We all know very well what a semiconductor diode is, but few of us know about the principle of operation of a diode. Today, especially for beginners, I will explain the principle of its operation. As is known, a diode passes current well on one side, but very poorly on the opposite direction. The diode has two terminals - anode and cathode. Not a single electronic device can do without the use of diodes. The diode is used to rectify alternating current, with the help of a diode bridge which consists of four diodes, you can turn alternating current into direct current, or using six diodes you can turn three-phase voltage into single-phase, diodes are used in a variety of power supplies, in audio-video devices, almost everywhere . Here you can see photos of some.

At the output of the diode, you can notice a drop in the initial voltage level by 0.5-0.7 volts. For lower voltage power supply devices, a Schottky diode is used; on such a diode the smallest voltage drop is observed - about 0.1V. Schottky diodes are mainly used in radio transmitting and receiving devices and in other devices operating mainly at high frequencies. The operating principle of a diode is quite simple at first glance: a diode is a semiconductor device with one-way conductivity of electric current.

The diode terminal connected to the positive pole of the power source is called the anode, and the negative terminal is called the cathode. The diode crystal is mainly made of germanium or silicon, one region of which has n-type electrical conductivity, that is, a hole region, which contains an artificially created lack of electrons, the other - n-type conductivity, that is, contains an excess of electrons, the boundary between them is called a n-n junction , n is the first letter of the word positive in Latin, n is the first letter of the word negative. If a positive voltage is applied to the anode of the diode, and a negative voltage is applied to the cathode, then the diode will pass current, this is called direct connection, in this position the diode is open, if reverse is applied, the diode will not pass current, in this position the diode is closed, this is called reverse connection .

The reverse resistance of the diode is very high and in circuits it is considered a dielectric (insulator). To demonstrate the operation of a semiconductor diode, you can assemble a simple circuit that consists of a power source, a load (for example, an incandescent lamp or a low-power electric motor) and the semiconductor diode itself. We connect all the components of the circuit in series, we supply plus from the power source to the anode of the diode, in series to the diode, that is, we connect one end of the light bulb to the cathode of the diode, and connect the other end of the same lamp to the minus of the power source. We observe the glow of the lamp, now we turn the diode over, the lamp will no longer glow because the diode is connected back, the transition is closed. I hope this will help you in some way in the future, newbies - A. Kasyan (AKA).

Semiconductor devices were used in radio engineering even before the invention of vacuum tubes. The inventor of radio, A. S. Popov, first used a coherer (a glass tube with metal filings) and then contact of a steel needle with a carbon electrode to detect electromagnetic waves.

This was the first semiconductor diode— detector. Later, detectors were created using natural and artificial crystalline semiconductors (galena, zincite, chalcopyrite, etc.).

Such a detector consisted of a semiconductor crystal soldered into a holder cup and a steel or tungsten spring with a pointed end (Fig. 1). The position of the tip on the crystal was found experimentally, achieving the highest volume of the radio station transmission.

Rice. 1. Semiconductor diode - detector.

In 1922, O. V. Losev, an employee of the Nizhny Novgorod Radio Laboratory, discovered a remarkable phenomenon: a crystal detector, it turns out, can generate and amplify electrical oscillations.

This was a real sensation, but the lack of scientific knowledge and the lack of the necessary experimental equipment did not allow at that time to deeply explore the essence of the processes occurring in a semiconductor and to create semiconductor devices capable of competing with an electron tube.

Semiconductor diode

Semiconductor diodes denoted by a symbol that has been preserved in general terms since the days of the first radios (Fig. 2.6).

Rice. 2. Designation and structure of a semiconductor diode.

The top of the triangle in this symbol indicates the direction of greatest conductivity (the triangle symbolizes the anode of the diode, and the short line perpendicular to the lead lines is its cathode).

The same symbol denotes semiconductor rectifiers, consisting, for example, of several diodes connected in series, parallel or mixed (rectifier columns, etc.).

Diode bridges

Bridge rectifiers are often used to power radio equipment. The outline of the same diode connection diagram (a square, the sides of which are formed by diode symbols) has long become generally accepted, therefore, to designate such rectifiers, a simplified symbol began to be used - a square with the symbol of one diode inside (Fig. 3).

Rice. 3. Designation of the diode bridge.

Depending on the value of the rectified voltage, each arm of the bridge can consist of one, two or more diodes. The polarity of the rectified voltage is not indicated on the diagrams since it is clearly determined by the diode symbol inside the square.

Bridges are structurally combined in one housing and are depicted separately, showing that they belong to one product in a positional designation. Next to the positional designation of diodes, like all other semiconductor devices, their type is usually indicated.

Based on the diode symbol, symbols for semiconductor diodes with special properties are built. To obtain the desired symbol, special characters are used, either on the basic symbol itself or in the immediate vicinity of it, and in order to focus attention on some of them, the basic symbol is placed in a circle - a symbol for the body of a semiconductor device.

Tunnel diodes

A sign resembling a straight bracket denotes the cathode of tunnel diodes (Fig. 4a). They are made from semiconductor materials with a very high impurity content, as a result of which the semiconductor turns into a semimetal. Due to the unusual shape of the current-voltage characteristic (it has a section of negative resistance), tunnel diodes are used to amplify and generate electrical signals and in switching devices. An important advantage of these diodes is that they can operate at very high frequencies.

Rice. 4. Tunnel diode and its designation.

A type of tunnel diodes are reverse diodes, in which, at a low voltage at the pn junction, the conductivity in the reverse direction is greater than in the forward direction.

Such diodes are used in reverse connection. In the symbol for a reversed diode, the cathode dash is depicted with two dashes touching it with their middle (Fig. 4.6).

