The most common AC to DC rectifier circuits. Main characteristics of rectifiers

Content:

In the modern variety of electrical appliances, both for domestic purposes and for other tasks, most contain a rectifier. This is due to their continuous complication due to the increase in functionality. And for multifunctionality, electronics that consume direct current are needed. It is provided by a power source. It always has a rectifier. Next, we will talk about this device in more detail.

What were the first rectifiers

The development of electricity supply started from scratch. And this means that there was neither the knowledge nor, moreover, the equipment for this. It took almost a century for modern semiconductor rectifiers to appear. They are a consequence of the historically established power supply infrastructure. And it, as you know, developed on the basis of alternating voltage.

DC power supply is more efficient, since losses in power lines due to inductance and capacitance of wires do not affect. But almost everywhere the electricity in the network corresponds to alternating voltage. This is because power supply is impossible without changing the voltage value. And this problem is still most effectively solved only by a transformer. The difference in the properties of electrical circuits with alternating and constant voltage was immediately noticed by the researchers.

And since the secondary winding of the transformer is an effective source of electricity, one way or another it was necessary to obtain some kind of constant voltage based on it. At the first stage of the development of electrical engineering, only electromagnetic machines appeared. They were adapted to rectify the voltage. The phenomenon of electrolysis was also known. It was also used for the manufacture of rectifiers - electrolytic.

Mechanical voltage rectification

The definition of rectification means obtaining a unidirectional electrical current. Its value in this case will depend on the shape of the alternating voltage in each half-cycle. But a unidirectional electric current is obtained, both with a positive half-cycle of the voltage, and with its negative value. In this case, when the voltage passes through zero, the load must be disconnected from the unnecessary voltage half-wave. The first rectifiers performed this task with mechanical contacts.

They were either driven by a synchronous motor or moved by a fairly fast solenoid. In both circuits, the voltage switching contacts move in sync with the voltage. In the circuit with the engine, they rotate, closing at the right time.

The node designed to rectify the voltage, when rotated, is similar to the collector of a DC motor. The number of lamellas - contacts is determined by the number of revolutions of the synchronous motor. When the sinusoid of the rectified voltage passes through zero, both brushes are in contact with either the beginning or the end of the lamella. The beginning of the lamella coincides with the tip of the arrow indicating the direction of rotation of the engine.

The contact time of the brushes with the lamella coincides with the duration of half the period of the rectified voltage. A synchronous motor rotates precisely and in multiples of the frequency of the supply voltage, which it rectifies by a collector attached to it. But its inertia will not allow rectifying the abrupt change in the frequency of the supply voltage. Therefore, it is effective only as a rectifier for the mains voltage.

The rectifier on the solenoid closes the contact either for the time the core is retracted or vice versa. It can only work at a certain minimum voltage, which is enough to move the contacts. Therefore, the part of the half-wave near the zero crossing of the voltage will not be processed properly. But such a rectifier can be made quite small. Therefore, it was widely used in its time.

It is obvious that without switching the electrical circuit, there can be no voltage rectification. And the possibilities of mechanical contact are limited by the power of the spark that occurs at the moment of breaking the electrical circuit. It gradually destroys this contact the faster, the greater the electrical power when it is opened.

This device works without switching. However, it was invented only after the advent of sufficiently pure aluminum. It is known that this metal forms a thin film of strong oxide on its surface. Aluminum oxide is almost an insulator. If an aluminum plate is immersed in a certain solution and a negative potential is applied to it, the film will collapse. In this case, the current in the solution should come from an iron plate immersed nearby - the anode.

A film of aluminum oxide will instantly dissolve in a solution of, for example, sodium phosphate. Therefore, the cathode surface will be made of pure aluminum. And the current will flow freely between the immersed electrodes. But as soon as the polarity of the electrodes is reversed, the surface of the aluminum plate will instantly oxidize. A film with high resistance will not pass an electric current.

The energy characteristics of an electrolytic rectifier depend on the volume of the vessel, as well as on the size and number of plates. The pure aluminum plate is efficient for a long time. Such a rectifier can be disabled only by mechanical destruction. From the increase in current, it is "insured" by the properties of the electrolyte. Too high a voltage will simply not straighten out. But when it returns to the nominal value, this rectifier will continue to work. He just doesn't kill.

Lamp options

Such mechanical and electrolytic rectifying devices existed for several decades before the introduction of vacuum tubes. But they were also limited by power losses. Although not related to switching. The fact is that a pre-created supply of electrons is necessary for the lamp to work.

And they did not learn how to get it in lamps otherwise than by heating a filament. So it turned out that, despite the speed, a conventional diode lamp consumed too much electricity to rectify the voltage. But over time, a powerful mercury lamp was invented - a mercury rectifier. It differed in that it produced a controlled electric discharge in mercury vapor. The discharge existed only at one voltage half-wave.

This made it possible to bring the power of the rectifier to values ​​acceptable for industrial use. And on the basis of mercury rectifiers, the first power lines operating at constant voltage were built. And in all other electrical appliances, electronic lamps-diodes were used. In the 1930s, the first semiconductor rectifiers based on selenium appeared. They were called so - "selenium rectifiers".

