Voltmeter

A voltage measuring device (also called a voltmeter or colloquially as a voltmeter called ) is used to measure electrical voltages.

In the measurement, the measured variable is transformed into a display of their multiples of the unit volts. The voltmeter consists of the measuring mechanism or the electronic measuring system and, optionally, a series resistor or voltage divider to adjust the measurement range. For laboratory applications, there are switchable multiple instruments with different temperature ranges, which are called multimeter.

For industrial applications, there are display type measuring devices.

• 3.1 Ideal - Real Meter
• 3.2 For devices with moving coil movement
• 3.3 For digital electronic devices

Embodiments

Digital Meters

In today's conventional digital voltmeters the voltage is shown in numeric form. The analog voltage to be measured is high impedance tapped and converted by analog -to-digital converter into a digital signal that controls the numerical display.

The advantage of digital voltmeters is their mechanical ruggedness, easy readability of the display, operation possibility in all spatial positions, higher measurement accuracy, significantly higher input resistances, especially at small voltage ranges, and lower manufacturing costs. Next offer many digital multimeter automatic range selection, and they can next DC voltage or AC voltage or mixed voltage - often with an RMS measurement - measure.

The disadvantage numerically instrument pointer is that temporal profiles can not be effectively tracked. Therefore, some digital voltmeters also offer a dial indicator in the form of a bar graph or a pointer on the display graphically represented (weakly visible in the image of the multi- meter from the numeric keypad ). The resolution of the numeric display is limited to 1 digit; see Digital measurement technology and digital multimeter.

Analogue Meters

Electrostatic measurement works

In an electrostatic voltmeter the voltage without current flow leads to a deflection of the pointer. The mechanical force produced by the repulsion of like or unlike charges attract. The simplest electrostatic voltmeter is the electroscope and was mainly used for the measurement of higher DC voltages. Exact gauges had three electrodes, of which one was a plate movably mounted between the other two electrodes. The voltage was ever connected to both fixed electrodes and with a pole to the mobile. The devices often have a light pointer ( display principle of the mirror galvanometer, but this is not one of the electrostatic measuring instruments ). They are also suitable for AC.

Moving iron and Drehspulmesswerke

Here, the voltage measurement is carried out indirectly via the measurement of a current is, possibly supplemented by the resistance of the drive coil around a resistor is proportional to the voltage. Thus, the device measures with his pointer deflection actually a stream, but the scale is labeled with the corresponding voltage values. When moving coil gauge the Lorentz force generates the pointer deflection. When moving iron it is the magnetic repulsion of iron parts, which are located inside a stationary coil. Moving coil always have a series resistor, moving iron works often do not - with these the coil can be made ​​sufficiently high impedance. Moving coil measuring the correct polarity mean; to measure AC voltage, they must possess a measuring rectifier. Moving iron works measure the rms value; they do not need a rectifier. Gauges with these measuring stations are usually marked on the scale with an accuracy class.

Tube voltmeter

Tube voltmeter, voltage measuring devices, which are equipped with electron tubes and have a much higher input resistance, as has been their time otherwise possible. They have been replaced today by fully digital meters that are equipped with an electrometer amplifier ( op-amp with JFET or MOS input ).

Transmitters

In industrial measurement technology and automation technology is also used no -indicating instruments, but transmitters that provide a standardized electrical signal to the central processing. This may technically be a unit analog signal, such as a 4 ... 20 mA. It can also be a digital output signal for transmission over a data bus, the bus is referred to in this context, the fieldbus. These gauges with digital measuring signal at the output then are called measurement converter.

As a transmitter for measuring range matching and for electrical isolation at high AC voltages are voltage converter ( special measuring transformers) in use. The nominal output voltage is preferably at 100 V.

Compensator

For precision measurements and for measuring current without any load on the test object (at least in balance) to voltage compensators are. However, they are replaced by electronic devices with comparable qualifications.

Use

The voltmeter is connected to the two points of a circuit, between which the voltage is to be measured. If one wants to measure the voltage drop over a component or object of measurement, the voltmeter is connected in parallel therewith. This can happen for short tests with test probes without the need to intervene to in the circuit. Therefore, the voltage measurement is the most common form of electrical control. Power measurements can often be obtained indirectly by measuring the voltage when the value of the resistance R is known, is measured with the ( current I = V / R V = measured voltage ).

With digital voltmeters have this problem of congestion in that form, because their internal resistance is very high and therefore implements only little power. Exceeding the measuring range upper limit is automatically switched depending on the version, a multi-stage voltage divider to the next higher range.

A safety reasons, maximum allowable voltage is usually in the range from 700 V to 1000 V and is partially printed on the meter, partially specified in the instructions.

Measuring range adjustment, errors due to self-consumption

Ideal - Real Meter

An ideal instrument has no internal consumption by the test object; this means that the voltage meter, the internal resistance must be infinite. Real, however, it draws a current like a resistor. This one features the equivalent circuit diagram of a parallel circuit of the ideal instrument with its internal resistance. Historically grown - due to the long time standard moving coil instrument - is still partially drawn a series circuit. Then - when the switching character to stand for an ideally lossless meter - whose resistance have the value zero, which is not consistent but with an identification as a voltmeter.

For devices with moving coil movement

To customize a moving-coil voltage meter to the desired measuring range, it is connected in series with a suitable series resistor Rv. Of the measurable at full scale voltage value then renamed some share of Umax on the meter, the rest Uv = renamed - Umax on the series resistor.

In multimeters on Moving- base Imax is mostly a valid for all ranges constant, or vice versa, the internal resistance is in every area of ​​another, - the greater, the greater the range. For ease of calculation of the internal resistance of a voltage- related resistance is at voltmeters ρ = 1/Imax stated specifically in Ω / V ( ohms per volt ), also as a constant for all measurement ranges. This information is to be multiplied by the respective scale value to get the actual internal resistance.

The fact that flows through the voltage measuring device, the determining of the measurement current results in each measurement, the initial conditions on the measurement object distorted, as for measuring a (additional ) stream is withdrawn. Therefore, this should be kept as small as possible, ie the resistance of the voltmeter RU should be as high as possible.

Target in the image shown in the upper circuit, the voltage drop across R1 is V1 is measured, it should be noted that the source of this power a source resistance Rq = R1 | R2 has |. (Note: The source of U0 has as an ideal voltage source to source resistance is zero. ) The lower screen circuit is electrically equivalent to the upper. If the meter connected to it, so a smaller measured value is obtained instead of U1 order because Rq and RU form a voltage divider. The resulting feedback deviation is expressed as relative measurement deviation f,

In digital electronic devices

With digital voltage measuring range extension is not common with a series resistor; the internal resistance of these instruments is typically from 1 to 20 M in all areas; Default is 10 M. Rather, we used here the voltage divider; multimeters for this is built internally. Due to the high internal resistance occurs, the problem of retroactivity deviation ( circuit influence error ) not to the same extent as in the case of moving-coil voltmeters.

The disadvantage of such a high internal resistance is that the voltage indicator is not unique in measurements of non- connected circuit elements; it is then influenced by fields of environment, reload the circuit part ( for example, a disconnected or broken wire or even the non-connected measurement cables ) by induction or induction.