Zener diodes

Semiconductor zener diodes, which also operate on the reverse branch of the current-voltage characteristic, have won a strong place in power supplies, especially low-voltage ones.

These are planar silicon diodes made using special technology. When they are turned on in the opposite direction and at a certain voltage, the junction “breaks through”, and subsequently, despite the increase in current through the junction, the voltage across it remains almost unchanged.

Rice. 5. Zener diode and its designation on the diagrams.

Thanks to this property, zener diodes are widely used as independent stabilizing elements, as well as sources of reference voltages in transistor stabilizers.

To obtain small reference voltages, the zener diodes are switched on in the forward direction, with the stabilization voltage of one zener diode equal to 0.7... 0.8 V. The same results are obtained when conventional silicon diodes are switched on in the forward direction.

To stabilize low voltages, special semiconductor diodes - stabistors - have been developed and are widely used. Their difference from zener diodes is that they operate on the direct branch of the current-voltage characteristic, i.e. when switched on in the forward (conducting) direction.

To show a zener diode in the diagram, the cathode dash of the basic symbol is supplemented with a short dash directed towards the anode symbol (Fig. 5a). It should be noted that the location of the stroke relative to the anode symbol should be unchanged regardless of the position of the zener diode symbol on the diagram.

This fully applies to the symbol of a two-anode (double-sided) zener diode (Fig. 5.6), which can be connected to an electrical circuit in any direction (in fact, these are two identical zener diodes connected back to back).

Varicaps

An electron-hole junction to which a reverse voltage is applied has the properties of a capacitor. In this case, the role of the dielectric is played by the pn junction itself, in which there are few free charge carriers, and the role of the plates is played by the adjacent layers of the semiconductor with electric charges of different signs - electrons and holes. By changing the voltage applied to the pn junction, you can change its thickness, and therefore the capacitance between the layers of the semiconductor.

Rice. 6. Varicaps and their designation on circuit diagrams.

This phenomenon is used in special semiconductor devices - varicapah[from English words vari(able) - variable and cap(acitor) - capacitor]. Varicaps are widely used for tuning oscillatory circuits, in automatic frequency control devices, and also as frequency modulators in various generators.

The conventional graphic designation of a varicap (see Fig. 6, a) clearly reflects their essence: parallel lines at the bottom are perceived as a symbol of a capacitor. Kick and variable capacitors, varicaps are often made in the form of blocks (they are called matrices) with a common cathode and separate anodes. For example in Fig. 6.6 shows the designation of a matrix of two varicaps, and Fig. 6,c - out of three.

Thyristors

Based on the basic symbol of the diode, conditional thyristor designations(from Greek Thyra– door and English (resi) stor- resistor). These are diodes, which are alternating layers of silicon with electrical conductivity types p and n. There are four such layers in the thyristor, i.e. it has three pn junctions (pnpp structure).

Thyristors have found wide application in various alternating voltage regulators, relaxation generators, switching devices, etc.

Rice. 7. Thyristor and its designation on circuit diagrams.

Thyristors with leads only from the outer layers of the structure are called dynistorimn and are designated by a diode symbol crossed out by a line segment parallel to the cathode line (Figure 7, a). The same technique was used in constructing the designation of a symmetrical dinistor (Fig. 7, b), conducting current (after switching on) in both directions.

Thyristors with an additional (third) output (from one of the internal layers of the structure) are called thyristors. Control along the cathode in the designation of these devices is shown by a broken line attached to the cathode symbol (Fig. 7, c), along the anode - by a line extending one of the sides of the triangle symbolizing the anode (Fig. 7, d).

The symbol for a symmetrical (bidirectional) triistor is obtained from the symbol for a symmetrical dinistor by adding a third terminal (Fig. 7, (5).

Photodiodes

Main part photodiode is a junction operating under reverse bias. Its body has a window through which the semiconductor crystal is illuminated. In the absence of light, the current through the pn junction is very small - it does not exceed the reverse current of a conventional diode.

Rice. 8. Photodiodes and their representation on diagrams.

When the crystal is illuminated, the reverse resistance of the junction drops sharply, and the current through it increases. To show such a semiconductor diode in a diagram, the basic symbol of the diode is placed in a circle, and next to it (top left, regardless of the position of the symbol) the sign of the photoelectric effect is depicted - two oblique parallel arrows directed towards the symbol (Fig. 8a).

In a similar way, it is not difficult to construct a symbol for any other semiconductor device that changes its properties under the influence of optical radiation. As an example in Fig. 8.6 shows the designation of the photodinistor.

LEDs and LED indicators

Semiconductor diodes that emit light when current passes through a pn junction are called LEDs. Such diodes are turned on in the forward direction. The conventional graphic symbol of an LED is similar to the photodiode symbol and differs from it in that the arrows indicating optical radiation are placed to the right of the circle and directed in the opposite direction (Fig. 9).

Rice. 9. LEDs and their representation on diagrams.

To display numbers, letters and other characters in low-voltage equipment, LED character indicators are often used, which are sets of light-emitting crystals arranged in a certain way and filled with transparent plastic.

ESKD standards do not provide symbols for such products, but in practice they often use symbols similar to those shown in Fig. 10 (seven-segment indicator symbol for displaying numbers and a comma).

Rice. 10. Designation of LED segment indicators.

As you can see, such a graphic designation clearly reflects the actual location of the light-emitting elements (segments) in the indicator, although it is not without a drawback: it does not carry information about the polarity of the inclusion of the indicator terminals in the electrical circuit (indicators are produced both with an anode terminal common to all segments and and with a common cathode terminal).