However, the characteristics of these rectifiers left much to be desired. Therefore, the search for more efficient technical solutions continued and ended with the appearance of a semiconductor diode. But its benefits are also relative. The semiconductor temperature cannot exceed 130–150 degrees Celsius. For this reason, all previous types of rectifiers have their own niche for conditions with high temperature and radiation. For other operating conditions, diode rectifiers are used.

semiconductor circuits

Any rectifier is a circuit. It includes the secondary winding of the transformer, the rectifying element, the electrical filter and the load. In this case, it is possible to obtain voltage multiplication. Rectified voltage is the sum of DC and AC voltages. A variable component is an undesirable component that is reduced in one way or another. But since half-waves of alternating voltage are used, it cannot be otherwise.

The influence of the variable component is estimated by the ripple factor.

It can be reduced in two ways:

• improving the efficiency of the electric filter;
• improving the parameters of the rectified alternating voltage.

The simplest half-wave rectifier. It cuts off one of the half-waves of the alternating voltage. Therefore, the ripple coefficient in such a circuit is the largest. But if a three-phase voltage is rectified with one diode in each phase, as well as the same filter, one will get three times less ripple. However, full-wave rectifiers have the best performance.

There are two ways to use both half-waves of alternating voltage:

• according to the bridge scheme;

• according to the scheme with the midpoint of the winding (Mitkevich's scheme).

Let's compare both these circuits for the same value of the rectified voltage. The bridge circuit uses fewer turns of the secondary winding of the transformer, which is an advantage. But at the same time, four diodes are needed in a single-phase rectifier bridge. In a mid-point circuit, twice as many turns of the mid-point secondary winding are needed, which is a disadvantage. Another disadvantage of this scheme is the need for symmetry of the parts of the winding about the midpoint.

Asymmetry will be an additional source of pulsations. But in this circuit, only two diodes are needed, which is an advantage. When rectified, there is voltage across the diode. Its value almost does not change depending on the strength of the current flowing through this diode. Therefore, the power dissipated by a semiconductor diode increases as the strength of the rectified current increases. This is very noticeable at high current strength, and therefore semiconductor diodes are placed on cooling radiators and, if necessary, blown.

When rectifying a large current, the two diodes of the mid-point circuit will be more economical and compact compared to the four diodes of the rectifier bridge. Rectifier circuits at one time did not appear out of nowhere. Engineers invented them. Therefore, rectifier circuits in the literature are sometimes named in connection with the names of their discoverers. The bridge circuit is referred to as the "full Graetz bridge". The circuit with the midpoint is the "Mitkevich rectifier".

Semiconductor diodes, together with capacitors, make it possible to create circuits in which capacitors are charged in half a cycle and discharged into a load in half a cycle. In this case, the stresses that accumulate on them are summed up. In this way, circuits for voltage multiplication can be created. The simplest and most efficient rectifier circuit that will double the voltage contains two diodes and two capacitors. It is called the Latour-Delon scheme. Its analogue is the Grenacher scheme.

By creating the required number of cells containing capacitors and diodes, you can get any voltage that is a multiple of their number. The circuit corresponding to this solution is shown below. In it, each of the cells contains a capacitor and a diode.

In the article, only some types of rectifiers that are most widely used were considered in detail.

When choosing a particular device, it is necessary to be guided by the parameters of the load voltage. Only in this way is effective voltage rectification obtained.

In the lighting electrical network, from which all household electrical appliances are powered, as a rule, alternating current flows. A rare exception is small rural settlements, where power plants provide direct current.

Radio receivers, tape recorders, electric players and other devices operate on vacuum tubes or semiconductor devices, the electrodes of which must be supplied with DC voltage. Batteries can only be charged with direct current. A number of production processes in factories, such as chromium plating, cannot be carried out if there is no constant voltage.

Why do our power plants give alternating current? After all, electric heaters and electric motors will also work well on direct current? This is mainly explained by the fact that alternating current can be easily transformed (transformed) into various voltages, which cannot be done with direct current. The transmission of alternating current energy through a power line can be carried out with much lower losses than with direct current, due to the fact that the voltage in the line in this case can be tens and hundreds of thousands of volts. At the place of consumption, the voltage is reduced at transformer substations and alternating voltage of 127 or 220 V is supplied to our apartments and factories.

How to get the constant voltage necessary for the normal operation of some devices?

A rectifier is used to convert alternating voltages to direct voltages. To understand how a rectifier works, you can only clearly understand what alternating current is. An alternating current is a current whose direction and magnitude change with time.

In the lighting network, according to the standard adopted in our country, the direction of the current changes 50 times per second, or, as they say, the frequency of the industrial current is 50 periods (hertz). This means that for some period of time the current in the network is 0, then the current begins to increase smoothly, reaches the maximum (amplitude) value, after which the current in the network gradually decreases and becomes equal to zero. After that, the direction of the current changes again and the current again smoothly increases to a maximum value, and then again decreases to zero. This process resembles a seesaw, which, swinging around the equilibrium position (zero current value), rises to the maximum height (maximum current value), then lowers, rises again, etc. Such a process of current change is called periodic. In our electrical network, such a process is repeated fifty times per second, i.e., the current (voltage) has fifty periods per second, changing its value according to a sinusoidal law.

Graphically, the picture of the current change in the network is shown in fig. 1. Such a graph is obtained if the values ​​of current or voltage are plotted on the vertical axis, and the time intervals counted from some moment taken as the origin of the reference are plotted along the horizontal axis.

The task of the rectifier is to obtain a constant voltage from an alternating one; A constant voltage can be graphically depicted as shown in Fig. 2. Direct current does not change either its direction or its magnitude.

