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Variable designation. Alternating electric current

Among the types of electric current, there are:

D.C:

Designation (-) or DC (Direct Current = direct current).

Alternating current:

Designation (~) or AC (Alternating Current = alternating current).

When direct current(-) Current flows in one direction. Direct current is supplied, for example, by dry batteries, solar panels and accumulators for appliances with low current consumption. The electrolysis of aluminium, electric arc welding and the operation of electrified railways require high direct current. It is created using AC rectification or using DC generators.

As the technical direction of the current, it is assumed that it flows from the contact with the “+” sign to the contact with the “-” sign.

In the case of alternating current (~), a distinction is made between single-phase alternating current, three-phase alternating current and high-frequency current.

With alternating current, the current constantly changes its magnitude and its direction. In the Western European power grid, the current changes direction 50 times per second. The frequency of oscillation changes per second is called the frequency of the current. The unit of frequency is hertz (Hz). Single-phase alternating current requires a voltage-carrying conductor and a return conductor.

Alternating current is used on the construction site and in industry to operate electrical machines such as hand-held grinders, electric drills and circular saws, as well as to light construction sites and construction site equipment.

Three-phase alternators generate on each of their three windings AC voltage frequency 50 Hz. Three separate networks can be supplied with this voltage, and at the same time, only six wires can be used for direct and return conductors. If you combine the return conductors, then you can limit yourself to only four wires

The common return wire will be the neutral conductor (N). As a rule, it is grounded. The other three conductors (outer conductors) are abbreviated LI, L2, L3. In the German power grid, the voltage between the outer conductor and the neutral conductor, or earth, is 230 V. The voltage between the two outer conductors, for example between L1 and L2, is 400 V.

High-frequency current is said to be when the oscillation frequency is much higher than 50 Hz (from 15 kHz to 250 MHz). High-frequency current can be used to heat conductive materials and even melt them, such as metals and some synthetic materials.

Alternating current– or AC ( Alternating Current). Designation (~).

Electricity called variables, if it changes its direction over time and continuously changes in magnitude.

Alternating current, which is used to connect household or industrial electrical appliances, changes according to a sinusoidal law:

i = Im sin(2πft)

AC Graph

i - instantaneous current value
Im - amplitude or highest value current
f is the value of the frequency of the alternating current
t - time

Widely used alternating current due to the fact that electricity alternating current can be technically simply and economically converted from lower voltage energy to higher voltage energy and vice versa. This property alternating current allows the transmission of electricity over long distances.

AC period

Industrial variable electricity are obtained using electric generators, the principle of operation of which is based on the law of electromagnetic induction. The rotation of the generator is carried out by a mechanical engine using thermal, hydraulic or nuclear energy.

Variable single-phase electricity has the following main characteristics:

f - AC frequency determines the number of cycles or periods per unit time. Hertz (Hz) is taken as the unit for measuring the frequency of alternating current:

1Hz = 10 3 kHz = 10 6 MHz

Τ - period - the time of one complete change of the variable.

If 1 period Τ occurs in 1 second, then the frequency f = 1 Hz(Hertz).

1c = 10 3 ms = 10 6 µs = 10 12 ns

AT Russian Federation the period Τ of the alternating current is taken equal to 0.02 seconds, therefore, according to the formula
f \u003d 1 / Τ, you can determine the frequency of the alternating current:

f = 1/0.02 = 50 Hz

ω - angular velocity

In addition to the frequency f when studying circuits alternating current the concept of angular velocity is introduced ω . The angular velocity ω is related to the frequency f by the following relationship:

At a frequency 50 Hz the angular velocity is 314 rad/s (2 × 3.14 × 50 = 314).

Instant value(i,u,e,p) - the value of the quantity at the moment, instantaneous.

Maximum or peak value(Im,Um,Em,Pm).

RMS current- this is the magnitude of the alternating current, equal to such a current, which on the resistance R, creates a heat release equal to the given alternating current, for the same time t (I, U, E, P).

