Voltmeter is a measuring device that is designed to measure voltage direct or alternating current in electrical circuits.
The voltmeter is connected in parallel to the terminals of the voltage source using remote probes. According to the method of displaying measurement results, voltmeters are divided into dial and digital ones.
The voltage value is measured in Voltach, indicated on instruments by the letter IN(in Russian) or Latin letter V(international designation).
On electrical diagrams, a voltmeter is designated by the Latin letter V surrounded by a circle, as shown in the photograph.
Voltage can be constant or alternating. If the voltage of the current source is alternating, then the sign " is placed in front of the value ~ "if constant, then the sign" – ".
For example, the alternating voltage of a household network of 220 Volts is briefly designated as follows: ~220 V or ~220 V. When marking batteries and accumulators, the sign " – " is often omitted, a number is simply printed. The voltage of the vehicle's or battery's power supply is indicated as follows: 12 V or 12 V, and batteries for a flashlight or camera: 1.5 V or 1.5V. The housing must be marked near the positive terminal in the form of a " + ".
The polarity of alternating voltage changes over time. For example, the voltage in household electrical wiring changes polarity 50 times per second (the frequency of change is measured in Hertz, one Hertz is equal to one voltage polarity change per second).
The polarity of direct voltage does not change over time. Therefore, different measuring instruments are required to measure AC and DC voltage.
There are universal voltmeters that can be used to measure both alternating and direct voltage without switching operating modes, for example, the E533 type voltmeter.
How to measure voltage in household electrical wiring
Attention! When measuring voltages above 36 V, it is unacceptable for a person to touch the exposed wires, as they may receive an electric shock.
According to the requirements of GOST 13109-97, the effective voltage value in the electrical network must be 220 V ±10%, that is, it can vary from 198 V to 242 V. If light bulbs in the apartment begin to burn dimly or often burn out, or household appliances begin to work unstably, then in order to take action, you must first measure the voltage value in the electrical wiring.
When starting measurements, it is necessary to prepare the device: – check the reliability of the insulation of conductors with tips and probes; – set the switch of measurement limits to the position of measuring alternating voltage of at least 250 V; 
– insert the connectors of the conductors into the sockets of the device, guided by the inscriptions next to them;
– turn on the measuring device (if necessary).
As you can see in the picture, the limit for changing the alternating voltage is 300 V in the tester, and 700 V in the multimeter. In many tester models, you need to set several switches to the required position at once. Type of current (~ or –), type of measurement (V, A or Ohms) and also insert the ends of the probes into the desired sockets.
In a multimeter, the black end of the probe is inserted into the COM socket (common for all measurements), and the red end into V, common for changing DC and AC voltage, current, resistance and frequency. The socket marked ma is used to measure small currents, 10 A when measuring current reaching 10 A.
Attention! Measuring voltage while the plug is inserted into the 10 A socket will damage the device. In the best case, the fuse inserted inside the device will blow out; in the worst case, you will have to buy a new multimeter. They especially often make mistakes when using instruments to measure resistance, and, forgetting to switch modes, measure voltage. I've met dozens of such faulty devices with burnt resistors inside.
After all the preparatory work has been completed, you can begin measuring. If you turn on the multimeter and no numbers appear on the indicator, it means that either the battery is not installed in the device or it has already exhausted its resource. Typically, multimeters use a 9 V Krona battery with a shelf life of one year. Therefore, even if the device has not been used for a long time, the battery may not work. When using the multimeter in stationary conditions, it is advisable to use a ~220 V/–9 V adapter instead of the crown.
Insert the ends of the probes into the socket or touch them to the electrical wires.
The multimeter will immediately show the voltage in the network, but you still need to be able to read the readings in a dial tester. At first glance, it seems difficult, since there are many scales. But if you look closely, it becomes clear on which scale to read the device. The TL-4 type device in question (which has served me flawlessly for more than 40 years!) has 5 scales.
The upper scale is used to take readings when the switch is in positions that are multiples of 1 (0.1, 1, 10, 100, 1000). The scale located just below is multiples of 3 (0.3, 3, 30, 300). When measuring AC voltages of 1 V and 3 V, 2 additional scales are applied. There is a separate scale for measuring resistance. All testers have a similar calibration, but the multiplicity can be any.
Since the measurement limit was set to ~300 V, it means that the reading must be made on the second scale with a limit of 3, multiplying the readings by 100. The value of a small division is 0.1, therefore, it turns out 2.3 + the arrow is in the middle between the lines, which means take the reading value 2.35×100=235 V.