The process of rectifying alternating current (voltage) consists in the fact that on the path of the current in the electrical circuit, an element is switched on - a valve that passes current in only one direction (of one sign). Schematically, an alternating current circuit with a valve is shown in fig. 3. One-sided conduction of the valve leads to the fact that only in positive half-cycles, the current passes through the valve, and in negative half-cycles (marked in Fig. 1 with the sign "-") there is no current in the circuit. Graphically, the current in such a circuit can be depicted as shown in Fig. 4. With a positive half-wave, the resistance of the valve is small and the current passes freely through it. With a negative half-wave, the current encounters great resistance, since in the opposite direction the resistance of the valve is hundreds and even thousands of times greater and the current does not pass through it. Thus, having included a valve in an alternating current circuit, we no longer receive alternating current in this circuit. The current in this circuit will only change in magnitude and will not change its direction. Such a current is called pulsating. You can use it, for example, to charge batteries. This current is not suitable for powering radio equipment. Further smoothing is required in order for the current to turn from pulsating to direct. This is achieved by using a filter.

In the simplest case, the role of a filter can be performed by a capacitor of a sufficiently large capacity. On fig. 5 shows a circuit diagram with a valve and capacitor C, which is a filter. Ripple smoothing (filtering) of the rectified current is carried out due to the fact that the capacitor is charged by the current passing through the valve and stores electrical energy. As soon as the current through the valve begins to decrease and the voltage at the load Rn of the rectifier begins to fall - and this happens at the end of each positive half-cycle - the capacitor releases the energy accumulated by it over the positive half-cycle. Graphically, this is shown in Figure 6. As can be seen from the figure, the current has not yet become completely constant and sharp ripples are noticeable. A better filter is needed, which would provide a constant current at the load with very small ripple, which will not significantly affect the operation of the device powered by the rectifier.

There are several types of rectifiers. The simplest of them is a half-wave, the circuit of which is shown in Fig. 7. In such a rectifier, only positive half-cycles of the rectified current are used. The ripple frequency of this current is equal to the frequency of the mains voltage, and to smooth out the ripples, the rectifier, assembled according to a half-wave circuit, requires a good filter. Such rectifiers are used to power equipment that consumes little current, since as the current increases, it will be necessary to complicate the rectifier filter.

More common is a full-wave rectification circuit, where (see Fig. 8) two valves B1 and B2 are used. The current in the load flows all the time in one direction. Rectification of voltage occurs as follows. At some point in time, one (upper, according to the scheme) output of the secondary winding of the transformer Tr1 will have a positive voltage with respect to the second (lower) end. The current will go through valve B1, and having a small resistance in the forward direction, then through the load to the midpoint of the secondary winding of the transformer. On fig. 8 the passage of current is shown by a solid arrow. This will continue for the first positive half cycle. When the direction of the current in the network changes, there will already be a negative voltage at the upper end of the transformer and the current will not flow through the valve B1, since the valve will have a very high resistance. At the lower end of the secondary winding of the transformer, there will now be a positive voltage and the current will go through valve B2, the load and to the middle point of the secondary winding - transformer Tr1.

With this switching on of the valves, both half-cycles of the rectified voltage are already used. The ripple frequency in such a rectifier is twice as high, and therefore the filtering of the rectified voltage is greatly facilitated. Almost all rectifiers for radios, televisions and tape recorders are assembled according to a full-wave circuit.

There is also a bridge circuit for switching on a rectifier. In this case, rectification occurs according to a full-wave circuit, but the transformer has a simpler design, its secondary winding contains half as many turns and no output from the midpoint is required. However, a bridged rectifier requires twice as many valves as a full-wave rectifier. The bridge rectifier circuit is shown in fig. 9. The arrows indicate the passage of current in both half-cycles.

As a valve for rectifying alternating current, selenium or cuprox washers, kenotrons, gastrons or semiconductor diodes can be used.

To power mass radio equipment, kenotron and selenium rectifiers are most widely used. Recently, germanium power diodes of the DG-Ts21-27 type have been increasingly used.

The kenotron is a vacuum, usually glass, radio lamp with two electrodes - an anode and a cathode. A two-anode kenotron has two anodes. The valve property of the kenotron is manifested in the fact that the current through the kenotron can only go in one direction - from the anode to the cathode. In the opposite direction, the current will not flow, since the electrons fly out only from the surface of the heated cathode and can only move to the anode if it currently has a positive voltage relative to the cathode.

The simplest half-wave rectifier circuit using a kenotron as a valve is shown in Fig. 10. The direction of current I is shown by an arrow. Capacitors C1 and C2 and inductor Dr1 make up a filter for smoothing ripples. Filters will be discussed in more detail below.

There are many different types of kenotrons, each of which is designed for certain operating conditions: some allow you to get a large rectified current at a relatively low voltage, others, on the contrary, work in a rectifier that gives a high voltage at a negligible current.

When designing a rectifier, first of all, it is necessary to choose the right type of kenotron. To do this, you need to know what current and voltage the load powered by the rectifier consumes, and in accordance with these data, select the appropriate type of kenotron from the reference book. Let it be required to choose a kenotron, which is supposed to be installed in the rectifier to power the receiver. The receiver has four lamps, not counting the kenotron.

The direct voltage required to power the radio tubes of the receiver is 250 V. The total current consumed by the anode-screen circuits of all receiver lamps is about 40 mA.