Getting a sinusoidal curve

In the system of Cartesian rectangular coordinates, a trigonometric circle and a curve reflecting the change in the value of the trigonometric sin functionsβ on the angle β between the 0x axis and the radius vector r . The radius vector r rotates counterclockwise. Let us rotate the radius vector by an angle β and draw a dotted line from the end of the vector r parallel to the 0x axis. From the circle (point a) along the 0x axis, we set aside a segment on a scale. From the end of the segment we construct a perpendicular to the intersection with the dotted line. We get a point c at the intersection of the perpendicular and the dashed line.

AC sine wave

We will carry out a similar construction by increasing the angle β until the radius vector rotates through the angle β = 360°, and we will obtain points similar to point c. We connect the points of a smooth curve, which will reflect the sinusoidal law of change in the magnitude of the alternating current.

The concept of phase

If two variables simultaneously pass their zero and maximum values, then they are in phase.

If two variables do not simultaneously pass through their zero and maximum values, then they are out of phase.

In radio engineering, the concepts are used:

1. Active resistance (R a)
2. Inductive reactance (X L - reactance)
3. Capacitance (X C - reactance)

The concept of active resistance

If a current flows through the conductor, then due to the phenomenon of self-induction, the electrons do not propagate uniformly over the cross section of the conductor, as a result of which the resistance of the conductor increases.

The phenomenon of uneven distribution of charges over the cross section of the conductor is called the surface effect. The higher the frequency, the greater the resistance.

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First, let's remember what types of current exist:

Alternating current (letter designation AC) - is produced due to the magnetic effect. This is the same current that we have in our homes. It does not have any poles because it changes them many times per second. This phenomenon (reversal of polarity) is called frequency and is expressed in hertz (Hz). At the moment, our network uses an alternating current of 50 Hz (that is, a direction change occurs 50 times per second). The two wires that enter the dwelling are called phase and zero, since there are no poles here.

Direct current (letter designation DC) is the current that is obtained by a chemical method (for example, batteries, accumulators). It is polarized and flows in a certain direction.

Basic physical quantities:

Potential difference (designation U). Since generators act on electrons like a water pump, there is a difference in its terminals, which is called the potential difference. It is expressed in volts (designation B). If you and I measure the potential difference at the input and output connections of an electrical appliance with a voltmeter, we will see readings of 230-240 V on it. Usually this value is called voltage.
Current strength (designation I). For example, when a lamp is connected to a generator, an electrical circuit is created that passes through the lamp. A stream of electrons flows through the wires and through the lamp. The strength of this current is expressed in amperes (designation A).
Resistance (designation R). Resistance is usually understood as a material that allows electrical energy to be converted into heat. Resistance is expressed in ohms (notation Ohm). Here you can add the following: if the resistance increases, then the current decreases, since the voltage remains constant, and vice versa, if the resistance decreases, then the current increases.
Power (designation P). Expressed in watts (notation W) - it determines the amount of energy consumed by the device that is currently connected to your outlet.

Types of consumer connections

Conductors, when included in a circuit, can be connected to each other in various ways:

Consistently.
Parallel.
mixed way

A connection is called serial, in which the end of the previous conductor is connected to the beginning of the next.

A connection is called parallel, in which all the beginnings of the conductors are connected at one point, and the ends at another.

A mixed conductor connection is a combination of series and parallel connections. Everything we have said in this article is based on the basic law of electrical engineering - Ohm's law, which states that the current strength in a conductor is directly proportional to the applied voltage at its ends and inversely proportional to the resistance of the conductor.

In the form of a formula, this law is expressed as follows:

Ohm's law formula.

Despite the outward strangeness, the question is far from being idle, although we are more accustomed to the fact that in typical sockets of our houses alternating current. That is why the question of what current in the outlet is direct or alternating, without hesitation, we will answer - of course, variable! Well, we decided to figure out whether this is so at the same time in the standards of sockets, the designations of direct and alternating current, and some related issues.

The main types and characteristics of sockets

In fact, the main characteristics are not what kind of direct or alternating current is in the outlet, the main thing is the level of protection and the contact group, that is, the shape of the plug (plug), as well as the permissible currents. Let's list what we should consider when choosing an outlet:

Place of installation (flush-mounted, outdoor, indoor, outdoor, etc.).
The actual shape of the socket and plug, as well as child protection.
Network parameters and load on the line where the outlet will work.

If you place the flush-mounted socket in a dry room, but not high from the floor, remember that this is a risk of water ingress (when washing floors, etc.). Therefore, such outlets must have an increased level of protection.