It turned out that the measured voltage value is 235 V, which is within acceptable limits. If during the measurement process there is a constant change in the value of the least significant digits, and the tester’s needle constantly fluctuates, it means that there are bad contacts in the electrical wiring connections and it is necessary to inspect it.
How to measure battery voltage
battery or power supply
Since the voltage of DC sources usually does not exceed 24 V, touching the terminals and bare wires is not dangerous to humans and no special safety precautions are required.
In order to assess the suitability of a battery, accumulator or the health of the power supply, it is necessary to measure the voltage at their terminals. The terminals of round batteries are located at the ends of the cylindrical body, the positive terminal is indicated by a “+” sign.
Measuring DC voltage is practically not much different from measuring AC voltage. You just need to switch the device to the appropriate measurement mode and observe the polarity of the connection.
The amount of voltage that a battery creates is usually marked on its body. But even if the measurement result showed sufficient voltage, this does not mean that the battery is good, since the EMF (electromotive force) was measured, and not the capacity of the battery, on which the operating life of the product in which it will be installed depends.
To more accurately estimate the battery capacity, you need to measure the voltage by connecting a load to its poles. An incandescent flashlight light bulb rated for a voltage of 1.5 V is well suited as a load for a 1.5 V battery. For ease of operation, you need to solder conductors to its base.
If the voltage under load decreases by less than 15%, then the battery or accumulator is quite suitable for use. If there is no measuring device, then you can judge the suitability of the battery for further use by the brightness of the light bulb. But such a test cannot guarantee the battery life of the device. It only indicates that the battery is currently still usable.
We take electricity for granted and hardly anyone thinks about what electrical voltage is and what its physical essence is when they turn on the light, computer or washing machine. In fact, it deserves much more attention, and not only because it can be deadly, but also because Humanity, having mastered this type of energy, has made a qualitative leap in civilization.
Let's remember one of the most interesting moments in a school physics lesson, when the teacher rotated the disk of an electric machine, and a spark jumped between the metal balls. This is the visible reflection of a natural phenomenon called electric current. It arises due to the fact that there are more negatively charged ions on one ball and fewer on the other, which is why a potential difference arises, that is, a fact that violates the basic law of Nature - conservation of energy.
Negatively charged particles tend to move to where there are fewer of them, thereby nullifying the difference. Of course, electrons do not travel all the way between the charged balls, called poles. Their range is limited by a crystal lattice, the nodes of which they cannot leave. But they are capable of hitting neighboring particles and transmitting momentum further along the chain, creating a domino effect. Each such collision generates a burst of energy, due to which the system passes from a resting state to an excited one, which is usually called electrical voltage.
The force that moves charged particles
In order to put electric voltage and current at his service, man had to find a force that could restore the potential difference between the poles, generating a continuous collision of particles of the crystal lattice. There were three of them:
- Electromagnetic induction is the generation of current as a result of the interdependent movement of metals in a magnetic field. Used in direct and alternating current generators.
- Electrochemical interaction generated by the potential difference between the crystal lattices of substances. Used in batteries, DC batteries.
- A thermochemical reaction that increases the activity of electrons as a result of heating.
The force that generates the movement of charged particles is called “electromotive” (abbreviation EMF) and is indicated on diagrams by the letter “E”, usually accompanying the mnemonic symbols of the connectors to which the power source is connected.
Volts and Amps
EMF and voltage are measured in volts - a conventional unit named after the Italian Alessandro Volta, the officially recognized inventor of the galvanic battery - a direct current source. This is the amount of work that is done when moving a unit of charge (coulomb), if 1 joule of conventional energy was spent.
However, there is a second unit of measurement of electric current - the ampere, named after the French physicist Andre-Marie Ampere. Traditionally, it is called current strength, although it is more correct to use the term “magnetomotive force”, which most fully reflects the dual physical essence of a charged particle.
The magnetic and electric fields of the electron tend to mutually compensate, and their dependence is determined by Ohm's law, described by the formula I = U / R. If the resistance of the medium drops sharply (for example, during a short circuit), then the current increases exponentially. This causes a response voltage drop, causing the system to return to equilibrium. A similar effect can be noticed during operation of a welding transformer, when when an arc occurs, the incandescent lamps almost go out.
