometer, preventing damage.
Formula for Shunt Resistance
The value of the shunt resistance (Rₛ) is calculated using the formula:
Where:
✔ G = Galvanometer resistance
✔ I₉ = Full-scale deflection current of the galvanometer
✔ I = Total current to be measured
✔ Rₛ = Shunt resistance
The shunt resistor must have very low resistance so that the majority of the current passes through it, leaving only a small current for the galvanometer.
Important Questions on Conversion of Galvanometer into Ammeter
1. Concept-Based Questions
✔ What is a galvanometer?
✔ Why can’t a galvanometer measure large currents?
✔ What modifications are required to convert a galvanometer into an ammeter?
✔ What is a shunt resistor, and why is it used?
✔ How does a shunt resistor protect a galvanometer?
✔ What happens if a shunt resistor is not used in the conversion?
2. Multiple-Choice Questions (MCQs)
Q1: What is the function of a shunt resistor in an ammeter?
- To increase the voltage rating
- To bypass excess current
- To reduce the resistance of the galvanometer
- Both B and C
✔ Answer: D) Both B and C
Q2: The shunt resistance should have a:
- High value
- Low value
- Same resistance as the galvanometer
- Variable resistance
✔ Answer: B) Low value
Q3: If the galvanometer has a resistance of 50Ω and the full-scale deflection current is 2mA, what will happen if 10A flows through it without a shunt?
- The galvanometer will function normally
- The galvanometer will burn out
- The resistance of the galvanometer will increase
- The needle will not move
✔ Answer: B) The galvanometer will burn out
3. True or False Questions
✔ A galvanometer can measure large currents without modification. (False)
✔ A shunt resistor is connected in series with the galvanometer. (False)
✔ An ammeter should have very low resistance. (True)
✔ The formula for the shunt resistance is Rₛ = (G * I₉) / (I – I₉). (True)
4. Fill in the Blanks
✔ A galvanometer is used to detect ____________. (small currents)
✔ The resistance of an ideal ammeter is ____________. (zero)
✔ A ____________ is connected in parallel with a galvanometer to convert it into an ammeter. (shunt resistor)
✔ The total current flowing through the ammeter is shared between the ____________ and the ____________. (galvanometer, shunt)
5. Numericals on Conversion of Galvanometer into Ammeter
Problem 1
Given:
✔ Galvanometer resistance (G) = 100Ω
✔ Full-scale deflection current (I₉) = 5mA
✔ Total current to be measured (I) = 5A
Find: Shunt resistance (Rₛ)
Solution:
Using the formula:
Thus, a shunt resistor of 0.1Ω is required.
Problem 2
Given:
✔ Galvanometer resistance (G) = 50Ω
✔ Full-scale deflection current (I₉) = 2mA
✔ Total current to be measured (I) = 10A
Find: Shunt resistance (Rₛ)
Solution:
Thus, a shunt resistor of 0.01Ω is required.
Practical Applications of Ammeter Conversion
✔ Used in laboratories to extend the measuring range of galvanometers.
✔ Important for electrical circuit analysis and instrumentation.
✔ Helps in calibrating and testing ammeters for accuracy.
✔ Used in industrial and home electrical panels for current measurement.
The conversion of a galvanometer into an ammeter is essential for measuring large currents. This is done by connecting a low-value shunt resistor in parallel with the galvanometer, ensuring that most of the current bypasses it.
Students should practice conceptual questions, numerical problems, and MCQs to master this topic. By understanding the role of shunt resistance, they can confidently answer exam questions and apply the concept in real-world electrical circuits.