All these properties are described by marking, and understanding how to read it will never be superfluous. But before that, for reference, we give symbol sockets and switches on drawings and schematic diagrams -

Let's decipher what is written on such devices using the example of such an abbreviation.

According to the degree of protection, sockets are distinguished by an IP code.. The IP is followed by two digits. The first (from 0 to 6) is the protection of the device from intrusion. Dust, fingers, objects, etc. The second (from 0 to 8) protection against water. That is, a socket marked IP68 is protected from all influences, and IP00 is actually a bare non-insulated contact. Type, sockets are marked in Latin letters. Appearance can be seen in this image -

In Russia, types C are used, without grounding and F with grounding. Some types of appliances are equipped with a different type of plug and can be used in our networks using an adapter. Pay special attention to the diameter of the plug in the plug. Soviet plugs will not fit into the Euro socket, because the pins on the plug are thicker. As a rule, the diameter marking has not been applied to sockets for a long time, it's just worth remembering that it is 4 mm, and the Soviet plug has a diameter of 4.8 mm.

Designation of direct and alternating current. Many have heard about the AC / DC group, and this is exactly the same thing - direct alternating current. Beautiful name. The designation of direct current is less common and it is worth understanding what the symbols mean:

(—) or DC(Direct Current translated as direct current). This means that you should not try to connect a conventional device that requires AC power to such an outlet. On the diagrams, I designate the direction arrow and the symbols "+" and "-" as polarity. The simplest example is a conventional battery.

Alternating current will be designated as follows: (~) or AC (Alternating Current, that is, alternating current). If you think about it, the designation of direct and alternating current in the name contains important information - a current of a constant direction, and a current whose direction changes. This picture illustrates it well.

In addition to this information, you can find markings in hertz on the outlet - the permissible current frequency. This is just a value that tells how many times per second the "direction" of the current changes. The standard is 50 Hz.

And now we have come to the most important characteristic, which we will talk about separately, since this is a more important issue than what kind of current in the outlet is direct or alternating.

Power characteristics and applicability of sockets for domestic purposes

So, on the outlet it will be written, for example: C (CEE 7/16) (Euro socket without grounding) or F (CEE 7/4) (Euro socket with grounding) IP44 (for the bathroom it's the best), AC (~) 220V 50Hz. For example - "IP44 AC 230V CEE7/4 50 Hz". Or "IP44 ~ 230V CEE7/4 50Hz".

On the same outlet there will be two more designations, three more to be exact. One of them is the image on the circuit diagram, which we posted above. This pictogram may be absent, it is not required to indicate, what current in the outlet, direct or alternating, and in general what this outlet is for, but many manufacturers (honor and praise for this) help ordinary buyers make a decision.

Even on the outlet, the marking "permanent connection" can be applied. Or "removable outlet with extension cord" or "removable". Do not make round eyes - we ourselves were in shock. Let us explain in order - a one-piece connection is protection from children. Special ways to plug the plug into the socket so that the one who knows the secret will take it out, but the children will not be able to. A removable socket, as a rule, is floor-mounted (photo at the beginning of the article), which can be closed if necessary, and if necessary, removed from the socket. An element of the “plinth” type will take its place, and until the next time no one will guess that a socket can be installed there.

Plug-in socket- a new fashionable thing. You plug the appliance in, turn the outlet socket and pull it out, a kind of extension cord hidden in the wall. One-piece sockets are equipped with locks from the swivel socket to the structural elements of the plug. We do not provide pictograms, because so far, in fact, there is no standard for such exotics.

But on any outlet there will definitely be a designation - 10A. Or 6A, or 16A, or 32A. This is the current allowed for the end device in this section of your power grid. The designation of direct and alternating current in this case does not matter, it is more important to understand the total total power of the devices that can be included in this outlet. A professional may object to us that there are no questions here, but we still repeat - it doesn’t matter what current is AC or DC in the outlet, allowable current is one of the most important characteristics .

What should be the total power of the outlet

You can estimate the total load in the line where the socket will work without knowing higher mathematics - add up the power of all devices that, even hypothetically, can be turned on at the same time. Let's say it's 4 kilowatts per line. Do not be surprised, the iron and kettle in the kitchen, turned on at the same time as the microwave, are the everyday realities of our apartments.