There is another effect: with a high resistance of the medium, a charge of the same sign accumulates on any surface until the voltage reaches a critical level, after which a breakdown (current occurs) occurs in the direction of the surface with the greatest potential difference. Static voltage is extremely dangerous because at the moment of discharge it can generate currents of hundreds of amperes. Therefore, metal structures that are exposed to a magnetic field for a long time must be grounded.
Constant or variable?
Voltage is the static component of electricity, and current is dynamic, because its direction changes along with the polarity at the ends of the conductor. And this property turned out to be very useful for the spread of electricity throughout the World. The fact is that any current is damped due to the internal resistance of the medium, according to the same law of conservation of energy. But it turned out that it is very difficult to amplify a flow of electrons moving in one direction, but one that changes direction cyclically is simple; for this, a transformer with two windings on one core is used.
To obtain alternating current, it is necessary to turn inside out the principle discovered by Faraday, who, in his prototype of an electric generator, rotated a copper disk in the field of a permanent magnet. Nikola Tesla did the opposite - he placed a rotating electromagnet inside a stationary winding, obtaining an unexpected effect: at the moment the poles pass through the neutral of the magnetic field, the voltage amplitude drops to zero, and then increases again, but with a different sign. During one revolution, the direction of movement of electrons in the conductor changes twice, constituting the working phase. Therefore, alternating current is also called phase current. And the voltage generating it is sinusoidal.
Nikola Tesla created a generator with two windings located at an angle of 90 0 to each other, and the Russian engineer M.O. Dolivo-Dobrovolsky improved it by placing three on the stator, which increased the stability of the electric machine. As a result, industrial alternating current became three-phase.
Why 220 volts 50 Hz?
In our country, a household single-phase network has ratings of 220 volts and 50 hertz. The reason for the appearance of these particular numbers is very interesting.
The palm in the domestic development of electricity belongs to Thomas Edison. He used exclusively direct current, since Nikola Tesla's brilliant invention of alternating current had not yet happened.
The first electrical device was an incandescent lamp with a carbon filament. It was experimentally found that it works best at a voltage of 45 volts and a ballast resistance included in the circuit, which ensures the dispersion of another twenty. An acceptable operating time was ensured by switching on two lamps in series. In total, in the household network, according to Edison, there should have been 110 volts.
However, the transmission of direct current from power plants to consumers was accompanied by great difficulties: after one or two miles it died out completely. According to the Joule-Lenz Law, the amount of heat dissipated by a conductor during the passage of current is calculated by the following formula: Q = R. I 2. To reduce losses by four times, the voltage was increased to 220 volts, and the power line was built from three conductors - with two “pluses” and one “minus”. The consumer received the same 110 volts.
The confrontation between Nikola Tesla and Thomas Edison, called the “War of Currents,” was decided in favor of alternating current, since it could be transmitted over long distances with minimal losses. Nevertheless, the voltage between the power conductors remains 220, and the linear voltage supplied to the consumer is 127 volts, since due to a phase shift of 120 degrees, the voltage amplitudes do not add up arithmetically, but are multiplied by 1.73 - the square root of three.
In the USSR, the network rating of 127 volts in one phase was used until the early 60s. During the improvement of electrical lines, carried out in order to increase the transmitted power, the designers followed the same path as Edison - they increased the voltage.
The reference point was 220 volts, which were measured between phases. It has become commonplace. And the industrial phase-to-phase voltage of 380 volts was obtained by multiplying 220 by 1.73. A frequency of 50 Hz is 3 thousand vibrations per minute, that is, the optimal number of revolutions of the crankshaft of a diesel engine or other internal combustion engine that drives an alternating current machine.
Now you know what voltage and electric current are, in what units they are measured and how they depend on each other, and also why there is 220 volts in your outlet. The facts presented are not of an academic nature and do not claim to be the ultimate truth. You can learn more about the nature of this phenomenon in textbooks on electrical engineering.
What is voltage and current
Voltage and current are quantitative concepts that should always be kept in mind when dealing with an electronic circuit. They usually vary over time, otherwise the operation of the circuit is of no interest.