The most suitable for our rectifier will be the 6Ts4P kenotron, which, according to reference data, can provide a current of up to 70 mA with a full-wave rectification circuit. In terms of voltage, this kenotron is also quite suitable, since for a full-wave rectification circuit, the reverse voltage that occurs in the rectifier does not exceed the triple voltage at the load and is equal to 250x3 = 750 V, and the 6Ts4P kenotron can withstand up to 1000 V reverse voltage.

In a selenium rectifier, selenium washers are used as a valve.

A selenium washer is an iron disk or a rectangular iron plate, on which a thin layer of a semiconductor, selenium, is deposited on one side. From above, the selenium layer is covered, to create a contact, with a thin layer of fusible metal.

The valve properties of selenium are manifested in the fact that it has one-sided conductivity. When the positive pole of the current source is applied to the iron plate, the selenium washer has negligible resistance, and, conversely, when the polarity is reversed, the resistance of the washer increases hundreds of times.

The choice of selenium valve for the rectifier is also made according to the current and voltage required for the load. It must be remembered that one selenium washer can withstand voltages up to 20 V, therefore, if a voltage develops on the load greater than this value, then the selenium washers must be connected in series.

For our example, it is enough to put 13 washers in each arm of a full-wave rectifier, since the voltage at the load is 250 V and the number of washers will be obtained if 250 V is divided by 20 V. The resulting fractional number must be rounded up to the nearest integer. To determine what diameter washers need to be placed, it must be remembered that a current of 30 mA is allowed per square centimeter of the surface of a selenium washer. Therefore, in order to determine the area of ​​selenium washers for our rectifier, it is necessary to divide the amount of current consumed by the receiver by the allowable current density (the amount of current allowed per 1 cm 2). The area of ​​the washer is 40/30 = 1.33 cm. The diameter of the washer is easy to determine using the well-known formula for the area of ​​a circle

Splosh \u003d 0.25 * π * D 2,

whence the diameter of the washer is

D \u003d (4 * S / π) 0.5 \u003d (4 * 1.33 / 3.14) 0.5 ≈ 1.3 cm.

You can not make such a calculation and take the diameter of the washer directly from the directory. If the radio amateur has washers of some other diameter, then they can be used in this rectifier. If the washers have a larger diameter than calculated, they can be installed as a valve without any changes in the rectifier circuit, remembering only that the allowable voltage for each washer should not exceed 20 V.

If the diameter of the existing washers is less than that obtained by calculation, then the washers can be connected in parallel in such a way that the total area of ​​two washers connected in parallel is equal to or greater than that obtained by calculation. When washers are connected in parallel, their number doubles, since it is necessary to observe the condition of permissible stress for each washer.

The calculation of the valve, which is used as a germanium diode (Fig. 11), is carried out in a similar way. Knowing the load current and voltage on it, choose the appropriate type of diode from the reference book. It may happen that the available germanium diodes of the DG-Z type are not suitable for the allowable current or voltage. If the diodes are not suitable for the current (the load current is greater than the permissible), then it is necessary to install several diodes connected in parallel. If the diodes are not suitable for voltage, they are connected in series. The calculation of the number of series-connected diodes comes down to choosing the number of diodes at which the voltage drop across each of them does not exceed the allowable one.

When diodes of the DG-Ts type are connected in series, each of them should be shunted with a resistance of at least 100 kOhm with a power of up to 1 W. Shunting diodes is necessary to equalize the voltage drop across each of them. Produced diodes have a significant spread of parameters, and there may be such a case when the voltage drop on one of them will be several times greater than on the other, which disables the diodes. This will not happen if each diode is shunted with resistance and the voltage drop is distributed evenly between each diode.

With a parallel connection of semiconductor diodes of the DG-Ts type, their number is calculated using simple formulas. So, for diodes of the type DG-Ts21 - 24, the number of diodes connected in parallel will be equal to

For diodes of type DG-Ts25 - 27 the number of diodes connected in parallel

n = 15.4I0 - 0.54.

In these formulas, I0 means the rectified current in amperes. It may happen that the number of diodes n calculated according to these formulas turns out to be fractional. Then this number should be rounded up to the nearest higher whole number. Sometimes the calculation results in 0 or a negative number. This means that only one diode needs to be installed and no parallel connections need to be made, as the selected diode will provide the required amount of rectified current.

Smoothing filter

As mentioned above, to smooth out ripples after the rectifier, a filter is switched on at its output. Typically, the filter consists of a filter choke Dr1 (Fig. 12), the winding of which, made of several thousand turns of thin wire, is located on a steel core. The filter also includes two or more filter capacitors. In place of these capacitors, in the vast majority of cases, electrolytic capacitors are used, which have relatively small dimensions and a large capacity (10 ... 50 microfarads).

The filter significantly attenuates the AC component of the rectified voltage and has little effect on the DC component that is used to power the anode-shield circuits of the receiver.

The quality of a filter is determined by its filter coefficient, which shows how many times the variable component at the filter output is attenuated relative to the variable component at its input.

The permissible value of the variable component at the filter output depends on the equipment that is powered by this rectifier. For low-frequency amplifiers, the anode voltage ripple amplitude should not exceed 0.5-1% of the useful signal voltage measured in the anode circuit of this stage. For high and intermediate frequency amplification stages, this amplitude should not exceed 0.05-0.1% (0.1-0.2 V).

The operation of the filter depends on the product of the inductance of the inductor and the capacitance of the filter capacitor at the output. The capacitance of this capacitor is usually taken in the range of 10-40 microfarads. The choke inductance for a low-power rectifier usually does not exceed 20-30 H.