In your kitchen there may be two sockets twice, but they can “hang” on one machine, which means this is one line. New buildings are especially guilty of this, in which the project of the apartment network is made by no one knows who.

So, we take the total power and divide it by the DC notation. A joke, of course, but there is some truth in it. Divide by the voltage to get the current. We talked about this in more detail in our article, we recommend reading the details there. But we are talking about sockets, so let us recall that the current strength even with normal consumers (kettle, microwave, iron, etc.) can vary significantly when the device is turned on. The most difficult for sockets are Microwave ovens and ovens high power, dishwashers and washing machines . Not only is it very desirable to draw a separate line to such devices, but the sockets must be marked at least 16A, of course, with the designation of direct or alternating current and other details, and of course from a reliable manufacturer. will take a separate place electric stove. This will require not only a separate line, on which there will be no other consumers, but also an outlet marked at least 25A, and preferably 32A. For those who move into an apartment with an electric stove, this is not a problem, GOST 30988.2.4-2003 not only describes in detail all sockets for household and not only purposes, but also provides for liability for dishonest installation just for currents over 16A. By the way, about this figure - 16A, it is worth remembering all home-grown electricians. And for currents over 32A, sockets are not really collapsible.

A few words about new sockets with additional features

Having considered the details of the use of sockets, we came to the conclusion that if we see the marking on our socket "IP44 ~ 230V CEE7/4 50Hz 16A". We know that this socket is protected from foreign objects, can withstand short-term watering, the European standard with grounding, is designed for a network not higher than 230 volts with a frequency of 50 hertz and is rated for current up to 16 amperes. The icon (if available) will help you find it on the wiring diagram and understand additional features.

As they say on the Internet - now you know everything. Well, except that we didn’t talk about sockets with a USB power function, built-in shutdown timers, current switches (the designation of direct and alternating current is most relevant for them). There are also sockets with line load indication (an indicator that changes color from green if everything is fine to red when everything is gone). The natural evolution of such sockets has become sockets with built-in RCDs. Complemented this line of sockets with automatic blocking. This is when the outlet is turned off with incorrect current parameters without turning off the circuit breakers. As well as sockets controlled via the Internet. But this exotic is a separate story, we will someday return to it.

Posted on 09/13/2016 08:48 - a miniature device designed to measure various electrical parameters, as well as to test semiconductor devices and electronic components. Roughly speaking, a multimeter is the same measuring instrument as a ruler or, for example, scales, only it measures not centimeters and grams, but Ohms, Volts and Amps. By the way, the fact that it can measure several quantities is evidenced by the prefix "multi".

The appearance of the device is shown in the photo. As you can see, it has a large switch on its front panel. With its help, the parameter is selected, as well as the measurement limit. In addition, the multimeter has a liquid crystal display, which displays the measurement result. About, how to use a multimeter will be discussed in this article.

In fairness, it should be noted that the indication in the multimeter is not necessarily liquid crystal. The market still sells many outdated models with an arrow scale. And although these devices are not as accurate as digital ones, and they are not as convenient to use, many radio amateurs prefer them. And yet, in this article we will focus on devices with liquid crystal display.

All multimeters, without exception, allow you to measure voltage, current and resistance. These values will be discussed in more detail below. In addition, most devices are equipped with a circuit probe, some multimeters have the ability to measure temperature. The circuit probe allows you to quickly establish the integrity of the conductor. In the event that the circuit resistance is less than 30 ohms, a beep will sound. This is very convenient - there is no need to look at the indication, and the resistance value, when checking an elementary circuit, is not so important.

Another useful function of multimeters is to test semiconductor diodes. Anyone who has worked with them knows that a diode passes current in one direction. If there is conductivity in another, then the device is faulty. The multimeter analyzes these parameters and displays the result on the screen. In addition, in the case when there is no marking on the body of the diode, using a tester, you can easily determine its polarity. Unfortunately, not all multimeters have this function.

More expensive and advanced models of devices have the ability to measure such quantities as the inductance of coils and the capacitance of capacitors. But since only special multimeters can do this, they will not be considered in this article.

