voltmeter

Demonstration voltmeter from a physics class

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. Portable instruments, usually equipped 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.

• The red wire carries the current to be measured.
• The restoring spring is shown in green.
• N and S are the north and south poles of the 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 so 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 measured 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 ohms 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 with a permanent-magnet field respond only to direct current. Measurement 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.

Vacuum Tube Voltmeter (VTVM)

The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier instead of the circuit under test. A once-popular form of voltmeter of this type was the vacuum tube voltmeter, frequently referred to as a VTVM. Today these instruments use a solid-state amplifier using field-effect transistors. The electronic amplifier between input and meter gives two benefits; a rugged moving coil instrument can be used, since its sensitivity need not be high, and the input resistance can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent of the range selected.

Digital voltmeters (DVM)

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 are usually designed around a special type of analog-to-digital converter called an integrating converter. Voltmeter accuracy is affected by many factors, including temperature and supply voltage variations. To ensure that a digital voltmeter's reading is within the manufacturer's specified tolerances, they should be periodically calibrated against a voltage standard such as the Weston cell.

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

One may also measure voltage using a potentiometer in the null-balance method. The potentiometer's resistance is changed at the wiper until the null detector shows zero voltage between the two circuits.

where

• V_t is the voltage across test points
• V_k is the known voltage
• R_e is the potentiometer resistance from one end terminal to the other end terminal
• R_w is the 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 traveling 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.

Voltmeter Schematic Symbol

The voltmeter symbol is seen in this example circuit diagram. A voltmeter (V) and an ammeter (A) are shown measuring a voltage and a current respectively, in a simple series circuit.

References

This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (April 2009)

See also

• Electrical measurements
• Solenoid voltmeter
• Ohmmeter
• Metrology
• Electrometer
• Meter (electronics)
• Electronic test equipment
• Voltage divider