Voltage(symbol: U, sometimes E). Voltage between two points is the energy (or work) expended in moving a unit positive charge from a point of low potential to a point of high potential (i.e., the first point has a more negative potential compared to the second). In other words, it is the energy that is released when a unit charge “slides” from high potential to low. Voltage is also called potential difference or electromotive force(e.d.s). The unit of measurement for voltage is the volt. Typically, voltage is measured in volts (V), kilovolts (1 kV = 10 -3 V), millivolts (1 mV = 10 -3 V) or microvolts (1 µV = 10 -6 V). In order to move a charge of 1 coulomb between points having a potential difference of 1 volt, it is necessary to do 1 joule of work. (A coulomb is a unit of electrical charge and is equal to the charge of approximately 6 * 10 18 electrons.) Voltage measured in nanovolts (1 nV = 10 -9 V) or megavolts (1 MV = 10 6 V) is rare.
Current(symbol: I). Current is the speed at which an electric charge moves at a point. The unit of measurement for current is ampere. Current is usually measured in amperes (A), milliamps (1 mA = 10 -3 A), microamps (1 µA = 10 -6 A), nanoamps (1 nA = 10 -9 A) and sometimes picoamps (1 pA = 10 -12 A). A current of 1 ampere is created by moving a charge of 1 coulomb in a time of 1 s. It is agreed that current in a circuit flows from a point with a more positive potential to a point with a more negative potential, although the electron moves in the opposite direction.
Remember: voltage is always measured between two points of the circuit, current always flows through point in the circuit or through any element of the circuit.
You can’t say “voltage in a resistor” - it’s ignorant. However, we often talk about voltage at some point in the circuit. In this case, they always mean the voltage between this point and the “ground”, that is, a point in the circuit whose potential is known to everyone. You will soon get used to this method of measuring voltage.
Voltage is created by influencing electrical charges in devices such as batteries (electrochemical reactions), generators (interaction of magnetic forces), solar cells (photovoltaic effect of photon energy), etc. We obtain current by applying voltage between the points of the circuit.
Here, perhaps, the question may arise: what exactly are voltage and current, what do they look like? In order to answer this question, it is best to use an electronic device such as an oscilloscope. It can be used to observe voltage (and sometimes current) as a function that changes over time.
In real circuits, we connect elements to each other using wires, metal conductors, each of which at each point has the same voltage (relative to, say, ground). In the region of high frequencies or low impedances, this statement is not entirely true. Now let’s take this assumption on faith. We mention this to make you understand that the actual circuit does not have to look like the schematic diagram, since the wires can be connected in different ways.
Remember a few simple rules regarding current and voltage:
The sum of currents flowing into a point is equal to the sum of currents flowing out of it (conservation of charge). This rule is sometimes called Kirchhoff's law for currents. Engineers like to call this point in the circuit a node. A corollary follows from this rule: in a series circuit (which is a group of elements that have two ends and are connected by these ends to one another), the current at all points is the same.
When connecting elements in parallel (Fig. 1), the voltage on each element is the same. In other words, the sum of the voltage drops between points A and B, measured along any branch of the circuit connecting these points, is the same and equal to the voltage between points A and B. Sometimes this rule is formulated as follows: the sum of the voltage drops in any closed loop of the circuit is zero. This is Kirchhoff's law for stress.
The power (work done per unit time) consumed by the circuit is determined as follows:
P=UI
Let's remember how we determined voltage and current, and we find that power is equal to: (work/charge)*(charge/unit of time). If the voltage U is measured in volts and the current I is measured in amperes, then the power P will be expressed in watts. Power of 1 watt is 1 joule of work done in 1 s (1 W = 1 J/s).
Power is dissipated as heat (usually) or sometimes expended in mechanical work (motors), converted into radiant energy (lamps, non-transmitters) or stored (batteries, capacitors). When developing a complex system, one of the main issues is determining its thermal load (take, for example, a computer in which the by-product of several pages of results of solving a problem are many kilowatts of electrical energy dissipated into space in the form of heat).
In the future, when studying periodically changing currents and voltages, we will generalize the simple expression P = UI. In this form, it is valid for determining the instantaneous value of power. By the way, remember that you don’t need to call current current intensity - it’s illiterate.
Essentially, the term refers to potential difference, and the unit of voltage is the volt. Volt is the name of the scientist who laid the foundation for everything we now know about electricity. And this man's name was Alessandro.
But this is what concerns electric current, i.e. the one with the help of which our usual household electrical appliances operate. But there is also the concept of a mechanical parameter. This parameter is measured in pascals. But this is not about him now.
What is a volt equal to?
This parameter can be either constant or variable. It is alternating current that “flows” into apartments, buildings and structures, houses and organizations. Electrical voltage represents amplitude waves, indicated on graphs as a sine wave.
