Jun 042012
 

Voltmeters are instruments for measuring the difference in the potentials of two different points of the circuit. Voltage is the reason for the directed movement of electrons. The unit for measuring voltage is called Volt – after the name of the Italian scientist Alessandro Volta. By definition, one Volt stands for voltage in a section of the circuit, in which the flowing current equals one Ampere and the dissipated power is one Watt. According to their structure voltmeters can be electromechanical and electronic – analog and digital. Electromechanical in turn are divided into magneto-electric, electromagnetic, electrodynamic, electrostatic, thermoelectric, diode-compensatory, etc.

Electronic voltmeters can be analog and digital. Depending on the place where the measurements are done, voltmeters are divided into stationary, mounted in panels or portable, for measuring the potential difference between two conductors during the work of electricians or in laboratory conditions. Voltmeters, as a rule, are connected in parallel with the source of electrical energy, as opposed to ammeters, which are connected successively in the circuit. If we need to make the simplest circuit, connecting both a voltmeter and an ammeter, it will look like this, for example:

The perfect voltmeter has infinitely large internal resistance. In the actual voltmeter the size of its internal resistance and its accuracy are in a direct ratio because the greater its own resistance, the less the measured quantity is influenced by external factors.
Depending on the nature of the measured quantity voltmeters are divided into: DC, AC, pulse, phase-sensitive, selective and multi-use.

 

Electromechanical voltmeters usually have a quantity indicator in the form of a pointer on a scale, although there are voltmeters in which the continuous (analogue) indication is „imitated“ by a digital display. In order to increase the measuring range of the voltmeter, additional resistances are added. The accuracy of voltmeters varies from a few percent to thousandths parts of a percent in laboratory equipment.

Electromagnetic voltmeters are the cheapest, the most reliable and have the simplest operational principle. Their most common application is as stationary devices installed in switchboards or control panels in power plants, industrial plants and power stations. The disadvantage of these voltmeters is their relatively high consumption (3-6W) and the high inductance of the coil, which makes the accuracy of the measured quantity dependable on the frequency.
The most accurate (most sensitive) are magneto-electric voltmeters, but they can only be used in DC circuits. In order to be used in AC circuits they need to be used together with thermoelectric, semiconductor or electronic-lamp converters of DC into AC. Such voltmeters, due to the use of these converters, are called thermoelectric, correctional, or electronic and are mainly used in laboratories. Correctional voltmeters are used for measurements in the range of audio frequencies, and electronic and thermoelectric ones – for high frequencies. The main disadvantage of the accuracy of these voltmeters is the preciseness of the curve of the measured voltage.

Electronic voltmeters are more complicated.  There is a variety of small-size resistors and capacitors, as well as electronic tubes in the past. This makes these voltmeters insufficiently stable and reliable, but because of their very high input resistance, they are indispensable when measuring low-power radar loops in a wide frequency range – from 50Hz to 100MHz with precision below 3%.

Electronic analog voltmeters consist of magneto-electric measuring tool, additional resistances, a differential amplifier with small DC interference, small drift, low noise, very high amplification ratio of the signal at an open loop of the reverse connection, very high reduction ratio of the cophasial signal and high input resistance. The differential measuring amplifier is used to achieve high accuracy of measurement and stability of the scheme. The measuring amplifier is a type of two-stage amplifier.
Digital electronic voltmeters convert the measured direct or slowly alternating voltage in discrete electrical code through an analog-digital converter. Thus the measured value appears on the display in a digital form.
The principle of operation of diode-compensatory voltmeters is connected with comparing the measured voltage, through a vacuum diode, with the reference DC type voltage, which is adjustable and whose source is the very voltmeter. The advantage of this method is the wide operating range of the frequency of the measured voltage – from 1 Hz to hundreds of MHz with high accuracy.
Pulsed voltmeters are used to measure the amplitude of periodic pulse signals with a large duty factor and high amplitude of the individual pulses.
Selective voltmeters can differentiate the individual harmonics in a complicated signal and calculate their arithmetic mean value.