In this section, a small educational program for those who were not previously familiar with these quantities. It is immediately worth noting that special values \u200b\u200bare invented for their measurement. If we draw an analogy with the distance, then it will be measured in meters and denoted by the English letter “m”. Exactly the same abbreviations were invented for electrical quantities.

Voltage is the force that causes current to flow through a conductor. The higher the voltage, the faster the electrons move. Voltage is usually measured in volts, abbreviated to a capital letter "V". But since it is impossible to find a multimeter with a Russified front panel on the market, you need to look for the English “V” on it.

The intensity of current flow through an electrical circuit is determined by its strength. Here it is appropriate to use the plumbing analogy to imagine an electrical circuit in the form of a pipe filled with water. High pressure in this pipe is not yet a reason for water to flow through it. Maybe at the other end of the pipe the valve is simply closed. And as it opens, the flow rate will increase. Here is the speed electrical circuit, and will be the current strength. It is measured in amperes "A".

Resistance shows how difficult it is for current to pass through a particular section of an electrical circuit. Returning to the plumbing allegory, resistance can be compared to some kind of narrow pipe section, such as a blockage. The smaller the diameter of the pipe in this place (read more resistance), the lower the speed of the water flow (current strength). This is very well illustrated in a hilarious picture. The unit of measurement is the ohm, which is denoted by the Greek letter omega (?).

direct current For those who know English, it will not be difficult to translate. Literal translation, directed current. It is an electric current that flows in one direction. In Russian, he received the name permanent. Most small household appliances run on direct current. It is issued by batteries of all classes and sizes, car and telephone batteries. Direct current is given the abbreviation DC.

Depending on the manufacturer on the multimeter, the corresponding positions may be indicated either DCA and DCV(measurement of direct current and voltage, respectively), or “A” and “V”, and next to the line and below it is a dotted line.

Alternating current ( alternating current) changes its direction dozens of times per second. For example, in home outlets, the frequency is 50 hertz. This means that the direction of the current changes 50 times per second. But you should not try to measure high voltage socket. It is very dangerous.

Alternating current has received the abbreviation "AC". There are 2 options on the multimeter switches:
“ACA" and " ACV” measurement of AC current and voltage; A ~ and V ~.

Measuring direct voltage has its own nuances - be sure to observe the polarity. This is especially true for pointer devices. In this case, their measuring head may fail. Digital - endure it painlessly, just a minus sign appears on the screen. This must be taken into account before using the multimeter in voltage measurement mode.

When working with a multimeter, it is very important to know how to connect it when measuring. There are only two options: in series or in parallel, depending on what value needs to be measured. At serial connection The same current flows through all elements of the circuit. Therefore, in series, they also say “into a circuit break”, you need to measure the current strength. If we consider parallel connection, then here the same voltage is applied to each element, and becoming probes parallel to any of them, you can measure it. So, voltage is measured in parallel, current is measured in series, this must be remembered and never confused.

The figure shows the circuits of parallel and serial connection. It should be noted that in series, the current flowing through each of the elements will be the same if their resistances are equal. The same condition will ensure equal voltage across the elements, in the case of a parallel connection.

Not an experienced user, tricky symbols printed on the main switch of the multimeter. But there is nothing complicated here, just remember how the units of measurement of voltage, current and resistance are indicated:

Volt - "V";
Ampere - "A";
OM - "Ω"

All manufacturers, without exception, use only these icons. True, there is one but. It is not always necessary to measure integer values. Sometimes the result is thousandths of a unit of measurement, and sometimes, on the contrary, millions. Therefore, the corresponding measurement limits are included in the multimeter and manufacturers use metric prefixes to designate them. There are only four main ones:

µ (micro) - 10-6 units;
m (miles) - 10-3 units;
k (kilo) - 103 units;
M (mega) - 106 units.

These prefixes are added to the main units of measurement and in this form are applied to the switch of the device operation modes: µA (microampere), mV (millivolt), kOhm (kiloohm), mOhm (megaohm).

Before measuring any value, you need to set the appropriate limit. To do this, you need to at least approximately know what the result will be, and set a number slightly higher than it on the device. If even in the first approximation it is impossible to predict the value of the measured current or voltage, it is better to start from the maximum limit. The result will be very approximate, but will allow you to draw a conclusion about how to set the limit. Measurements can now be taken with greater accuracy.

