Short Answer:
A voltmeter is an electrical measuring instrument used to measure the potential difference (voltage) between two points in an electric circuit. It operates based on the principle that current flows through a coil in a magnetic field and creates a deflection proportional to the voltage applied across its terminals. The deflection is shown on a calibrated scale as voltage.
The voltmeter is always connected in parallel with the component across which voltage is to be measured. It uses very high internal resistance so that it does not draw significant current from the circuit, ensuring accurate voltage measurement without affecting the original operation of the circuit.
Detailed Explanation:
Principle of operation of a voltmeter
A voltmeter works on the principle of Ohm’s Law and electromagnetic deflection. According to Ohm’s Law, the voltage across a resistance is directly proportional to the current flowing through it. In the case of analog voltmeters (like the PMMC type), when a voltage is applied across the voltmeter terminals, a small current flows through its internal coil, which is suspended in a magnetic field. The interaction between the current and magnetic field produces a torque that causes the coil to rotate, and the pointer connected to the coil moves over a calibrated scale to show the voltage value.
The more the voltage applied, the more the current, and hence the greater the deflection of the pointer. In digital voltmeters, the voltage is converted into a digital signal using analog-to-digital converters (ADC), and the voltage reading is displayed on a digital screen. In both types, the principle remains the same: the measured voltage produces an effect proportional to its magnitude.
Working Mechanism:
- Connection Across Load
The voltmeter is connected in parallel with the electrical device or load so that it can measure the voltage drop directly across it. - High Internal Resistance
The internal resistance of a voltmeter is very high. This ensures that it does not allow significant current to pass through it, thereby avoiding any major change in the original circuit’s current. - Current through Coil
The applied voltage causes a very small current to pass through a fine coil placed within a magnetic field. This coil is attached to a pointer that rests over a scale. - Magnetic Deflection
Due to the interaction between the magnetic field and the current-carrying coil, a deflection occurs. The amount of deflection is directly proportional to the input voltage. - Calibrated Reading
The pointer deflects over a graduated scale marked in volts. The final reading on the scale gives the voltage between the two points.
Types of Voltmeters:
- Analog Voltmeters: Use electromagnetic deflection (e.g., PMMC type) to move a pointer across a scale.
- Digital Voltmeters: Use ADC circuits to convert voltage into a digital signal for numerical display.
- AC and DC Voltmeters: Designed to measure alternating or direct voltages respectively.
- Moving Iron and Electrodynamometer Voltmeters: Can measure both AC and DC voltages, often used in power systems.
Key Characteristics:
- Always connected in parallel to the component.
- Should have high input resistance to avoid drawing current.
- Should give linear and accurate readings.
- Can be portable or panel-mounted for continuous monitoring.
Practical Use:
Voltmeters are widely used in laboratories, homes, industries, and electronics. They are essential for troubleshooting electrical faults, testing batteries, checking power supplies, and verifying circuit conditions. In most digital multimeters, the voltmeter function is built-in and can measure both AC and DC voltages.
Conclusion
The voltmeter works on the principle that voltage applied across a known resistance causes a proportional current, which can be detected and displayed. Analog voltmeters use electromagnetic deflection, while digital ones use electronic conversion. A good voltmeter has high internal resistance and must be used properly to avoid circuit disturbances. Understanding its working helps in effectively using it in real-world electrical and electronic measurements