In electrical engineering, a voltmeter is a crucial instrument used to measure voltage across components in a circuit. This article explains the working principle of a voltmeter, its types, construction, how to extend its range, and answers key FAQs relevant for exams like SSC JE, RRB JE, UPPCL JE, and others.
🔌 What is a Voltmeter?
A voltmeter is an instrument used to measure the potential difference (voltage) between two points in an electric circuit. It is connected in parallel to the component across which voltage is to be measured.
⚙️ Construction of Voltmeter
- It is based on a moving coil galvanometer, just like the ammeter.
- A high resistance is connected in series with the galvanometer to limit the current flow.
- The pointer deflection is proportional to the voltage applied across its terminals.
💡 Working Principle of Voltmeter
A voltmeter works on the principle of the moving coil mechanism where a magnetic field interacts with the current in the coil to produce deflection. Since the galvanometer coil is sensitive, a large current can damage it. Therefore, a high resistance is used in series so that only a small current passes, proportional to the applied voltage.
📐 Range Extension of Voltmeter
To measure voltages higher than the rating of the basic galvanometer, its range is extended by connecting a high resistance in series. This limits the current and protects the instrument while allowing higher voltages to be measured accurately.
🧮 Formula for Series Resistance (Rs)
Let,
V = Total voltage to be measured
Ig = Current required for full-scale deflection of galvanometer
G = Resistance of galvanometer
Rs = Series resistance to be added
Then,
Rs = (V / Ig) - G
🧲 Why is Series Resistance Used?
- To protect the galvanometer coil from excessive current.
- To allow measurement of higher voltages accurately.
- To ensure proper working in the linear range of the instrument.
🔁 Connection of Voltmeter in Circuit
A voltmeter is always connected in parallel across the component or portion of the circuit whose voltage is to be measured.
Important Point: The resistance of an ideal voltmeter should be infinite to ensure that it does not draw any current and does not affect the circuit conditions.
🧾 Types of Voltmeters
- DC Voltmeter
- AC Voltmeter
- Moving Coil Voltmeter
- Moving Iron Voltmeter
- Digital Voltmeter
- Electrostatic Voltmeter
⚡ Applications of Voltmeter
- Used in labs and panels to measure potential difference.
- Helps monitor voltage in electronic circuits.
- Important for battery and power supply testing.
- Used in fault finding and voltage regulation analysis.
✅ Advantages of Voltmeter
- Accurate voltage measurement.
- Quick response and sensitivity.
- Digital voltmeters offer high precision and easy readability.
- Works for both AC and DC circuits.
⚠️ Limitations of Voltmeter
- Has high resistance which may slightly affect high-impedance circuits.
- Incorrect connection may damage the instrument.
- Analog voltmeters may have parallax error.
🔢 Numerical Questions on Voltmeter with Solutions
Q1. A galvanometer has a resistance of 100 Ω and requires 1 mA for full-scale deflection. What resistance is needed to convert it into a 10 V voltmeter?
Solution: Rs = (10 / 0.001) - 100 = 10000 - 100 = 9900 Ω
✅ Answer: 9900 Ω
Q2. A 50 µA galvanometer has 20 Ω resistance. What series resistance is required for 5 V range?
Solution: Rs = (5 / 0.00005) - 20 = 100000 - 20 = 99980 Ω
✅ Answer: 99.98 kΩ
Q3. A voltmeter has an internal resistance of 10 kΩ and measures 5 V. What current flows through it?
Solution: I = V / R = 5 / 10000 = 0.0005 A = 0.5 mA
✅ Answer: 0.5 mA
Q4. A digital voltmeter shows 9.8 V for a 10 V source. What is the percentage error?
Solution: Error = ((10 - 9.8)/10) × 100 = 2%
✅ Answer: 2%
Q5. A galvanometer with 60 Ω resistance requires 2 mA for full scale. Find series resistance for 20 V voltmeter.
Solution: Rs = (20 / 0.002) - 60 = 10000 - 60 = 9940 Ω
✅ Answer: 9940 Ω
❓ FAQs on Voltmeter
Q1. Why is voltmeter connected in parallel?
Answer: Because it measures potential difference across two points without disturbing the circuit current.
Q2. What happens if a voltmeter is connected in series?
Answer: Being high-resistance, it will block current, and the circuit won’t function properly.
Q3. What is the difference between voltmeter and ammeter?
Answer: Voltmeter measures voltage and is connected in parallel; ammeter measures current and is connected in series.
Q4. How to increase the range of a voltmeter?
Answer: By connecting a suitable high resistance in series with the meter.
Q5. What is the ideal resistance of a voltmeter?
Answer: Infinite, to avoid drawing current and affecting the circuit.
🎯 Conclusion
A voltmeter is an indispensable tool in electrical measurements. By understanding its working, construction, and proper usage, one can ensure accurate voltage monitoring in both academic and industrial applications.
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