Other types of voltmeters are: killovoltmeter, millivoltmeter – for measuring low voltages mV, microvoltmeter – for measuring very low voltages – less than 1 mV, nanovoltmeter – for measuring voltages below 1 μV.
Phase-sensitive voltmeters (vectormeters) measure the component voltages of the first voltage harmonic. They have two scales – one for the active component and another for the reactive one. Thus, phase-sensitive voltmeters can identify complex voltage, as well as its components, defining a certain initial value of reference voltage as zero.

These voltmeters are used to determine the amplitude-phase characteristics of amplifiers, as well as for determining whether a network has an active character. As a standard, cosφ must be greater than 0.8 in electricity networks for domestic subscribers. The deterioration of the power factor is caused by reactive cargos, like different engines, inductances, etc. By a phase-sensitive voltmeter we can measure and compare the two components of voltage – active and reactive.

Voltmeters for measuring DC voltage have on their hull their polarity of switching „+“ and „-“ in order to avoid errors in their switching.
Modern portable voltmeters have a switch of the measuring range. Often the voltmeter is included in the apparatus of the multi-meter, but when used in laboratories, where high precision is required, it can be a single device. Voltmeters designed for DIN rail mounting also find application.
In practice, a voltmeter or a multi-meter, configured to measure voltage, is used for measuring the voltage of the secondary winding of a transformer, which is part of the circuit. First you need to observe the elementary safety measures. For example, when the voltage is above 24V you shouldn’t touch the bare ends of the wires, because current may flow through your body. For this purpose you can use alligator clips, with which you can catch the free ends of the secondary winding, before powering the primary winding, thus your hands remaining free. The voltage, transformed by the transformer, is variable in nature, because the transformer transforms its magnitude, but not its nature, so it is good to have this in mind when placing the switch at the defined position before measuring.

If the measured voltage is greater than that of a transformer, do not think there is a problem with it. This is because the secondary circuit is not closed by a load. When a load is connected, the measured voltage will drop slightly. The more low-powered the measured transformer, the bigger the difference between the voltage at idle running and the voltage with a connected consumer. Another common usage of voltmeters as separate devices or as part of a multi-meter in everyday life is when we want to measure the degree of dilution of batteries or accumulators. In this case, we measure the DC voltage. The aim is to compare how the voltage is different from the voltage that is indicated on the housing. The lower the indicated voltage, the more diluted the battery.

Let’s have a look at the case in which you need to measure the voltage drop of a resistor which is part of an electronic circuit. Although we connect the voltmeter parallel to the load (resistor), if it is small, some reverse current will flow through the voltmeter. The voltmeter creates a parallel path for the current, flowing through the circuit, i.e. there is a shunt. Thus, part of the current is diverted from the circuit and passes through the shunt – voltmeter, then again returns into the circuit.
According to Ohm’s law, the voltage in the measured part of the circuit will be determined by the current which flows through this part of the circuit and the circuit’s resistance. Yes, but we have shunted the circuit and part of the current flows through the shunt. As long as the resistance is invariable and the current is smaller, the voltage will also change. In other words, we measure voltage, which is altered by the connection of a voltmeter to the circuit. Therefore, no matter how accurate the voltmeter is, the measurement is not precise.

How do we overcome this problem? How can we reduce the impact of the measuring device upon the circuit to which it is connected? As mentioned above, it is necessary to increase the input resistance of the voltmeter, because the higher the input resistance of the device, the less current will flow through it and the more accurate measuring results we will have. Modern digital voltmeters have resistance that is high enough and they don’t affect the measured quantity.
When measuring voltage using tentacle probes, it is important that the wire, which they get in contact with, has been carefully cleaned. It is not enough to remove its insulation, but also clean the lacquer layer that is on it, because poor contact may also compromise the measurement. It is recommended that the wire is twisted around the terminals of the probes to achieve better contact and larger contact area.

Voltmeters are a basic measuring device which is widely used in households and industry, often as part of multi-meters. Voltmeters are used daily by millions of people worldwide. As shown in the material, they can be of diverse types depending on the nature of the measured quantity, on their structure and application.

 

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