Some multimeters are equipped with an “auto-rangin” feature. Thanks to it, the measurement limit is set automatically. This is very convenient, since using a multimeter, in this case, is much easier. The figure shows a simple multimeter (left) and a device equipped with an auto-ranging” function (right).

Instrument manufacturers rarely, if ever, adhere to standards, so the same function may be labeled differently on different multimeters. Of course, it is impossible to list everything here. possible options characters, but the main ones are listed below.

So, a wavy line represents alternating current. And note that both current and voltage can be measured. It can be alternating current (current strength), or it can be alternating current voltage.

The horizontal line, with a dotted line below it, denotes direct current and direct voltage.

Designation of current and voltage using the abbreviation “AC” and “DC”. The example shows that sometimes letters are duplicated by signs. It should also be noted that the designations AC, DC can be either before A or V, or after.

This icon indicates the continuity of the chains. If the circuit is intact, the multimeter will beep. Sometimes this function is combined with the resistance measurement mode. In this case, the beep will sound if the resistance is less than 30 ohms.

Diode test function. Allows you to determine the health of the diode and its polarity.

to measure voltage, you need:

connect the probes to the multimeter.
better right away, get used to doing it right: black to the nest COM and red to the nest V;
set the switch to the position corresponding to the measurement mode (variable or constant) and the limit;
now you can become probes in parallel with the circuit element on which it is supposed to measure the voltage.

The figure shows an example of measuring the voltage drop on a nine-volt battery "krone";

Now the screen of the device should show voltage. In the event that the display shows "1", the measurement limit is small, you need to set it smaller. But in this example, the switch is in the correct position, set to the limit of 20 volts DC. The red wire is positive, it is connected to the plus of the battery, and the black one, respectively, is a minus, inserted into the connector COM on a multimeter. It connects to the battery negative.

We connect the probes, do not forget about the color; Here you need to pay attention to the following: when measuring low currents, the red cord is connected to the same socket as when measuring voltage, and currents up to 10 amperes - to the “10A” connector.
Now you need to select the measurement mode and its limit.

Unlike voltage, current is measured sequentially. To do this, you have to break (that's why they say "break") the chain. If everything is done correctly, the display will show the current value. In the event that zeros are displayed on the screen, there may be several reasons: the voltage is not turned on, there is no contact on the probes, and, most likely, the limit is large. If the unit is displayed on the screen, the limit is small. The figure shows a circuit for measuring direct current flowing through a light bulb.

Connect the probe to the “COM” and “?” connectors. Of course, it is not necessary to observe the polarity here, and yet it is better to connect black to the COM connector. Set the limit and measurement mode.

We measure the resistance of the resistor or the spiral of the light bulb, as shown in the figure. It must be borne in mind that the measured element must be excluded from the circuit. Otherwise, the measurements will not be correct. If the indicator in front of the figure shows several zeros, the measurement limit was taken, for greater accuracy it must be reduced. If the limit is small, the indicator will still show the same unit.

Set the device to beep mode. There is a corresponding icon on the switches. It is also shown as an example in the table above.

Install the probes in the sockets by analogy with measuring resistance. Measure the desired circuit element. If an electric current flows between the probes, i.e. it is working, a sound signal with a frequency of about 1 kHz should be heard. in this case, it is necessary to disconnect the power supply from the circuit. By the way, if there is no sound signal, then it is not at all necessary that it is faulty. Perhaps its normal resistance exceeds 30 ohms.

A multimeter tests a diode by passing current through it and measuring the voltage drop across it. With some skill, the device can even check bipolar transistors. Sometimes semiconductor devices do not even need to be soldered from the circuit. So, the sequence of actions is as follows.

The probes are connected in the same way as measuring resistance. The device switch is set to measure the diode. Most often, this icon is a schematic designation of a diode. We measure the diode by touching its anode and cathode with probes. The readings of the device should be: for a silicon diode -500-700 mV, for a germanium diode - 200-300mV, a working LED should show 1.5-2 V.

Now we change the polarity on the diode. The device should show zeros, otherwise it is faulty. That, in general, is all that can be briefly told about working with a multimeter. Everything else will come with experience. The main thing is not to forget about safety and before using a multimeter, be sure to study the safety rules.