However, this usually does not cause any particular difficulties, since the connection of the common output of the indicator (as well as the microcircuits) is specified in the diagram.

Optocouplers

Light-emitting crystals are widely used in optocouplers - special devices used to connect individual parts of electronic devices in cases where their galvanic isolation is necessary. In the diagrams, optocouplers are depicted as shown in Fig. eleven.

The optical connection of the light emitter (LED) with the photodetector is shown by two parallel arrows perpendicular to the lead lines of the optocoupler. The photodetector in an optocoupler can be not only a photodiode (Fig. 11,a), but also a photoresistor (Fig. 11,6), photodinistor (Fig. 11,c), etc. The mutual orientation of the symbols of the emitter and photodetector is not regulated.

Rice. 11. Designation of optocouplers (optocouplers).

If necessary, the components of the optocoupler can be depicted separately, but in this case, the optical connection sign should be replaced with the signs of optical radiation and photoelectric effect, and the belonging of the parts to the optocoupler should be shown in the positional designation (Fig. 11, d).

Literature: V.V. Frolov, Language of radio circuits, Moscow, 1998.

D iodine- the simplest in design in the glorious family of semiconductor devices. If you take a semiconductor plate, for example germanium, and introduce an acceptor impurity into its left half and a donor impurity into the right half, then on one side you will get a semiconductor of type P, respectively, on the other, type N. In the middle of the crystal you will get the so-called P-N junction, as shown in Figure 1.

The same figure shows the conventional graphic designation of a diode in the diagrams: the cathode terminal (negative electrode) is very similar to the “-” sign. It's easier to remember that way.

In total, in such a crystal there are two zones with different conductivities, from which two outputs come out, so the resulting device is called diode, since the prefix “di” means two.

In this case, the diode turned out to be a semiconductor, but similar devices were known before: for example, in the era of electronic tubes there was a tube diode called a kenotron. Now such diodes are a thing of history, although adherents of “tube” sound believe that in a tube amplifier even the anode voltage rectifier should be tube-based!

Figure 1. Diode structure and diode designation on the diagram

At the junction of semiconductors with P and N conductivity, it turns out P-N junction, which is the basis of all semiconductor devices. But unlike a diode, which has only one transition, they have two P-N junctions, and, for example, they consist of four junctions at once.

P-N junction at rest

Even if the P-N junction, in this case the diode, is not connected anywhere, interesting physical processes still occur inside it, which are shown in Figure 2.

Figure 2. Diode at rest

In the N region there is an excess of electrons, it carries a negative charge, and in the P region the charge is positive. Together these charges form an electric field. Since unlike charges tend to attract each other, electrons from the N zone penetrate into the positively charged P zone, filling some holes. As a result of such movement, a current, albeit very small (several nanoamperes), appears inside the semiconductor.

As a result of this movement, the density of the substance on the P side increases, but up to a certain limit. Particles usually tend to spread evenly throughout the entire volume of a substance, just as the smell of perfume spreads throughout the entire room (diffusion), so sooner or later the electrons return back to the N zone.

If for most consumers of electricity the direction of the current does not matter - the light bulb lights up, the tile heats up, then for a diode the direction of the current plays a huge role. The main function of a diode is to conduct current in one direction. It is this property that is provided by the P-N junction.

Turning the diode in reverse

If a power source is connected to a semiconductor diode, as shown in Figure 3, then no current will pass through the P-N junction.

Figure 3. Diode reverse connection

As can be seen in the figure, the positive pole of the power source is connected to area N, and the negative pole is connected to area P. As a result, electrons from region N rush to the positive pole of the source. In turn, positive charges (holes) in the P region are attracted by the negative pole of the power source. Therefore, in the region of the P-N junction, as can be seen in the figure, a void is formed, there is simply nothing to conduct current, there are no charge carriers.

As the voltage of the power source increases, electrons and holes are increasingly attracted by the electric field of the battery, while in the region of the P-N junction there are fewer and fewer charge carriers. Therefore, in reverse switching, no current flows through the diode. In such cases it is customary to say that The semiconductor diode is reverse-voltage locked.

An increase in the density of matter near the battery poles leads to occurrence of diffusion, - the desire for a uniform distribution of matter throughout the entire volume. This is what happens when the battery is disconnected.

Semiconductor Diode Reverse Current

This is where the time has come to remember the non-mainstream media that have been conventionally forgotten. The fact is that even in the closed state, a small current passes through the diode, called reverse. This reverse current and is created by minor carriers, which can move in exactly the same way as the main ones, only in the opposite direction. Naturally, such movement occurs under reverse voltage. The reverse current is usually small, which is due to the small number of minority carriers.

As the temperature of the crystal increases, the number of minority carriers increases, which leads to an increase in the reverse current, which can lead to the destruction of the P-N junction. Therefore, operating temperatures for semiconductor devices - diodes, transistors, microcircuits are limited. To prevent overheating, powerful diodes and transistors are installed on heat sinks - radiators.

Turning on the diode in the forward direction

Shown in Figure 4.

Figure 4. Direct connection of the diode

Now let's change the polarity of the source: connect the minus to area N (cathode), and the plus to area P (anode). With this inclusion in the N region, electrons will be repelled from the negative of the battery and move towards the P-N junction. In region P, positively charged holes will be repelled from the positive terminal of the battery. Electrons and holes rush towards each other.

Charged particles with different polarities gather near the P-N junction, and an electric field arises between them. Therefore, the electrons overcome the P-N junction and continue to move through the P zone. In this case, some of them recombine with holes, but most of them rush to the plus of the battery; current Id flows through the diode.

