A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an Analog to digital converter.
Voltmeters are made in a wide range of styles. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Small portable instruments, usually equipped with facilities to also measure current and resistance in the form of a multimeter, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant.
General purpose analog voltmeters may have an accuracy of a few per cent of full scale, and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Meters using amplifiers can measure tiny voltages of microvolts or less.
Part of the problem of making an accurate voltmeter is that of calibration to check its accuracy. In laboratories, the Weston Cell is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.
Analog voltmeter
A moving coil galvanometer of the d'Arsonval type. Wire carrying current to be measured
Restoring spring
N and S are poles of magnet
A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with the instrument. It employs a small coil of fine wire suspended in a strong magnetic field. When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.
One of the design objectives of the instrument is to disturb the circuit as little as possible and hence the instrument should draw a minimum of current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.
The sensitivity of such a meter can be expressed as 'ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale value. For example a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be 200,000 and at full scale the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges.
Moving-coil instruments respond only to direct current; measuremnent of AC voltage requires a rectifier in the circuit so that the coil deflects in only one direction. Moving-coil instruments are also made with the zero position in the middle of the scale instead of at one end; these are useful if the voltage reverses its polarity.
Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.
Digital voltmeters
Two digital voltmeters. Note the 40 microvolt difference between the two measurements, an offset of 34 parts per million.
The first digital voltmeter was invented and produced by Andrew Kay of Non-Linear Systems (and later founder of Kaypro) in 1954.
Digital voltmeters usually employ an electronic circuit that acts as an integrator, linearly ramping output voltage when input voltage is constant (this can be easily realized with an opamp). The dual-slope integrator method applies a known reference voltage to the integrator for a fixed time to ramp the integrator's output voltage up, then the unknown voltage is applied to ramp it back down, and the time to ramp output voltage down to zero is recorded (realized in an ADC implementation). The unknown voltage being measured is the product of the voltage reference and the ramp-up time divided by the ramp-down time. The voltage reference must remain constant during the ramp-up time, which may be difficult due to supply voltage and temperature variations.
Digital voltmeters necessarily have input amplifiers and like vacuum tube voltmeters generally have a constant input resistance of 10 megohms regardless of set measurement range.
Potentiometer
A simple, passive voltmeter implementation according to the null-balance method
An important laboratory technique is measurement of voltage with a potentiometer in the null-balance method. The potentiometer's voltage divider is changed at the wiper until the null detector shows zero voltage between the two circuits.
where
Vt: Voltage across test points
Vk: Known voltage
Re: Potentiometer resistance from one end terminal to the other end terminal
Rw: Potentiometer resistance from wiper to end terminal
There are many implementations for null detectors, including moving-coil galvanometers, nanovolt-sensitive integrated circuits, and simple audio circuits that click to indicate voltage difference. The null detector need only be sensitive to small voltage differences but does not need to be linear or accurate. The voltage divider can be made with high uniformity and accuracy, with calculable sources of error. While the method was originally used with manually-adjusted potentiometers, automatic and recording analog instruments are commonly made with the same principle of operation.
Oscilloscope
The oscilloscope method of measuring voltage employs the deflection of the ray in a cathode ray tube (CRT). The ray is actually a beam of electrons travelling in the vacuum inside the tube. The deflection of the beam is either caused by the magnetic field of a coil mounted outside the tube or by the electrostatic deflection caused by the voltage on plates inside the tube. By comparing the deflection caused by an unknown voltage with that caused by a known reference voltage the unknown voltage can easily be deduced. Such measurements can be done for signals too high in frequency for measurement by an analog or digital multimeter.
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