When estimating filter data, you can use the following rule: the product of the inductance of the filter inductor, expressed in Henry, and the capacitance of the capacitor at the filter output, expressed in microfarads, should be 200.

To improve filtering, you can compose a smoothing filter from several links. Filtration improvement can also be achieved by using a tuned inductor; for this, a constant capacitor is connected in parallel with the filter inductor (in Fig. 12 this connection is shown by a dotted line).

The capacitance of the capacitor is taken in the range of 0.05-0.1 microfarads and in each individual case is found empirically.

The filter inductor can be turned on both in the “+” and in the “-” of the rectifier, this will not affect the quality of the filter. In some cases, when it is desirable to use the voltage drop across the filter choke winding to supply a negative bias to the control grids of the receiver amplifier lamps, the choke is included in the negative rectifier circuit.

When powering low-tube receivers, instead of a filter inductor, you can turn on the windings (or winding) of a low-frequency transformer.

Structurally, the choke for smoothing filters is similar to a low-power power transformer. The difference lies in the fact that the transformer has several windings, the choke has only one. The inductor core must have an air gap, which eliminates the possibility of magnetic saturation of the core with direct current flowing through the inductor winding.

Magnetic saturation reduces the inductance of the inductor, which degrades the performance of the filter.

Structurally, the filter inductor and the power transformer of the rectifier can be calculated, guided by the article printed in Appendix No. 1 for beginners, "Calculation and Manufacturing of a Power Transformer" (sent out with Radio magazine No. 5, 1957). It should only be taken into account that, when setting the voltage at the rectifier output, it is necessary to take into account the voltage drop across the filter inductor and that in the case of using a full-wave kenotron rectifier with a capacitor filter, the effective voltage and current of the step-up winding are related to the voltage and current at the rectifier output by the following relationships: voltage on the secondary winding, 2..2.2 times more voltage is taken at the rectifier output, and the current in the winding is 1..1.2 I0. The currents and voltages of the windings for the glow of lamps and the kenotron are determined by the data of the glow of the kenotron and lamps, for which the calculated rectifier is intended to power.

Instead of a filter inductor, an active resistance is sometimes used, which must have a significant value to obtain good filtration.

The disadvantage of such a filter is a large voltage drop across the filter resistance, so such a filter can only be used in low-power amplifiers. When calculating a rectifier with such a filter, the allowable drop in the rectified voltage across the resistance included in the filter, Upad, is set, after which the value of this resistance R is found by the formula

where I0 is the current in mA taken from the rectifier.

Very often, various constant voltages are used to power a particular equipment. In order to use the same rectifier for this purpose, a chain of several series-connected constant resistances of several thousand ohms is connected to its input. These resistances should not be very large, since otherwise the voltage taken from the divider will greatly depend on the magnitude of the load. They should also not be too small so as not to overload the rectifier.

A rectifier is a device for converting AC voltage to DC. It is one of the most common parts in electrical appliances, ranging from hair dryers to all types of power supplies with DC output voltage. There are different schemes of rectifiers, and each of them copes with its task to a certain extent. In this article we will talk about how to make a single-phase rectifier, and why you need it.

Definition

A rectifier is a device that converts AC to DC. The word "constant" is not entirely correct, the fact is that at the output of the rectifier, in the sinusoidal alternating voltage circuit, in any case, there will be an unstabilized pulsating voltage. In simple words: constant in sign, but changing in magnitude.

There are two types of rectifiers:

half wave. It rectifies only one half-wave of the input voltage. Strong ripples and low relative to the input voltage are characteristic.

full wave. Accordingly, two half-waves are straightened. The ripple is lower, the voltage is higher than at the rectifier input - these are the two main characteristics.

What does stabilized and unstabilized voltage mean?

A stabilized voltage is a voltage that does not change in magnitude regardless of either the load or the input voltage surges. For transformer power supplies, this is especially important, because the output voltage depends on the input voltage and differs from it by Ktransformation times.

Unstabilized voltage - varies depending on surges in the supply network and load characteristics. With such a power supply, due to drawdowns, the connected devices may malfunction or be completely inoperable and fail.

Output voltage

The main values ​​\u200b\u200bof alternating voltage are the amplitude and effective value. When they say “in the 220V network,” they mean the current voltage.

If they talk about the amplitude value, then they mean how many volts are from zero to the top point of the half-wave of the sinusoid.

Omitting the theory and a number of formulas, we can say that 1.41 times less than the amplitude. Or:

The amplitude voltage in the 220V network is:

The first scheme is more common. It consists of a diode bridge - interconnected by a "square", and a load is connected to its shoulders. The bridge type rectifier is assembled according to the diagram below:

It can be connected directly to a 220V network, as done in, or to the secondary windings of a mains (50 Hz) transformer. Diode bridges according to this scheme can be assembled from discrete (separate) diodes or you can use a ready-made assembly of a diode bridge in a single package.

The second circuit - a mid-point rectifier cannot be connected directly to the network. Its meaning is to use a transformer with a tap from the middle.

In essence, these are two half-wave rectifiers connected to the ends of the secondary winding, the load is connected with one contact to the junction point of the diodes, and the second - to the tap from the middle of the windings.

Its advantage over the first circuit is a smaller number of semiconductor diodes. And the disadvantage is the use of a transformer with a midpoint or, as they also call it, a tap from the middle. They are less common than conventional non-tapped secondary transformers.

