Voltmeter

The voltmeter is an apparatus which makes it possible to measure the tension (or electric difference of Potentiel) between two points, size of which the measuring unit is the Volt (V). The great majority of the current measuring devices is built around a numerical voltmeter, the physical size to measure being converted into tension using a suitable sensor.

Various types of voltmeter

Analogical voltmeters

They in the process of disappearance, although are still used as rapid indicators of about size or of the variation of the measured tension.

Magnetoelectric voltmeters

A magnetoelectric voltmeter consists of a Galvanomètre, therefore a very sensitive Ampèremètre magnetoelectric, in series with an additional resistance of high value. One carries out a voltmeter with several gauges of measurement by changing the value of additional resistance. For measurements in Alternative course, a rectifying bridge with diodes is inserted but this process makes it possible to measure only sinusoidal tensions.

Voltmeters ferroelectric

A voltmeter ferroelectric consists of a Ampèremètre ferroelectric in series with an additional resistance of high value. As the ammeters of the same type do it for the currents, they make it possible to make effective measurements of values of tensions of an unspecified form (but of low frequency < 1 Khz).

Numerical voltmeters

They generally consist of a analog-to-digital Converter doubles slope, of a system of treatment and a display system.

Measure median values of continuous tensions

The tension to be measured is applied to the entry of the analog-to-digital converter through a resistance whose value depends on the gauge chosen, then the processor, taking account of this gauge, makes it possible to post the median value of this tension.

Measure effective values of the alternating voltages

“Bottom-of-the-range” voltmeter
It is usable only for the measurement of the sinusoidal tensions in the electric field of frequency of the distribution networks. The tension to be measured is rectified by a bridge of diodes then treated like a continuous tension. The voltmeter posts then a value equalizes with 1,11 times the median value of the rectified tension. If the tension is sinusoidal, this value is the effective value of the tension .

Voltmeter “effective truth”
The majority of the marketed apparatuses take this measurement in three stages:
1. the tension is high squared by an analogical multiplior of precision.
2. the apparatus carries out the analog-to-digital conversion of the average of the square of the tension
3. the square root of this value is then carried out numerically.

The analogical multiplior of precision being component expensive, these voltmeters are 3 to 4 times more expensive than the precedents. The quasi total digitalization of calculation makes it possible to reduce the cost while improving the precision.

Other methods of measurement are also used, for example:

• analog-to-digital Conversion of the tension to be measured, then entirely digital processing of the calculation of the “square root of the average square”.
• Equalization of the heating effect generated by the variable tension and of that generated by a continuous tension which is then measured.

One distinguishes 2 types of voltmeters “effective truth”

* TRMS (True Root Mean Square= True square root of the average square)

It measures the true effective value of a variable tension.

* RMS (Root Mean Square= Racine square of the average square)

value RMS is obtained thanks to a filtering which eliminates the component continues (Median value) of the tension, and makes it possible to obtain the effective Valeur of the undulation (effective value RMS or AC) of the tension.

History

The first numerical voltmeter was designed and built by Andy Kay in 1953

Internal resistance

Measurement with a voltmeter is carried out by connecting it in parallel on the portion of circuit which one wishes to know the potential difference. Thus in theory, so that the presence of the apparatus does not modify the distribution of the potentials and the currents within the circuit, no current should circulate in its sensor. What implies that the Résistance at least interns known as sensor either Infini E, or or largest possible compared to the resistance of the circuit to be measured.

See too

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