This current is called direct current. It is limited by the technical data of the diode, a certain maximum value. If this value is exceeded, there is a risk of diode failure. It should be noted, however, that the direction of the forward current in the figure coincides with the generally accepted direction, opposite to the movement of electrons.

It can also be said that with the forward direction of switching on, the electrical resistance of the diode is relatively small. When turned on in reverse, this resistance will be many times greater; no current flows through the semiconductor diode (insignificant reverse current is not taken into account here). From all of the above, we can conclude that the diode behaves like an ordinary mechanical valve: turned in one direction - water flows, turned in the other - the flow stops. For this property the diode received the name semiconductor gate.

To understand in detail all the abilities and properties of a semiconductor diode, you should get acquainted with its volt-ampere characteristic. It is also a good idea to learn about the different diode designs and frequency properties, advantages and disadvantages. This will be discussed in the next article.

• Tutorial

Don't get in. Will kill! (With)

I will try to explain the work with diodes, LEDs, as well as zener diodes on the fingers. Experienced electronics engineers may skip the article because they will not discover anything new for themselves. I will not go into the theory of electron-hole conductivity of the pn junction. I believe this teaching approach will only confuse beginners. This is a bare theory that has almost no relation to practice. However, I offer this to those interested in the theory. Everyone is welcome under cat.

This is the second article in the electronics series. I also recommend reading, which tells you what electric current and voltage are.

A diode is a semiconductor device that has 2 terminals for connection. It is made, simply put, by connecting 2 semiconductors with different types of impurities, they are called donor and acceptor, n and p, respectively, so the diode contains a pn junction inside. The terminals, usually made of tinned copper, are called the anode (A) and cathode (K). These terms go back to the times of vacuum tubes and are used in writing to indicate the directionality of the diode. Graphic notation is much simpler. The names of the diode terminals will be remembered by themselves when used in practice.

As I already wrote, we will not use the theory of electron-hole conductivity of the diode. Let's just encapsulate this theory down to a black box with two terminals for connection. In much the same way, programmers encapsulate working with third-party libraries without going into the fucking details of their work. Or, for example, when, when using a vacuum cleaner, we do not go into detail about how it works inside, it just works and one of the properties of the vacuum cleaner is important to us - sucking dust.

Let's look at the properties of the diode, the most obvious:

• From anode to cathode, this direction is called direct, the diode passes current.
• From the cathode to the anode, in the opposite direction, the diode does not pass current. (Actually no. But more on that later.)
• When current flows in the forward direction, some voltage drops across the diode.

Perhaps these properties are already well known to you. But there are some additions. What is considered forward and what is backward? Direct connection is called when the voltage at the anode is greater than at the cathode. The opposite is the opposite. Direct and reverse inclusion is a convention. In real circuits, the voltage on the same diode can change from direct to reverse and vice versa.

A silicon diode begins to pass any significant current only when the voltage at the anode is approximately 0.65 V higher than at the cathode. No not like this. When any current flows, a voltage drop is formed across the diode, approximately equal to 0.65 V or higher.

A voltage of 0.65 V is called the forward voltage drop across the pn junction. This is only an approximate average value, it depends on the current, crystal temperature and diode manufacturing technology. When the flowing current changes, it changes nonlinearly. To somehow indicate this nonlinearity graphically, manufacturers take the current-voltage characteristics of the diode. In powerful high-voltage diodes, the voltage drop can be 2, 3, etc. greater. times. This means that several pn junctions are connected in series inside the diode.

To determine the voltage drop, you can use the current-voltage characteristic (CV) of the diode in the form of a graph. Sometimes these graphs are given in datasheets for real diode models, but more often they are not. The first graph I came across below shows the current-voltage characteristics of the KD243A, although this is not important, they are all approximately similar.

On the graph, Upr is the forward voltage drop across the diode. Ipr – current flowing through the diode. The graph shows what the voltage drop across the diode will be when the nth current flows. But most often, datalists do not show real current-voltage characteristics, but rather the forward voltage drop indicated at a certain current. In English literature, voltage drop is referred to as forward voltage.

How to use

The voltage drop across a diode is a bad characteristic for us, since this voltage does no useful work and is dissipated as heat on the diode body. The smaller the drop, the better. Typically, the voltage drop across a diode is determined based on the current flowing through the diode. For example, let's connect a diode in series with the load. Essentially, this will protect the circuit from over-reversing, in case the power supply is detachable. In the figure below, a 47 Ohm resistor is taken as the protected circuit, although in reality it can be anything, for example, a section of a large circuit. The power supply is a 12 V battery.

Let's say the load without a diode consumes 255 mA. In this case, this can be calculated using Ohm's law: I= U / R = 12 / 47 = 0.255 A or 255 mA. Although usually the consumption of a spherical circuit in a vacuum is already known, at least based on the maximum characteristics of the power supply. Let's find on the I-V curve indicated above the voltage drop for the KD243A diode at 0.255 A of flowing current, at 25 degrees. It is equal to approximately 0.75 V. These 0.75 V will drop across the diode, and 12 - 0.75 = 11.25 V will remain to power the circuit - sometimes it may not be enough. As a bonus, you can find the power released in the form of heat and losses on the diode using the formula P = I * U = 0.75 * 0.255 = 0.19 W, where I and U are the current through the diode and the voltage drop across the diode.

What to do when the current-voltage curve is not available? For example, for the popular 1n4007 diode, only the forward voltage is specified as 1 V at a current of 1 A. You need to use this value, or measure the actual drop. And if this value is not specified for any diode, then an average of 0.65 V will do. In reality, it is easier to measure this voltage drop with a voltmeter in the circuit than to look for it in graphs. I think there is no need to explain that the voltmeter should be switched on to a constant voltage if a direct current flows through the diode, and the probes should touch the anode and cathode of the diode.