Ripple smoothing

Power supply with pulsating voltage is unacceptable for a number of consumers, for example, light sources and audio equipment. Moreover, the permissible light pulsations are regulated in state and industry regulations.

To smooth out ripples, they use a parallel-mounted capacitor, an LC filter, various P- and G-filters ...

But the most common and simplest option is a capacitor installed in parallel with the load. Its disadvantage is that in order to reduce ripples on a very powerful load, it will be necessary to install capacitors of a very large capacity - tens of thousands of microfarads.

Its principle of operation is that the capacitor is charged, its voltage reaches an amplitude, the supply voltage after the point of maximum amplitude begins to decrease, from that moment the load is powered by the capacitor. The capacitor discharges depending on the resistance of the load (or its equivalent resistance if it is not resistive). The larger the capacitance of the capacitor, the smaller the ripple will be when compared with a capacitor with a smaller capacitance connected to the same load.

In simple words: the slower the capacitor discharges, the less ripple.

The discharge rate of the capacitor depends on the current drawn by the load. It can be determined by the time constant formula:

where R is the load resistance and C is the capacitance of the smoothing capacitor.

Thus, from a fully charged state to a fully discharged capacitor, it will be discharged in 3-5 t. It charges at the same rate if the charge occurs through a resistor, so in our case it does not matter.

It follows from this that in order to achieve an acceptable level of ripple (it is determined by the requirements of the load on the power source), a capacitance is needed that will be discharged in a time many times greater than t. Since the resistances of most loads are relatively small, a large capacitance is needed, therefore, in order to smooth out ripples at the output of the rectifier, they are used, they are also called polar or polarized.

Please note that it is highly not recommended to confuse the polarity of an electrolytic capacitor, because this is fraught with its failure and even explosion. Modern capacitors are protected from explosion - they have a stamping in the form of a cross on the top cover, along which the case will simply crack. But a jet of smoke will come out of the condenser, it will be bad if it gets into your eyes.

The capacitance is calculated based on what ripple factor needs to be provided. In simple terms, the ripple coefficient shows by what percentage the voltage sags (pulses).

C=3200*In/Un*Kp,

Where In is the load current, Un is the load voltage, Kn is the ripple factor.

For most types of equipment, the ripple factor is taken as 0.01-0.001. Additionally, it is desirable to install as large a capacitance as possible to filter out high-frequency interference.

How to make a power supply with your own hands?

The simplest DC power supply consists of three elements:

1. Transformer;

3. Capacitor.

This is an unregulated DC power supply with a smoothing capacitor. The voltage at its output is greater than the alternating voltage of the secondary winding. This means that if you have a 220/12 transformer (primary at 220V and secondary at 12V), then you will get 15-17V DC at the output. This value depends on the capacitance of the smoothing capacitor. This circuit can be used to power any load, if it does not matter to it that the voltage can “float” with changes in the mains voltage.

A capacitor has two main characteristics - capacitance and voltage. We figured out how to select the capacitance, but not with the selection of voltage. The voltage of the capacitor must exceed the amplitude voltage at the output of the rectifier by at least half. If the actual voltage on the capacitor plates exceeds the nominal voltage, there is a high probability of its failure.

Old Soviet capacitors were made with a good voltage margin, but now everyone uses cheap electrolytes from China, where at best there is a small margin, and at worst, it will not withstand the specified nominal voltage. So don't skimp on reliability.

A stabilized power supply differs from the previous one only in the presence of a voltage (or current) stabilizer. The simplest option is to use L78xx or others, such as the domestic ROOL.

So you can get any voltage, the only condition when using such stabilizers is that the voltage to the stabilizer must exceed the stabilized (output) value by at least 1.5V. Consider what is written in the 12V datasheet of the L7812 stabilizer:

The input voltage should not exceed 35V, for stabilizers from 5 to 12V, and 40V for stabilizers at 20-24V.

The input voltage should exceed the output voltage by 2-2.5V.

Those. for a stabilized 12V power supply with an L7812 series stabilizer, it is necessary that the rectified voltage lies within 14.5-35V to avoid drawdowns, it would be an ideal solution to use a transformer with a 12V secondary winding.

But the output current is quite modest - only 1.5A, it can be amplified using a pass transistor. If you have , you can use this scheme:

It shows only the connection of a linear stabilizer. The "left" part of the circuit with a transformer and a rectifier is omitted.

If you have NPN transistors like KT803 / KT805 / KT808, then this one will do:

It is worth noting that in the second circuit, the output voltage will be less than the stabilization voltage by 0.6V - this is a drop at the emitter-base junction, we wrote more about this. To compensate for this drop, a diode D1 was introduced into the circuit.

It is possible to install two linear stabilizers in parallel, but it is not necessary! Due to possible deviations in manufacturing, the load will be unevenly distributed and one of them may burn out because of this.

Install both the transistor and the linear regulator on a heatsink, preferably on separate heatsinks. They get very hot.

Regulated power supplies

The simplest adjustable power supply can be made with an adjustable linear stabilizer LM317, its current is also up to 1.5 A, you can amplify the circuit with a pass transistor, as described above.

Here is a more visual diagram for assembling an adjustable power supply.

With a thyristor regulator in the primary winding, essentially the same regulated power supply.

By the way, a similar scheme regulates the welding current:

Conclusion

A rectifier is used in power supplies to produce direct current from alternating current. Without his participation, it will not be possible to power a DC load, such as an LED strip or a radio.

