Voltmeter

Two digital voltmeters.

A voltmeter is an instrument used to measure the potential difference between two points in an electrical circuit.

Classification of voltmeters

We can classify voltmeters by the principles on which their operation is based and thus better understand how each one works:

Electromechanical voltmeters

These voltmeters, in essence, are made up of a galvanometer whose scale has been graduated in volts. There are models for direct current and alternating current.

Vector voltmeters

Used with microwave signals. In addition to the voltage module, they give an indication of its phase.

Digital voltmeters

They give a numerical indication of the voltage, usually on an LCD type display. They often have additional features such as memory, peak detection, true RMS, autoranging, and other functionality.

The measurement system employs analog-to-digital conversion techniques (usually using a dual-ramp integrator) to obtain the numerical value displayed on an LCD numeric display.

The first digital voltmeter was invented and produced by Andrew Kay of "Non-Linear Systems" (and later founder of Kaypro) in 1954..

Use

To measure the potential difference, the voltmeter must be placed in parallel; that is, in derivation on the points between which we try to carry out the measurement. This leads us to the fact that the voltmeter must have an internal resistance as high as possible, so that it does not produce an appreciable consumption, which would give rise to an erroneous measurement of the voltage. For this, in the case of instruments based on the electromagnetic effects of electric current, they will be equipped with coils of very fine wire and with many turns, so that with little current intensity through the device the necessary moment for the movement of the pointer.

Currently there are digital devices that perform the function of the voltmeter presenting quite high isolation characteristics using complex isolation circuits.

The figure shows the connection of a voltmeter (V) between the points a and b of a circuit, between which the potential difference is to be measured.

In some cases, to allow the measurement of voltages higher than those that would be supported by the windings and mechanical parts of the device or the electronic circuits in the case of digital ones, they are provided with a high-value resistor placed in series with the voltmeter, so that it is only subjected to a fraction of the full voltage.

The following is the formula for calculating the series resistance necessary to achieve this expansion or multiplication of scale:

Ra=Rv(N− − 1){displaystyle R_{mbox{a}}=R_{mbox{v}}(N-1),},

where N is the multiplication factor (N≠1)
R_a is the pull-up resistor of the voltmeter
R_v is the internal resistance of the voltmeter