A little about other characteristics

In the previous example, if you turn the battery over, I mean reverse the polarity, see the bottom figure, no current will flow and the voltage drop across the diode in the worst case will be 12 V - the battery voltage. The main thing is that this voltage does not exceed the breakdown voltage of our diode, also known as reverse voltage, also known as breakdown voltage. And one more condition is also important: the current in the forward direction through the diode does not exceed the rated current of the diode, also known as forward current. These are the two main parameters by which a diode is selected: forward current and reverse voltage.

Sometimes data sheets also indicate the diode's power dissipation or rated power (power dissipation). If it is specified, it cannot be exceeded. We have already figured out how to calculate it in the previous example. But if the power is not indicated, then you need to focus on the current.

They say that in the opposite direction no current flows through the diode, or almost no current flows. In fact, a leakage current flows through it, reverse current in English literature. This current is very small, from several nanoamperes for low-power diodes to several hundred microamperes for high-power ones. Also, this current depends on the temperature and applied voltage. In most cases, the leakage current does not play any role, for example, as in the previous example, but when you work with nanoamps and put some kind of protection diode at the input of the op-amp, then oh... The circuit will behave completely differently, as I intended.

Also a few words about the term “nominal”. Typically, current and voltage ratings indicate that if these parameters are exceeded, the manufacturer does not guarantee the operation of the product, unless otherwise stated. And this is for all electronic components, not just the diode.

What else can you do

There are many applications for diodes. Radio electronics designers usually invent their circuits from pieces of other circuits, the so-called building bricks. Here are some options.

For example, a circuit for protecting digital or analog inputs from overvoltage:

The diodes in this circuit do not pass current during normal operation. Only leakage current. But when an overvoltage with a positive half-wave occurs at the input, i.e. the input voltage becomes greater than Upit plus the forward voltage drop across the diode, then the upper diode opens and the input is closed to the power bus. If a negative half-wave voltage occurs, the lower diode opens and the input is shorted to ground. In this circuit, by the way, the less leakage and capacitance the diodes have, the better. Such protection circuits, as a rule, are already installed in all modern digital microcircuits inside the crystal. And external powerful TVS diode assemblies protect, for example, USB ports on motherboards.

You can also assemble a rectifier from diodes. This is a very common type of scheme and it is unlikely that any of the readers have not heard about them. Rectifiers are available in half-wave, full-wave and bridge types. We have already become acquainted with a half-wave rectifier in our very first long-suffering example, when we considered protection against overvoltage. It does not have any special advantages, except for the plus on the battery. One of the most important disadvantages that limits the use of a half-wave rectifier circuit in practice: the circuit only works with a positive half-wave voltage. Negative voltage is completely cut off and no current flows. “So what?”, you say, “Such power will be enough for me!” But no, if such a rectifier is located after the transformer, then the current will flow only in one direction through the windings of the transformer and, thus, the transformer iron will be additionally magnetized. The transformer can go into saturation and heat up much more than it should.

Full-wave rectifiers do not have this drawback, but they require the middle terminal of the transformer winding. Here, when the polarity of the alternating voltage is positive, the upper diode is open, and when the polarity is negative, the lower diode is open. The transformer efficiency is not fully utilized.

Bridge circuits do not have both disadvantages. But now two diodes are switched on in the current path at any given time: a forward diode and a reverse diode. The voltage drop across the diodes doubles and is not 0.65-1V, but an average of 1.3-2V. Taking this drop into account, the rectified voltage is calculated.

For example, we need to get 18 volts of rectified voltage, which transformer should we choose for this? 18 volts plus the drop across the diodes, let’s take the average 1.4 V, equals 19.4 V. We know from that the amplitude value of the alternating voltage is root 2 times its effective value. Therefore, in the secondary circuit of the transformer, the alternating effective voltage is 19.4 / 1.41 = 13.75V. Taking into account the fact that the voltage in the network can fluctuate by 10%, and also that under load the voltage will drop a little, we will choose a 230/15 V transformer.

The power of the transformer we need can be calculated from the load current. For example, let's say we're going to connect a one-amp load to a transformer. This is if with a reserve. Always leave a small margin, 20-40%. Simply using the power formula you can find P = U * I = 15 * 1 = 15 VA, where U and I are the voltage and current of the secondary winding. If there are several secondary windings, then their powers are added up. Plus transformation losses, plus margin, so we’ll choose a 20-40 VA transformer. Although transformers are often sold with the current of the secondary windings indicated, it doesn’t hurt to check the overall power.

After the rectifier bridge, a smoothing capacitor is required, not shown in the figure. Don't forget about him! There are clever formulas for calculating this capacitor depending on the number of ripples, but I recommend this rule: install a 10,000 µF capacitor per ampere of current consumption. The capacitor voltage is no less than the rectified voltage without load. In this example, you can take a capacitor with a nominal value of 25V.

We will select the diodes in this circuit for a current >=1A and a reverse voltage, with a margin, of more than 19.4 V, for example, 50-1000 V. You can use Schottky diodes. These are the same diodes, only with a very small voltage drop, which is often tens of millivolts. But the disadvantage of Schottky diodes is that they are not produced for more or less high voltages, more than 100V. More precisely, they have been releasing them recently, but their cost is exorbitant, and the advantages are no longer so obvious.

Light-emitting diode

The inside is structured completely differently than a diode, but has the same properties. It only glows when current flows in the forward direction.

