Voltmeter
Voltmeter
From Wikipedia, the free encyclopedia.
Many voltmeters are in fact very high resistance
ammeters.
Hence the design of the instrument is identical to that of an ammeter except
that 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 electric
current to operate. The moving coil galvanometer is one example of this
type of voltmeter. It employs a small coil of fine wire suspended in a strong
magnetic field. When electricity is applied it rotates and compresses a small
spring. The angular rotation is proportional to the current that is flowing
through the coil. For use as a voltmeter a series resistance is added so that
now the deflection becomes proportional to the applied voltage.
Other methods for measuring voltage include the potentiometer,
and the oscilloscope.
The potentiometer method of measuring voltage uses a length of uniform
resistance wire held straight and with a DC voltage applied across the wire. A
voltage source is connected to one end of the wires and to a sliding
connection on the wire via some form of current detector. The slider is
adjusted until a balance is obtained and no current is detected. At this
setting the length of wire from the end connected to the voltage source up to
the slider is measured. This procedure is repeated both for a source whose
voltage is accurately known (a voltage reference) and for a source whose
voltage is to be determined. The unknown voltage is then given by the product
of the reference voltage and the length of wire corresponding to the unknown
voltage, divided by the length of wire corresponding to the reference voltage.
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 defliction 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.
The first digital voltmeter was invented and produced by Andy Kay
of Non-Linear Systems (and later founder of Kaypro)
in 1954.
Digital voltmeters usually employ an electronic circuit that acts as an
integrator. (It ramps up or down linearly if a constant voltage is applied).
This "dual slope integrator method" applies a known reference
voltage to the integrator for a fixed time to ramp it up, and then the unknown
voltage is applied to ramp it back down and the time is measured. The unknown
voltage being measured is the product of the voltage reference and the ramp up
time divided by the ramp down time. Clearly the voltage reference has to
remain constant over time and with supply voltage and temperature variations.
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. Simple ones use band-gap diodes.
Another description of a volt meter is:
an instrument used to measure differences of electric potential,
commonly called voltage, in volts or units that are multiples or
fractions of volts. A voltmeter is usually combined with an ammeter
and an ohmmeter in a multipurpose instrument. Most voltmeters are
based on the d’Arsonval galvanometer
and are of the analog type, i.e., they give voltage readings that can
vary over a continuous range as indicated by a scale and pointer.
However, digital voltmeters, which provide voltage readings that are
composed of a group of digits, are becoming increasingly common. Since
an oscilloscope is capable of giving a calibrated visual indication of
voltage, it can be called a voltmeter. See also potentiometer.
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