The article discusses the working principle, construction, and characteristics of analog voltmeter, emphasizing components like the multiplier resistor, input resistance, and sensitivity. It also highlights practical examples, different voltage ranges, and comparisons between analog and digital voltmeters.
What is an Analog Voltmeter?
An analog voltmeter is an electrical instrument used to measure voltage in a circuit, displaying the reading on a scale with a pointer or needle. It operates on the principle of a moving coil mechanism, typically using a D’Arsonval galvanometer. The basic components include:
- Meter Movement: Converts the electrical current into mechanical motion, moving the needle across a calibrated scale.
- Multiplier Resistor: Connected in series to extend the voltage range of the voltmeter.
- Scale: A calibrated arc marked with voltage levels for measurement.
Analog Voltmeter Operating Principle
An analog voltmeter consists of a basic meter movement and a multiplier resistor connected in series (Figure 1). The resistance of the multiplier can be determined using series-circuit relationships and Ohm’s law because the current through the multiplier (Imt) is the same as the current through the meter movement (Im), and the multiplier voltage (Vmt) plus the meter-movement voltage (Vm) equals the voltmeter voltage (Vt).
Figure 1. The voltmeter multiplier extends the voltage range of the basic meter movement.
Like shunts, multiplier resistors have low tolerances and temperature coefficients. Unlike shunts, multipliers have very high resistances.
Analog Voltmeter Example 1
What value multiplier is needed to make a 5-V voltmeter from a 1-mA, 100-Ω meter movement?
Given:
Im = 1 mA
Rm = 100 Ω
Find:
Rmt
Pmt
Known:
Pmt = Imt Rmt
$$R_{mt}=\frac{V_{mt}}{I_{mt}}$$
Imt = Im
Vmt = Vt – Vm
Vm = Im Rm
Solution:
$$V_m=0.001A\ \times\ 100\Omega=0.1\ V$$
$$V_{mt}=5V-0.1V=4.9V$$
$$R_{mt}=\frac{4.9V}{0.001A}=4900\Omega$$
$$P_{mt}=0.001A\times4.9V=0.0049\ W=4.9\ mW$$
The multiplier’s resistance must be 4900 Ω, and its power rating must be greater than 4.9 mW.
Multirange Voltmeter Configuration
A multirange voltmeter, typical of those used in the voltmeter section of a VOM, is shown in Figure 2. The selector switch in Figure 2 is the non-shorting type. When the meter is switching ranges, the voltmeter is momentarily opened. A shorting switch would not be appropriate for a voltmeter. It would momentarily connect two multipliers in parallel. The total resistance of the paralleled multipliers would be less than that of the lowest-value multiplier. This could cause the meter movement to be overloaded.
Figure 2. Multirange voltmeter. Switching to a larger multiplier increases the range.
Analog Voltmeter Rating
The total internal resistance of a voltmeter is an important rating of the voltmeter. The internal resistance of a voltmeter is called its input resistance. For a multirange meter like the one in Figure 2, the input resistance is different for each range. Rather than an input resistance for each range being specified, one general rating for all ranges is used. This rating is specified in ohms per volt. The ohms-per-volt rating indicates the input resistance for each volt of that range. For example, a 1000-Ω/V voltmeter would have 1000 Ω of input resistance on the 1-V range. On the 10-V range, it would have 10,000 Ω of input resistance. The input resistance of a voltmeter on a given range is found by multiplying the range by the ohms-per-volt rating. The ohms-per-volt rating of a voltmeter is often referred to as its sensitivity.
Analog Voltmeter Example 2
What is the input resistance of a 10,000-Ω/V voltmeter on the 20-V range?
Given:
$$10,000\ \frac{\Omega}{V}\ voltmeter$$
Find:
Input resistance on 20-V range?
Known:
Input Resistance = Sensitivity x Range
Solution:
$$Input\ Resistance=10,000\ \frac{\Omega}{V}\ \times\ 20\ V=200,000\ \Omega=200\ k\Omega$$
The input resistance on the 20-V range is 200 kΩ.
Analog Voltmeter Sensitivity
The sensitivity (ohms-per-volt rating) of a voltmeter is determined by the full-scale current of the meter movement. Mathematically, the relationship is:
$$Sensitivity=\ \frac{1}{Full-Scale\ Current}$$
Therefore, the sensitivity of the voltmeter of Figure 2 is:
$$Sensitivity=\ \frac{1}{0.00005}=20,000\ \frac{\Omega}{V}$$
Notice that voltmeters have very high internal resistances and ammeters have very low internal resistances. A voltmeter should not significantly change the voltage distribution (or the load) of the circuit in which it is used. Therefore, its internal resistance must be high relative to the resistance of the load.
DMMs have the same input resistance for all voltage ranges. On the lower ranges, they usually have higher input resistance than the VOM. However, on the higher ranges, the opposite is often true. For example, a 100-kΩ/V VOM on the 500-V range has 50 MΩ of input resistance. This exceeds the input resistance of most DMMs, which is typically 10 MΩ.