The only difference from a diode is in some characteristics. The most important thing is the forward voltage drop. It is much larger than the 0.65 V of a conventional diode and depends mainly on the color of the LED. Starting from red, whose voltage drop is on average 1.8 V, and ending with a white or blue LED, whose voltage drop is about 3.5 V. However, in the invisible spectrum these values ​​​​are wider.

In fact, the voltage drop here is the minimum ignition voltage of the diode. At a lower voltage, there will be no current at the power source and the diode simply will not light up. For powerful lighting LEDs, the voltage drop can be tens of volts, but this only means that there are many series-parallel diode assemblies inside the crystal.

But now let's talk about indicator LEDs, as they are the simplest. They are produced in various cases, most often in semi-circular ones, with a diameter of 3, 5, 10 mm.

Any diode glows depending on the current flowing. Essentially it is a current device. The voltage drop is obtained automatically. We set the current ourselves. Modern indicator diodes more or less begin to glow at a current of 1 mA, and at 10 mA they already burn out the eyes. For powerful lighting diodes, you need to look at the documentation.

Application of LED

Having only the appropriate resistor, you can set the desired current through the diode. Of course, you will also need a constant voltage power supply, for example, a 4.5 V battery or any other power supply.

For example, let's set a current of 1 mA through a red LED with a voltage drop of 1.8 V.

The diagram shows the nodal potentials, i.e. voltage relative to zero. A multimeter in dialing mode will best tell us which direction to turn on the LED, since sometimes we come across Chinese LEDs with the legs mixed up. When you touch the multimeter probes in the right direction, the LED should glow faintly.

Since a red LED is used, the resistor will drop 4.5 - 1.8 = 2.7V. This is known from Kirchhoff’s second law: the sum of the voltage drops in successive sections of the circuit is equal to the emf of the battery, i.e. 2.7 + 1.8 = 4.5V. To limit the current to 1 mA, the resistor, according to Ohm's law, must have a resistance R = U / I = 2.7 / 0.001 = 2700 Ohms, where U and I are the voltage across the resistor and the current we need. Don't forget to convert the values ​​to SI units, amperes and volts. Since manufactured resistance ratings are standardized, we will choose the closest standard rating of 3.3 kOhm. Of course, in this case the current will change and it can be recalculated using Ohm’s law I = U / R. But often this is not important.

In this example, the current supplied by the battery is small, so the internal resistance of the battery can be neglected.

With lighting LEDs everything is the same, only the currents and voltages are higher. But sometimes they no longer need a resistor; you need to look at the documentation.

Something else about the LED

In fact, to shine is the main purpose of an LED. But there is another use. For example, an LED can act as a voltage reference. They are necessary, for example, to obtain current sources. Red LEDs are used as reference voltage sources because they are less noisy. They are included in the diagram in the same way as in the previous example. Since the battery voltage is relatively constant, the current through the resistor and LED is also constant, so the voltage drop remains constant. A tap is made from the anode of the LED, where 1.8V is present, and this reference voltage is used in other parts of the circuit.

To more reliably stabilize the current on the LED, with a pulsating voltage from the power source, instead of a resistor, a current source is placed in the circuit. But current sources and voltage reference sources are the topic of another article. Maybe someday I'll write it.

Zener diode

In English literature, a zener diode is called a Zener diode. Everything is the same as the diode, in direct connection. But now we’ll only talk about the reverse switching on. In reverse switching, under the influence of a certain voltage on the zener diode, a reversible breakdown occurs, i.e. current begins to flow. This breakdown is completely normal and operating mode of the zener diode, in contrast to the diode, where when the rated reverse voltage was reached, the diode simply failed. At the same time, the current through the zener diode in breakdown mode can change, but the voltage drop across the zener diode remains practically unchanged.

What does this give us? Essentially this is a low-power voltage stabilizer. A zener diode has all the same characteristics as a diode, plus the stabilization voltage Ust or nominal zener voltage is also added. It is indicated at a certain stabilization current Ist or test current. Also, the documentation for zener diodes indicates the minimum and maximum stabilization current. When the current changes from minimum to maximum, the stabilization voltage floats somewhat, but not significantly. See current-voltage characteristics.

The working area of ​​the zener diode is indicated in green. The figure shows that the voltage in the working area is almost constant, with a wide range of changes in the current through the zener diode.

To reach the working area, we need to set the zener diode current using a resistor in the same way as was done in the example with the LED (by the way, you can also use a current source). Only, unlike the LED, the zener diode is switched in the opposite direction.

At less current than Ist. min the zener diode will not open, but with more than Ist. max – an irreversible thermal breakdown will occur, i.e. The zener diode will simply burn out.

Zener diode calculation

Let's look at the example of our calculated transformer power supply. We have a power supply that produces a minimum of 18 V (in fact, there is more there, due to the 230/15 V transformer, it is better to measure in a real circuit, but that is not the point now), capable of delivering a current of 1 A. We need to power the load with the maximum consumption of 50 mA with a stabilized voltage of 15 V (for example, let it be some kind of abstract operational amplifier - op-amp, they have approximately the same consumption).

Such a light load was chosen for a reason. Zener diodes are rather low-power stabilizers. They must be designed so that the entire load current plus the minimum stabilization current Ist can pass through them without overheating. min. This is necessary because the current after resistor R1 is divided between the zener diode and the load. The current in the load may not be constant, or the load may be completely disconnected from the circuit. Essentially this is a parallel stabilizer, i.e. all the current that does not go to the load will be taken over by the zener diode. It’s like Kirchhoff’s first law I = I1 + I2, only here I = Iload + Ist. min.

