In electrical engineering, an ammeter is an essential measuring instrument used to measure electric current flowing through a circuit. This article explains the working principle of an ammeter, its types, construction, how to extend its range, and answers some important FAQs for competitive exams like SSC JE, RRB JE, UPPCL JE, and others.
🔌 What is an Ammeter?
An ammeter is an instrument used to measure the current flowing through a circuit in amperes (A). It is always connected in series with the load because current in a series circuit remains the same across all elements.
⚙️ Construction of Ammeter
- It is based on a moving coil galvanometer.
- It contains a coil placed between the poles of a permanent magnet.
- To convert a galvanometer into an ammeter, a low resistance called shunt is connected in parallel with it.
💡 Working Principle of Ammeter
A galvanometer deflects when current flows through it. But it can measure only a small amount of current. To measure large currents, most of the current is passed through a shunt resistor connected in parallel, and only a small current flows through the meter coil. This keeps the meter safe and allows current measurement over a larger range.
📐 Range Extension of Ammeter
To measure currents higher than the rating of the basic ammeter (galvanometer), we extend its range by connecting a low-value resistor called a shunt resistor in parallel with the ammeter.
🧮 Formula for Shunt Resistance (S)
Let,
I = Total current to be measured
IG = Current through galvanometer
G = Resistance of galvanometer
S = Shunt resistance
Then,
S = (IG × G) / (I - IG)
This formula helps in designing an ammeter for higher current ranges.
🧲 Why is Shunt Resistance Used?
- To protect the galvanometer coil from high current.
- To allow the majority of the current to pass through the shunt path.
- To extend the measurement range of the basic meter.
🔁 Connection of Ammeter in Circuit
Ammeter is connected in series with the load so that the entire circuit current passes through it.
Important Point: The resistance of an ideal ammeter should be zero to avoid voltage drop across it. Practically, it is made very small.
🧾 Types of Ammeters
- DC Ammeter
- AC Ammeter
- Moving Coil Ammeter
- Moving Iron Ammeter
- Digital Ammeter
- Clamp Ammeter
⚡ Applications of Ammeter
- Used in laboratories and industries to measure electric current.
- Used in power distribution and control panels.
- Common in electrical machines and battery monitoring.
- Important for fault analysis in power systems.
✅ Advantages of Ammeter
- Provides accurate measurement of current.
- Quick response time.
- Digital ammeters are easy to read.
- Can be used in both AC and DC systems.
⚠️ Limitations of Ammeter
- Has internal resistance, though small.
- Must be connected in series; wrong connection may damage the meter.
- Limited by the range unless extended by a shunt.
🔢 Numerical Questions on Ammeter with Solutions
Q1. A galvanometer of resistance 100 Ω gives full-scale deflection at 5 mA. What shunt resistance is needed to convert it into an ammeter of 5 A range?
Solution: S = (0.005 × 100)/(5 - 0.005) = 0.1001 Ω
✅ Answer: 0.1 Ω
Q2. A 50 µA galvanometer has a resistance of 20 Ω. What is the shunt resistance required to convert it into a 1 mA full-scale ammeter?
Solution: S = (0.00005 × 20)/(0.001 - 0.00005) ≈ 1.0526 Ω
✅ Answer: 1.05 Ω
Q3. A galvanometer with internal resistance 50 Ω shows full deflection at 10 mA. How can it be used to measure up to 2 A?
Solution: S = (0.01 × 50)/(2 - 0.01) ≈ 0.251 Ω
✅ Answer: 0.251 Ω
Q4. What will be the current through a galvanometer if a shunt of 0.2 Ω is connected in parallel to it, and total current is 4 A? Galvanometer resistance is 20 Ω.
Solution: Ig ≈ 0.0396 A = 39.6 mA
✅ Answer: 39.6 mA
Q5. An ammeter with internal resistance of 0.05 Ω reads 10 A. What is the voltage drop across it?
Solution: V = 10 × 0.05 = 0.5 V
✅ Answer: 0.5 V
Q6. A moving coil instrument has a resistance of 200 Ω and requires 1 mA for full-scale deflection. What is the voltage needed for full scale?
Solution: V = 0.001 × 200 = 0.2 V
✅ Answer: 0.2 V
Q7. An ammeter of range 0-5 A has a resistance of 0.1 Ω. What power is dissipated in it when full-scale current flows?
Solution: P = 5² × 0.1 = 2.5 W
✅ Answer: 2.5 W
Q8. A galvanometer of resistance 40 Ω gives full deflection at 1 mA. What shunt resistance is needed to convert it into 100 mA ammeter?
Solution: S = (0.001 × 40)/(0.1 - 0.001) ≈ 0.404 Ω
✅ Answer: 0.404 Ω
Q9. A galvanometer is rated for 2 mA and 30 Ω. What is the shunt resistance for 10 A range?
Solution: S = (0.002 × 30)/(10 - 0.002) ≈ 0.006 Ω
✅ Answer: 0.006 Ω
Q10. A digital ammeter shows a reading of 4.95 A when the actual current is 5 A. What is the percentage error?
❓ FAQs on Ammeter
Q1. Why is ammeter connected in series?
Answer: Because in series, the current remains the same, allowing accurate current measurement through the load.
Q2. What happens if an ammeter is connected in parallel?
Answer: It may get damaged due to large current flow, as it has very low resistance and cannot handle parallel voltage.
Q3. What is the difference between ammeter and voltmeter?
Answer: Ammeter measures current and is connected in series; voltmeter measures voltage and is connected in parallel.
Q4. How to increase the range of an ammeter?
Answer: By connecting a suitable shunt resistor in parallel with the meter coil.
Q5. What is the ideal resistance of an ammeter?
Answer: Zero, so that it doesn't drop voltage across it and affect circuit operation.
🎯 Conclusion
An ammeter is a vital instrument in the world of electrical engineering, enabling accurate current measurement. Its range can be extended using shunt resistors, allowing it to be used for higher currents. Understanding its working and proper connection is essential for safe and effective use in practical applications.
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