voltmeter
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voltmeter
Voltmeter
An instrument for the measurement of the electric potential difference between two conductors. Many different kinds of instruments are available to suit different purposes. Voltages of the order of picovolts (10-12 V) to megavolts (106 V) can be measured. Frequencies from zero (dc) to many megahertz and accuracies in the range from a fraction of part per million (ppm) to a few percent may be covered. See Electrical units and standards, Voltage measurement
Analog voltmeters
Where no great accuracy is required, a voltage may be indicated by a mechanical displacement of a pointer against a scale. There is a wide variety of principles on which instruments of this type can be based. The d'Arsonval movement (see illustration) is one of the most popular constructions. This is basically a current-sensing instrument and is used in conjunction with a suitable resistance in series to measure voltage. A further variant, taut-band suspension, uses a pair of resilient strips under tension to carry the current to the coil, locate it, and provide the rotational restoring force. See Ammeter, Multimeter
The permanent-magnet, moving-coil instrument is very sensitive, but by its nature is responsive only to the average value of the current flowing through the coil. It is therefore unsuitable for ac. A rectifier circuit can be used in order to combine the sensitivity of the movement with ac response. A transformer can be used to reduce the nonlinearity that results from the forward voltage drop of the diode rectifiers, at the expense of current drain.
Electronic voltmeters
The movements so far described require energy from the signal being measured to cause the deflection. The resulting current is liable to modify the voltage at the measurement point. To reduce this loading effect, active circuits are often used between the input terminals and the indicating movement. Once an independent source of power is available, electronic circuits can be used to provide other features, including a variety of kinds of signal processing and digital presentation of the results.
Digital voltmeters
Digital voltmeters (DVMs) are now the preferred instruments for ac and dc measurements at all levels of accuracy and at all voltages up to 1 kV. Essentially a digital voltmeter consists of a voltage reference, usually provided by a Zener diode, an analog-to-digital converter and digital display system, and a power supply, which may be derived from either the mains or a battery. The basic range of the instrument provides measurement from zero to 10 or 20 V. Additional lower ranges may be provided by amplifiers, whose gain is stabilized by precision resistors. These electronic input amplifiers often provide a very high input impedance, perhaps exceeding 1010 &OHgr;. Since this impedance is obtained by active means, a much lower impedance may be found when the instrument is switched off. Higher voltage ranges are provided by the use of resistive attenuators, usually limited to a value of 10 M&OHgr; by economic restraints. The best accuracy is always obtained on the basic range, where it is limited to that of the analog-to-digital converter.
Sampling voltmeters
A sampling voltmeter is an instrument that uses sampling techniques and has advantages at very low frequencies, that is, below 1 Hz, and also at very high frequencies, where conventional measuring circuits become difficult or even impossible. Low-frequency sampling instruments achieve uncertainties as small as 50 ppm with 10-V signals; high-frequency instruments can achieve a few percent with frequencies as high as 12 GHz and amplitudes as small as 1 mV. Measurements are generally of rectified-mean or root-mean-square voltage. Modern digital sampling voltmeters may also be capable of calculating and displaying voltages or energy density as a function of frequency. Sampling voltmeters, like conventional voltmeters, may use scale and pointer meters, graphic recorders, cathode-ray tubes, or digital indicators for readout of measured quantities. See Waveform determination
voltmeter
[′vōlt‚mēd·ər]Voltmeter
An instrument for the measurement of the electric potential difference between two conductors. Many different kinds of instruments are available to suit different purposes. Voltages of the order of picovolts (10-12 V) to megavolts (106 V) can be measured. Frequencies from zero (dc) to many megahertz and accuracies in the range from a fraction of part per million (ppm) to a few percent may be covered. See Voltage measurement
Analog voltmeters
Where no great accuracy is required, a voltage may be indicated by a mechanical displacement of a pointer against a scale. There is a wide variety of principles on which instruments of this type can be based. The d'Arsonval movement (see illustration) is one of the most popular constructions. This is basically a current-sensing instrument and is used in conjunction with a suitable resistance in series to measure voltage. A further variant, taut-band suspension, uses a pair of resilient strips under tension to carry the current to the coil, locate it, and provide the rotational restoring force. See Ammeter
The permanent-magnet, moving-coil instrument is very sensitive, but by its nature is responsive only to the average value of the current flowing through the coil. It is therefore unsuitable for ac. A rectifier circuit can be used in order to combine the sensitivity of the movement with ac response. A transformer can be used to reduce the nonlinearity that results from the forward voltage drop of the diode rectifiers, at the expense of current drain. See Rectifier, Transformer
Electronic voltmeters
The movements so far described require energy from the signal being measured to cause the deflection. The resulting current is liable to modify the voltage at the measurement point. To reduce this loading effect, active circuits are often used between the input terminals and the indicating movement. Once an independent source of power is available, electronic circuits can be used to provide other features, including a variety of kinds of signal processing and digital presentation of the results.