So, let's choose a zener diode with a stabilization voltage of 15 V. To set the current through the zener diode, a resistor (or current source) is always needed. 18 – 15 = 3 V will drop across resistor R1. Current Iload will flow through resistor R1. + Ist. min. Let's accept Ist. min = 5 mA, this is approximately sufficient current for all zener diodes with stabilization voltage up to 100 V. Above 100 V, 1 mA or less can be taken. You can take Ist. min and more, but it will only be useless to heat the zener diode.

So, Ir1 = Iload flows through R1. + Ist. min = 50 + 5 = 55 mA. Using Ohm's law, we find the resistance R1 = U / I = 3 / 0.055 = 54.5 Ohms, where U and I are the voltage across the resistor and the current through the resistor. Let's choose a resistance of 47 ohms from the nearest standard series; there will be a little more current through the zener diode, but that's okay. It can even be calculated, the total current: Ir1 = U / R = 3 / 47 = 0.063A, then the minimum zener diode current: 63 - 50 = 13 mA. Power of resistor R1: P = U * I = 3 * 0.063 = 0.189 W. Let's choose a standard 0.5 W resistor. By the way, I advise you not to exceed the resistor power of approximately Pmax/2, they will last longer.

The zener diode also dissipates power in the form of heat, and in the worst case it will be equal to P = Ust * (Iload + Ist.) = 15 * (0.050 + 0.013) = 0.945 W. Zener diodes are produced at different powers, the nearest 1W, but then the temperature of the case when consuming about 1W will be somewhere around 125 degrees C, it is better to take it with a reserve of 3 W. Zener diodes are produced at 0.25, 0.5, 1, 3, 5 W, etc.

The first Google search for “zener diode 3W 15V” yielded 1N5929BG. Next we look for “datasheet 1N5929BG”. According to the datasheet, it has a minimum stabilization current of 0.25 mA, which is less than 13 mA, and a maximum current of 100 mA, which is more than 63 mA, i.e. fits into his work regime, so it suits us.

In general, this is the whole calculation. Yes, the stabilizer is not ideal, its internal resistance is not zero, but it is simple and cheap and is guaranteed to work in the specified current range. And also since this is a parallel stabilizer, the power supply current will be constant. More powerful stabilizers can be obtained by powering the zener diode with a transistor, but this is the topic of the next article, about transistors.

As a rule, it is impossible to check the zener diode for breakdown with a conventional multimeter. With a more or less high-voltage zener diode, there is simply not enough voltage on the probes. The only thing that can be done is to test it for the presence of normal diode conductivity in the forward direction. But this indirectly guarantees the functionality of the device.

Zener diodes can also be used as reference voltage sources, but they are noisy. For these purposes, special low-noise zener diodes are produced, but their price, in my understanding, is off scale for a piece of silicon; it is better to add a little and buy an integrated source with the best parameters.

There are also many semiconductor devices similar to a diode: a thyristor (controlled diode), a triac (symmetrical thyristor), a dinistor (opened pulsed only when a certain voltage is reached), a varicap (with variable capacitance), something else. You will need the first ones in power electronics when building controlled rectifiers or active load regulators. And I haven’t encountered the latter for 10 years, so I leave this topic for independent reading on the wiki, at least about the thyristor.

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Diode- This is an element that has different conductivity. This property is used in various electrical and electronic circuits. Based on it, devices are created that are used in various fields.

Types of diodes: vacuum and semiconductor. The latter type is currently used in the vast majority of cases. It will never be superfluous to know how a diode works, what it is needed for, how it is indicated on the diagram, what types of diodes exist, the use of different types of diodes.

Electrovacuum diodes

Devices of this type are made in the form of electron tubes. The lamp looks like a glass container with two electrodes placed inside it. One of them is an anode, the other is a cathode. They are in a vacuum. Structurally, the anode is made in the form of a thin-walled cylinder. The cathode is located inside. It usually has a cylindrical shape. An insulated filament is laid inside the cathode. All elements have leads that are connected to the pins (legs) of the lamp. The legs of the lamp are brought out.

Principle of operation

When an electric current passes through the spiral, it heats up and heats the cathode inside which it is located. From the surface of a heated cathode, electrons that leave it, without an additional accelerating field, accumulate in the immediate vicinity of it. Some of them are then returned to the cathode.

When a positive voltage is applied to the anode, electrons emitted by the cathode rush towards it, creating an anode electron current.

The cathode has a limit on electron emission. When this limit is reached, the anode current stabilizes. If a small negative voltage is applied to the anode relative to the cathode, the electrons will stop moving.

The cathode material from which it is made has a high degree of emission.

Current-voltage characteristic (CVC)

The current-voltage characteristic of diodes of this type graphically shows the dependence of the anode current on the forward voltage applied to the cathode and anode terminals. It consists of three sections:

• Slow nonlinear increase in current;
• Working part of the characteristics;
• Anode current saturation region.

The nonlinear section begins after the anodic current cutoff region. Its nonlinearity is associated with the small positive potential of the cathode, which electrons left when it was heated by the filament.

The active section defines an almost vertical line. It characterizes the dependence of the anode current on increasing voltage.

The saturation section is a line of constant anode current with increasing voltage between the lamp electrodes. The electron tube in this area can be compared to an electric current conductor. The cathode emission reached its highest value.

Semiconductor diodes

The property of a p-n junction to pass electric current in one direction has found application in the creation of devices of this type. Direct connection is the supply of a negative potential to the n-region of the transition, in relation to the p-region, the potential of which is positive. When turned on in this way, the device is in the open state. When the polarity of the applied voltage changes, it will be in a locked state, and no current will pass through it.

Diodes can be classified according to their purpose, manufacturing features, and the type of material used in its manufacture.

