with high resistance. However, this resistance, in the order of a few k, is significantly less than the internal resistance of the digital voltmeters, usually equal to 10 M.
For this reason, analog voltmeters introduce a greater disturbance in the circuits in which they are introduced than digital voltmeters.
To limit this disturbance, we went so far as to use galvanometers with a sensitivity of 15 micro-ampers for full-scale on universal controllers (combination voltmeter-micro-ampermeter
-capacimeter) of high-end. (Metrix MX 205 A for example)
A magnetoelectric voltmeter consists of a galvanometer, thus a very sensitive magnetoelectric millimeter, in series with an additional resistance of high value (from a few k to a few hundred k).
A voltmeter is made at several calibers of measurement by changing the value of the additional strength. For alternating current measurements, a diode straightener bridge is interspersed, but this process can only measure sinusoidal tension. However, they have a number of advantages : they do not require a battery to operate.
Moreover, at the same price, their bandwidth is much wider, allowing AC measurements on several hundred kilohertz where a standard digital model is limited to a few hundred hertz.
This is why they are still widely used in testing on high-frequency electronic equipment (HI-FI)
A ferroelectric voltmeter consists of a series ferroelectric milli-ampmeter
with an additional resistance of high value (from a few hundred Ω to a few hundred k). As amps of the same type do for currents, they can measure the effective value of voltages of any shape (but of low frequency < 1 kHz).
They usually consist of a dual-ramp analog-digital converter, a processing system and a display system.
Measuring the effective values of alternative tensions
It can only be used to measure sinusoidal voltages in the frequency domain of electrical distribution networks. The voltage to be measured is straightened by a bridge of diodes and then treated as a continuous voltage. The voltmeter then displays a value equal to 1.11 times the average value of the upright voltage. If the tension is sinusoidal, the result displayed is the effective value of the tension; if it is not, it makes no sense.
True effective voltmeter
The majority of devices on the market do this in three steps :
1 - The voltage is high squared by an analog precision multiplier.
2 - The device performs analog-digital conversion of the average voltage square
3 - The square root of this value is then performed digitally.
Because the precision analog multiplier is an expensive component, these voltmeters are three to four times more expensive than the previous ones. The near-total digitization of the calculation reduces the cost while improving accuracy.
Other measurement methods are also used, for example :
- Analog-digital conversion of the voltage to be measured, then fully numerical processing of the calculation of the \square root of the middle square.\
- Equalization of the thermal effect generated by the variable voltage and that generated by a continuous voltage which is then measured.
There are two types of \true effective\ voltmeters :
- TRMS (From English True Root Mean Square meaning \true medium square root\) - It measures the true effective value of variable voltage.
- RMS (From English Root Mean Square meaning \average square root\) - The value RMS filtering that eliminates the continuous (medium value) component of the voltage, and achieves the effective value of the voltage ripple.
The first digital voltmeter was designed and built by Andy Kay in 1953.
The measurement with a voltmeter is done by plugging it in parallel to the portion of circuit whose potential is desired to be known.
Thus, in theory, in order for the presence of the device not to change the distribution of potentials and currents within the circuit, no current should circulate in its sensor. This implies that the internal resistance of the said sensor is infinite, or at least as great as possible in relation to the resistance of the circuit to be measured.