Digital voltmeters
Digital voltmeters (DVMs) are now the preferred instruments for ac and dc measurements at all levels of accuracy and at all voltages up to 1 kV. Essentially a digital voltmeter consists of a voltage reference, usually provided by a Zener diode, an analog-to-digital converter and digital display system, and a power supply, which may be derived from either the mains or a battery. The basic range of the instrument provides measurement from zero to 10 or 20 V. Additional lower ranges may be provided by amplifiers, whose gain is stabilized by precision resistors. These electronic input amplifiers often provide a very high input impedance, perhaps exceeding 1010 &OHgr;. Since this impedance is obtained by active means, a much lower impedance may be found when the instrument is switched off. Higher voltage ranges are provided by the use of resistive attenuators, usually limited to a value of 10 M&OHgr; by economic restraints. The best accuracy is always obtained on the basic range, where it is limited to that of the analog-to-digital converter. See Amplifier, Analog-to-digital converter, Electronic power supply, Zener diode
Sampling voltmeters
A sampling voltmeter is an instrument that uses sampling techniques and has advantages at very low frequencies, that is, below 1 Hz, and also at very high frequencies, where conventional measuring circuits become difficult or even impossible. Low-frequency sampling instruments achieve uncertainties as small as 50 ppm with 10-V signals; high-frequency instruments can achieve a few percent with frequencies as high as 12 GHz and amplitudes as small as 1 mV. Measurements are generally of rectified-mean or root-mean-square voltage. Modern digital sampling voltmeters may also be capable of calculating and displaying voltages or energy density as a function of frequency. Sampling voltmeters, like conventional voltmeters, may use scale and pointer meters, graphic recorders, cathode-ray tubes, or digital indicators for readout of measured quantities.
voltmeter
Voltmeter
an electrical instrument for the measurement of electromotive forces or voltages in electrical circuits. A voltmeter is connected in parallel with a load or with a source of electrical energy (Figure 1).
The world’s first voltmeter was the “electric force indicator” of the Russian physicist G. V. Rikhman (1745). The operating principle of the “indicator” is also used in the modern electrostatic voltmeter.
Moving-iron voltmeters are the simplest to manufacture, the least expensive, and the most reliable in service. They are used chiefly as the permanent instruments on control panels in power stations and industrial enterprises and only rarely as laboratory instruments. Their disadvantages are the relalively large inherent power consumption (3-7 watts) and the high inductance of the winding that makes their reading largely dependent on frequency.
Moving-coil voltmeters are the most sensitive and accurate but are only suitable for DC measurements. In combination with thermoelectric, semiconductor, or electron tube AC-to-DC converters they are used to measure AC voltage. Such voltmeters, which are called thermoelectric, rectifier, and electronic voltmeters, are used mainly in laboratory practice. Rectifier voltmeters are used for measurements in the audiofrequency range, and thermoelectric and electronic types are used at high frequencies. The shortcoming of these instruments is the considerable effect on the accuracy of the data of the waveform of the measured potential (voltage).
Electronic voltmeters have complicated circuits with somewhat unstable components (electron tubes, small electrical resistors and capacitors), which tends to reduce their reliability and accuracy. However, they are indispensable for measurements made in low-power radio engineering circuits because they have a high input impedance and operate over a wide frequency range (from 50 Hz to 100 MHz) with errors no greater than 3 percent of the full scale reading. Electronic voltmeters are also made to measure the amplitude of voltage pulses having a duration of tenths of a jitsec and an on-off time ratio of up to 2,500.
In the early 20th century both thermal and induction volt-meters were widely used; nowadays their industrial production has been discontinued because of instrinsic drawbacks such as the large power consumption and the effects of the surrounding temperature on their readings.
REFERENCES
Arutiunov, V. O. Elektricheskie izmeritel’nye pribory i izmereniia. Moscow-Leningrad, 1958.Shkurin, G. P. Spravochnik po elektroizmeritel’nym i radioizmeritel’nym priboram. Moscow, 1960.
N. G. VOSTROKNUTOV