Basically, silicon or germanium wafers, which have n-type electrical conductivity, are used for the manufacture of semiconductor devices. They contain an excess of negatively charged electrons.

Using different manufacturing technologies, it is possible to obtain point or plate diodes at the output.

When making point devices, a pointed conductor (needle) is welded to an n-type plate. A certain impurity is applied to its surface. For germanium wafers, the needle contains indium; for silicon wafers, the needle is coated with aluminum. In both cases, a p-n junction region is created. Its shape resembles a hemisphere (point).

For planar devices, the diffusion or fusion method is used. The area of ​​transitions obtained by this method varies widely. The further purpose of the product depends on its size. Wires are soldered to the p-n junction areas, which are used in the form of leads from the body of the finished product when installing various electrical circuits.

In the diagrams, semiconductor diodes are designated in the form of an equilateral triangle, to the upper corner of which is attached a vertical line parallel to its base. The terminal of the line is called the cathode, and the terminal of the base of the triangle is the anode.

Direct connection is such a connection in which the positive pole of the power source is connected to the anode. When turned back on, the “plus” of the source is connected to the cathode.

Volt-ampere characteristics

The current-voltage characteristic determines the dependence of the current flowing through a semiconductor element on the magnitude and polarity of the voltage applied to its terminals.

In the region of forward voltages, three regions are distinguished: a small forward current and a forward operating current through the diode. The transition from one region to another occurs when the direct voltage reaches the conductivity threshold. This value is on the order of 0.3 volts for germanium diodes and 0.7 volts for silicon-based diodes.

When a reverse voltage is applied to the terminals of a diode, the current through it is very small and is called reverse current or leakage current. This dependence is observed up to a certain value of the reverse voltage. It is called breakdown voltage. When it is exceeded, the reverse current increases like an avalanche.

Parameter Limits

For semiconductor diodes, there are parameter values ​​that cannot be exceeded. These include:

• Maximum forward current;
• Maximum reverse breakdown voltage;
• Maximum power dissipation.

A semiconductor element can withstand a limited amount of forward current through it. If it is exceeded, the pn junction overheats and fails. Planar power devices have the greatest margin for this parameter. The magnitude of the direct current through them can reach tens of amperes.

Exceeding the maximum breakdown voltage can turn a diode, which has unidirectional properties, into an ordinary conductor of electric current. The breakdown can be irreversible and varies widely, depending on the specific device used.

Power- this is a quantity that directly depends on the current and voltage that is applied to the diode terminals. Like exceeding the maximum forward current, exceeding the maximum dissipation power leads to irreversible consequences. The diode simply burns out and ceases to fulfill its purpose. To prevent such a situation, power devices install devices on radiators, which remove (dissipate) excess heat into the environment.

Types of semiconductor diodes

The property of a diode to pass current in the forward direction and not pass it in the reverse direction has found application in electrical engineering and radio engineering. Special types of diodes have also been developed to perform a narrow range of tasks.

Rectifiers and their properties

Their use is based on the rectifying properties of these devices. They are used to obtain a constant voltage by rectifying the input alternating signal.

A single rectifier diode allows you to obtain a pulsating voltage of positive polarity at its output. Using a combination of these, a wave-like output voltage waveform can be obtained. When using additional elements in rectifier circuits, such as high-capacity electrolytic capacitors and inductors with electromagnetic cores (chokes), a constant voltage can be obtained at the output of the device, reminiscent of the voltage of a galvanic battery, which is so necessary for the operation of most consumer equipment.

Semiconductor Zener diodes

These diodes have a I-V characteristic with a high steepness reverse branch. That is, by applying a voltage whose polarity is reversed to the zener diode terminals, you can use limiting resistors to enter it into the avalanche-controlled breakdown mode. The voltage at the point of avalanche breakdown has a constant value with a significant change in the current through the zener diode, the value of which is limited depending on the device used in the circuit. This results in the effect of stabilizing the output voltage at the desired level.

Technological operations in the manufacture of zener diodes achieve different values ​​of breakdown voltage (stabilization voltage). The range of these voltages is (3−15) volts. The specific value depends on the selected device from the large family of zener diodes.

Operating principle of detectors

To detect high-frequency signals, diodes made using point technology are used. The detector's job is to limit one half of the modulated signal. This allows you to subsequently use a high-pass filter to leave only the modulating signal at the output of the device. It contains low frequency audio information. This method is used in radio receivers that receive an amplitude modulated signal.

Features of LEDs

These diodes are characterized by the fact that when forward current flows through them, the crystal emits a stream of photons, which are a source of light. Depending on the type of crystal used in the LED, the light spectrum can be either in the range visible to the human eye or in the invisible range. Invisible light is infrared or ultraviolet radiation.

When choosing these elements, it is necessary to imagine the goal that needs to be achieved. The main characteristics of LEDs include:

• Power consumption;
• Rated voltage;
• Consumption current.

The current consumption of the LED used for indication in widely used devices is no more than 20 mA. At this current, the LED glow is optimal. The onset of luminescence begins at a current exceeding 3 mA.

The rated voltage is determined by the internal resistance of the junction, which is not a constant value. As the current through the LED increases, the resistance gradually decreases. The voltage of the power source used to power the LED must be used no less than the voltage specified in its data sheet.

Power consumption is a value that depends on the current consumption and the rated voltage. It increases with increasing quantities that determine it. It should be taken into account that powerful light diodes can contain 2 or even 4 crystals.

LEDs have undeniable advantages over other lighting devices. They can be listed for a long time. The main ones are:

• High efficiency;
• Great durability;
• High level of safety due to low supply voltages.

The disadvantage of their operation is the need for an additional stabilized DC power source, and this increases the cost.