Voltmeter

A voltmeter is a measuring instrument for measuring the voltage 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 electric current to operate. This is achieved by using a sensitive ammeter or microammeter in series with a high resistance.

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 that is flowing through the coil. For use as a voltmeter, a series resistance is added so that the angular rotation becomes proportional to the applied voltage.

Sensitivity

Idealised voltmeters have infinite input impedance, so that they have no effect on the circuit being measured. Real voltmeters have finite impedance, which the designer will try to make as high as possible. Voltmeter sensitivity is usually specified in ohms per volt FSD (full scale deflection), or just ohms / volt. The most common type of portable multi-range analog voltmeter or multimeter has a sensitivity of 20,000 ohms/volt, thus exhibiting an input resistance of 20Kohms on a 1V scale, 200Kohms on a 10V scale, and so on. The sensitivity of the meter depends on the FSD of its galvanometer; a 20Kohm/volt instrument requires a 50uA galvanometer movement.

Potentiometer

A voltmeter may also be realized using a potentiometer, which is a length of uniform resistance material (wire or carbon film, for instance) and a "wiper" that can short-circuit any portion of the material, thereby changing effective resistance between the wiper and an end terminal of the potentiometer. The unknown voltage source may be connected to a current detector, which is in turn connected to the potentiometer's wiper, while the known voltage source is connected to an end terminal of the potentiometer. Then the wiper position is adjusted to change the potentiometer's effective resistance until a balance is obtained and no current is detected. At this time, record the potentiometer's wiper position. For example, if our potentiometer were a length of very long wire and our wiper were some sort of metal wand in contant with that wire, record the length of wire between the wiper and the end of the wiper that is in our circuit. Now replace the unknown voltage supply with the known voltage supply and repeat the procedure. The unknown voltage is then given by the product of the known voltage and the recorded used length of wire corresponding to the unknown voltage, divided by the recorded length of wire corresponding to the reference voltage.

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.

Vt = \frac{Vk}{Re}Rw

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.

Vacuum Tube Voltmeter (VTVM)

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).

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 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.

Digital voltmeters

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 contant (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 (synonym for cadmium cell, or NiCad battery) is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.

Gourt Home::Gourt :: Voltmeter

Sensitivity

Potentiometer

Vacuum Tube Voltmeter (VTVM)

Oscilloscope

Digital voltmeters

See also