Also used in a variety of car battery chargers, there are a number of circuits using a transformer with a group of taps from the primary winding, which are switched by a jack switch, and only a diode bridge is installed in the secondary winding. The switch is installed on the high voltage side, since the current is several times lower there and its contacts will not burn from this.

According to the diagrams from the article, you can assemble the simplest power supply both for constant work with some kind of device, and for testing your electronic homemade products.

The circuits do not have high efficiency, but they produce a stabilized voltage without much ripple, you should check the capacitance of the capacitors and calculate for a specific load. They are perfect for low-power audio amplifiers, and will not create additional background. An adjustable power supply will be useful for motorists and auto electricians to test the generator voltage regulator relay.

An adjustable power supply is used in all areas of electronics, and if it is improved with short-circuit protection or a current stabilizer on two transistors, then you will get an almost full-fledged laboratory power supply.

One of the most common current converters are AC to pulsating (constant in the direction of movement of carriers, but variable in instantaneous value) current. They have a very wide application. Conventionally, they can be divided into low-power rectifiers (up to several hundred watts and high-power rectifiers (kilowatts and more)).

Its main part is the rectifying device B, formed from diodes combined in a special way. It is here that the conversion of alternating current to pulsating direct current occurs. Alternating voltage is supplied to the rectifying device through the transformer Tr. In some cases, there may not be a transformer (if the voltage of the power network corresponds to that which is necessary for the operation of the rectifier). The transformer (if any) in most also has features in the connection of its windings. The pulsating current, as a rule, is not constant in magnitude at every instant of time, and when it is necessary to have a smoother value than that obtained after the rectifier, F filters are used. If necessary, the rectifier is supplemented with a voltage or current stabilizer St, which maintains them at a constant level, if the parameters of the power network change for various reasons. The block diagram is completed by the load H, which significantly affects the operation of the entire device and is therefore considered an integral part of the entire converter.

The rectifier itself is that part of it, which is circled in the figure above by a dotted line and consists of a transformer and a rectifier.

This subsection deals with low power rectifiers, which are necessary to provide constant voltage to all devices in the fields of control, regulation, current amplifiers, low power generators, and so on. As a rule, they are powered by a single-phase AC voltage of 220 or 380 V with a frequency of 50 Hz.

Zero rectification circuit

It is advisable to consider the principle of operation of the simplest single-phase current rectifier on the so-called zero circuit. Although it is now relatively rare (which will be discussed later), knowledge of the physical processes that occur in this scheme is very important for understanding further material.

The null circuit looks like this:

The transformer Tp has two windings on the secondary side, connected in series in such a way that, with respect to the midpoint a voltage at the free ends of the windings in and With equal in magnitude but opposite in phase. The rectifying device is formed by two diodes D1 and D2, which are connected together by their cathodes, while each anode is connected to a respective winding. The load Zn is connected between the cathodes of the diodes and the point of the transformer.

Consider how a pulsating voltage occurs on a load. First, we will consider the load as a purely active resistance, Z n \u003d R n. When the voltage in the windings changes according to a sinusoidal law, then in that half-cycle, when a positive potential is applied to the anode of the diode, a direct current will flow. Since the voltage across the diode is a fraction of a volt, we will neglect it. Then the entire positive half-wave of the alternating voltage will be applied simply to the load R n. When the voltage is applied negatively to the anode, there will be no current (we will also neglect the small reverse current of the diode). Thus, only a positive half-wave of alternating voltage will reach the load during half the period. The second half of the period will be free of current.

The secondary windings are connected in antiphase, the load is common for both windings, thus, at the time when the current will pass in one of them (for example, in the upper one), the other will be free from it and vice versa.

Therefore, in the load, each half-cycle will be filled with a half-wave of alternating voltage:

And the rectified voltage U d will have the form of identical half-waves, which are repeated with a period half as long as the period of the alternating voltage in the power supply (2π radians). For generalization, which will be convenient, we will further assume that the period of change of the rectified voltage is less than 2π in m times and equals 2π/ m(in our case m-2). If the load is active resistance R n, then the current in it i d will repeat the voltage curve.

The considered circuit will have the disadvantage that in the secondary windings, in comparison with the primary, there are significant current ripples, because these windings work in turn. Since they are wound on one core, the magnetic flux in the latter will be variable, therefore, in the primary winding, the current will also be variable, having both positive and negative half-waves. As is known from the course of electrical engineering, the effective and average values ​​​​of current or voltage are the same only for direct current. The larger the ripple, the larger the effective value will be relative to the average. Therefore, the power of both sides of the transformer will not be the same. However, there is only one transformer, and the amount of iron for its core should be chosen based on a single power value.

Therefore, the concept of the typical power of the transformer was conditionally introduced, which is equal to the average power of both sides:

Rectifier bridge or Graetz circuit

This drawback can be corrected using a rectifying device in the form of a so-called bridge (Graetz scheme):

In this case, the first half-cycles will work, for example, diodes D2 and D4, and the second half-cycles - D1 and D3. On the load each time there will be a full half-wave of the secondary voltage:

The bridge circuit also has a less complex, lighter and cheaper transformer. it has several other advantages.

It is interesting that this circuit appeared historically earlier than zero, however, it did not receive distribution, because it had, firstly, four diodes instead of two. However, the main thing was not their number, but the fact that during operation every half-cycle, the current passes through two series-connected diodes, on which a double voltage falls. At that time, there were no semiconductor diodes, and vacuum or mercury diodes had a significant voltage drop during the passage of direct current, which significantly reduced the efficiency. It turned out that a more complex zero-circuit transformer, but with one diode in the current rectification circle, is economically more profitable than a bridge circuit with a double number of diodes and double the energy consumption for them. And only the appearance of relatively cheap semiconductor diodes with a very small forward voltage drop made it possible to turn to bridge circuits, which have now practically replaced the zero one (if you wish, you can see the manifestation of one of the dialectical laws - spiral development).