Alternating current is indicated in diagrams by the symbol “~”. And if we talk about what one volt is equal to, then we can say that this is an electrical action in a circuit where, when a charge equal to one coulomb (C) flows, work equal to one joule (J) is performed.
The standard formula by which it can be calculated is:
U = A:q, where U is exactly the desired value; “A” is the work that the electric field (in J) does to transfer charge, and “q” is precisely the charge itself, in coulombs.
If we talk about constant values, then they practically do not differ from variables (with the exception of the construction graph) and are produced from them, using a rectifying diode bridge. Diodes, without passing current to one side, seem to divide the sine wave, removing half-waves from it. As a result, instead of phase and zero, we get plus and minus, but the calculation remains in the same volts (V or V).
Voltage measurement
Previously, only an analog voltmeter was used to measure this parameter. Now on the shelves of electrical engineering stores there is a very wide range of similar devices already in digital design, as well as multimeters, both analog and digital, with the help of which the so-called voltage is measured. Such a device can measure not only the magnitude, but also the current strength, the resistance of the circuit, and it is even possible to check the capacitance of the capacitor or measure the temperature.
Of course, analog voltmeters and multimeters do not provide the same accuracy as digital ones, the display of which shows the voltage unit down to hundredths or thousandths.
When measuring this parameter, the voltmeter is connected to the circuit in parallel, i.e. if it is necessary to measure the value between phase and zero, the probes are applied one to the first wire, and the other to the second, in contrast to measuring current, where the device is connected in series to the circuit.
In circuit diagrams, a voltmeter is indicated by the letter V surrounded by a circle. Different types of such devices measure, in addition to volts, different units of voltage. In general, it is measured in the following units: millivolt, microvolt, kilovolt or megavolt.
Voltage value
The value of this parameter of electric current in our life is very high, because whether it corresponds to the required one depends on how brightly the incandescent lamps will burn in the apartment, and if compact fluorescent lamps are installed, then the question arises whether or not they will light at all. The durability of all lighting and household electrical appliances depends on its surges, and therefore having a voltmeter or multimeter at home, as well as the ability to use it, is becoming a necessity in our time.
Surely, each of us, at least once in our lives, has had questions about what current is, voltage, charge, etc. All these are components of one large physical concept - electricity. Let's try to study the basic patterns of electrical phenomena using simple examples.
What is electricity.
Electricity is a set of physical phenomena associated with the emergence, accumulation, interaction and transfer of electric charge. According to most historians of science, the first electrical phenomena were discovered by the ancient Greek philosopher Thales in the seventh century BC. Thales observed the effect of static electricity: the attraction of light objects and particles to amber rubbed with wool. To repeat this experiment yourself, you need to rub any plastic object (for example, a pen or ruler) on a wool or cotton fabric and bring it to finely cut pieces of paper.
The first serious scientific work that described the study of electrical phenomena was the treatise of the English scientist William Gilbert “On the Magnet, Magnetic Bodies and the Great Magnet - the Earth,” published in 1600. In this work, the author described the results of his experiments with magnets and electrified bodies. The term electricity is also mentioned here for the first time.
W. Gilbert's research gave a serious impetus to the development of the science of electricity and magnetism: during the period from the beginning of the 17th to the end of the 19th century, a large number of experiments were carried out and the basic laws describing electromagnetic phenomena were formulated. And in 1897, the English physicist Joseph Thomson discovered the electron, an elementary charged particle that determines the electrical and magnetic properties of matter. An electron (in ancient Greek, electron is amber) has a negative charge approximately equal to 1.602 * 10-19 C (Coulomb) and a mass equal to 9.109 * 10-31 kg. Thanks to electrons and other charged particles, electrical and magnetic processes occur in substances.
What is stress.
There are direct and alternating electric currents. If charged particles constantly move in one direction, then there is a direct current in the circuit and, accordingly, constant voltage. If the direction of movement of particles periodically changes (they move in one direction or another), then this is an alternating current and it arises, accordingly, in the presence of an alternating voltage (i.e., when the potential difference changes its polarity). Alternating current is characterized by a periodic change in the current strength: it takes on a maximum and then a minimum value. These current values are amplitude, or peak. The frequency of voltage polarity changes may vary. For example, in our country this frequency is 50 Hertz (that is, the voltage changes its polarity 50 times per second), and in the USA the frequency of alternating current is 60 Hz (Hertz).