Sources
Help build the largest human-edited directory on the web.
Submit a site - Become an editor
Wikipedia. Licensed under the GNU Free Documentation License.
Are you an expert in this subject? Join the discussion and share your knowledge at Wikipedia.org.
Voltmeter
From Wikipedia, the free encyclopedia
A voltmeter is an instrument used for measuring the electrical potential difference between two points in an electric circuit.
The voltage can be measured by allowing it to pass a current through a resistance; therefore, a voltmeter can be seen as a very high resistance ammeter. 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.
Note that voltmeters built on this principle show varying input resistance as the instrument is switched through its measuring range. The meter will generally specify a number of "Ohms/Volt" on the faceplate. Multiplying this number by the voltage range the meter is set to gives the input resistance of the instrument.
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 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.
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.
Contents |
Restoring spring
N and S are poles of magnet
Another popular form of voltmeter is the electronic voltmeter, or vacuum tube voltmeter, frequently referred to as a VTVM. This kind of voltmeter uses a tube (or valve in British English) or transistor circuit to amplify the input voltage, which facilitates two objectives: increased sensitivity, and/or increased input impedance (this equipment usually has an input resistance of 10 to 20 megohms).
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.
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. 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.
Digital voltmeters, like vacuum tube voltmeters, generally exhibit a constant input resistance of 10 megohms regardless of set measurement range.
A voltmeter may also be realized using a 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
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 nanovolt-sensitive integrated circuits, simple audio circuits that click to indicate voltage difference, and transformed ammeters, as discussed at the top of this article. For more on circuit transformations, note Thevenin's theorem and Norton's theorem.