Often, ac voltmeters have much lower sensitivity than dc voltmeters do. For example, a multimeter may have a 20-kΩ/V rating on the dc ranges and only a 5-kΩ/V rating on the ac ranges.
This disparity between dc and ac sensitivity in VOM ratings is caused by the rectification (Figure 3) required for the D’Arsonval to respond to ac. As shown in Figure 3, only one-half of each ac cycle is used to produce the magnetic field that provides rotational torque on the moving coil.
Figure 3. Principle of rectification in analog voltmeter.
Voltmeter Standards
Standards for voltmeters focus on ensuring accuracy, reliability, and safety in electrical testing applications. Examples include:
IEC 60051-2 __ Specifies requirements for analog ammeters, voltmeters, and their accessories.
ASTM A1013 __ Describes a method for measuring high-frequency core loss in soft magnetic components using voltmeter-ammeter-wattmeter method.
Analog vs. Digital Voltmeters
Analog voltmeters rely on a moving coil mechanism, where current passing through the coil interacts with a magnetic field to create torque. This torque deflects a needle on a calibrated scale, providing a voltage reading. Key characteristics include:
- Advantages: Simple design, suitable for observing trends and fluctuations, and no external power supply required.
- Disadvantages: Lower accuracy, parallax errors in reading, and limited resolution.
Digital voltmeters use analog-to-digital conversion (ADC) to display voltage readings on a digital screen. They measure voltage as discrete numerical values. Key characteristics include:
- Advantages: High accuracy, ease of reading, higher resolution, and additional features such as memory and auto-ranging.
- Disadvantages: Require external power and can be affected by electromagnetic interference.
Table. Comparison between Analog and Digital Voltmeters
| Feature | Analog Voltmeter | Digital Voltmeter |
| Accuracy | Moderate | High |
| Resolution | Limited (scale-dependent) | High (numerical display) |
| Ease of Use | Requires careful reading | Easy to read |
| Power Source | Not required | External or battery-powered |
| Response to Noise | Susceptible to fluctuations | Generally immune |
| Cost | Lower | Higher |
Each type has its specific applications, with analog voltmeters being suitable for monitoring trends in fluctuating signals and digital voltmeters being ideal for precise and detailed measurements.
FAQs for Analog Voltmeter
1. How does an analog voltmeter work?
An analog voltmeter operates based on the D’Arsonval movement, where an electric current passes through a coil in the presence of a permanent magnetic field, producing a torque that moves a needle across a calibrated scale to indicate voltage.
2. What is the role of a multiplier resistor in an analog voltmeter?
The multiplier resistor extends the voltage measurement range of the voltmeter by limiting the current flowing through the sensitive meter movement. It is connected in series with the movement and is designed to drop the excess voltage.
3. What factors determine the sensitivity of an analog voltmeter?
The sensitivity of an analog voltmeter, measured in ohms per volt (Ω/V), depends on the full-scale current of the meter movement. It is calculated as the reciprocal of the full-scale current (Sensitivity = 1 / Full-scale Current).
4. Why is the input resistance of an analog voltmeter important?
The input resistance of an analog voltmeter must be high relative to the circuit it is measuring to minimize the loading effect. A low input resistance can draw significant current from the circuit, altering the voltage being measured.
5. What causes parallax error in an analog voltmeter, and how can it be minimized?
Parallax error occurs when the needle is viewed from an angle rather than directly above. It can be minimized by ensuring the scale has a mirrored background and aligning the needle’s reflection with the actual needle during measurement.
6. How does the range selector switch work in a multirange analog voltmeter?
The range selector switch connects different multiplier resistors in series with the meter movement, allowing the user to measure voltages across multiple ranges. It is typically a non-shorting switch to prevent temporary overloading of the meter during range changes.
7. Can an analog voltmeter measure both AC and DC voltages?
Yes, but for AC voltage measurement, an analog voltmeter requires a rectifier circuit to convert the AC signal into a unidirectional current that can drive the meter movement.
8. What is the impact of temperature on an analog voltmeter’s performance?
Temperature changes can affect the resistance of the multiplier resistor and the coil, leading to measurement inaccuracies. High-quality analog voltmeters use resistors with low temperature coefficients to mitigate this effect.
9. How is the maximum voltage rating of an analog voltmeter determined?
The maximum voltage rating is determined by the breakdown voltage of the components, particularly the multiplier resistors and the insulation of the internal circuitry, to prevent damage or arcing.
10. Why do analog voltmeters have lower accuracy compared to digital voltmeters?
Analog voltmeters are less accurate due to mechanical limitations, such as needle friction, non-linear scale calibration, and susceptibility to parallax errors, while digital voltmeters provide direct numerical readings with higher resolution and precision.