Basic ratios for the rectifier

Let us derive some important formulas that describe the processes that exist in this scheme. We will assume that the given values ​​are the average values ​​of the voltage on the load U d and the average value of the current in it I d .

Let's remember this expression for later. In our case m=2 and . Since U d is considered given, then

From the previous expression we have:

This coefficient determines the ratio of the supply network to the voltage on the secondary side winding:

The effective value of the current of the secondary winding

The current of the secondary winding is at the same time the current in the load. Since the load is purely active and the current in it repeats the shape of the pulsating voltage, then between its average value and its effective value there is the same dependence as for voltages, that is

The effective value of the current of the primary winding

The current in the primary winding repeats, taking into account n, the current of the secondary winding:

Transformer power

The capacities of the primary and secondary sides of the transformer in this circuit are the same, therefore:

Rectified voltage ripple

The pulsating voltage consists of the average value U d and an infinite number of harmonic components, the amplitudes of which can be determined by the Fourier formulas. If you choose the origin of coordinates as in the figure, then only cosine harmonics will be present in the harmonic composition (because the curve is symmetrical about the coordinate axis). The amplitude of the k-th harmonic is determined by the formula:

Where: l – half-period π/m;

The first harmonic U (1) m will have the largest amplitude, so we will only determine it, assuming that k=1:

Replacing we get:

The ratio of the first harmonic to the mean value is called the ripple factor:

Let's remember this formula for the future, and now we note that in our case, for m - 2, q - 2/3. These are large ripples - the amplitude of the first harmonic is 67% of the average value of the rectified voltage.

Average diode current

As we have already seen, the diodes work in turn - each of them conducts on average half of the total current that is in the load. Therefore, each of the diodes must be rated for current I in \u003d I d / 2

The highest reverse voltage on the diode

While the diode B1 conducts it can be considered closed, and then the voltage of the secondary winding will be applied to the diode B2 in the opposite direction. Therefore, each of the diodes must be designed for its amplitude value:

An electric current rectifier is a special device that is designed to obtain an output direct electric current from an input alternating current. Most rectifiers accept filters to smooth out the unidirectional ripple voltages and currents they produce.

Why you need a rectifier

The main disadvantage of galvanic cells that power many electrical appliances is their short service life. These inconveniences are especially noticeable if the load requires high currents. The electric current of an industrial power supply is best suited for powering electronic consumers. But you cannot connect a device intended for battery power directly to the network. It is necessary to convert the AC voltage to DC. Therefore, it is very useful to understand how to make a rectifier. To power the equipment, voltages less than the mains voltage are usually used. This is achieved through the use of a power transformer. Then the AC voltage is converted to DC. The permanent is obtained in two stages:

first, the variable image is converted into a pulsating image, that is, changing from zero in only one direction. The filter then converts the ripple voltage to DC.

Types of rectifiers

• Half-wave - a rectifier consisting of a capacitor and one semiconductor diode. Its design is very simple. It has a low efficiency, therefore it is used only to power low-power consumers.
• Full-wave - rectifier, consists of transformer windings, a capacitor and four diodes. Usually it is performed on a bridge circuit. It is used to power radio equipment.

Diodes are selected according to the following parameters: the value of the direct (rectified) current at the output of the rectifier and the value of the reverse voltage. These parameters are taken from directories. The rectified current cannot be less than the current drawn by the load. The diodes will not heat up if the rectified current is 2 times greater than the current required by the consumer. The reverse voltage consists of the voltage of the secondary winding and the voltage across the capacitor.

Making a rectifier

• Take a half-liter glass jar or glass, plates with an area of ​​40x100 mm - aluminum and copper, a rubber pipe with a diameter of 2 cm. Cut off 2 cm from the pipe and put it on an aluminum plate. This is done because the electrolyte strongly corrodes aluminum during operation. If you put rubber on it, it will protect the metal from corrosion, and the rectifier will last much longer.
• We will use a solution of baking soda as an electrolyte. It will need 5-7 grams per 100 ml of water. For the positive pole we will take aluminum, and for the negative - lead. The current will flow if you connect the rectifier with a lead plate to the network. But the current will only flow in one direction. The aluminum plate will be a permanent positive voltage pole.
• If an aluminum plate is included in the network, then the lead plate will act as a negative pole. This will be a half-wave rectifier, through which only one half-cycle current flows. In this case, current will flow in the positive direction.
• Full-wave rectifiers are used to fully utilize the voltage. The number of elements that they consist of depends on the required amount of rectified current. They are connected to both phases of the mains.
• Use fuses when plugging in the appliance. With the help of a rheostat, you can adjust the voltage.

Rectifier calculation

• Let's determine the alternating voltage of the secondary winding of the transformer:

  Un - constant load voltage, V;

  B is a coefficient that depends on the load current.

• Determine the maximum current flowing through the diodes:

  Id \u003d 0.5 C In,

  Id - current flowing through the diode,

  In - the highest value of the current,

  C - coefficient depending on the load.

• Let's define the reverse voltage:

  Uar = 1.5 Un,

  Uobr - reverse voltage,

  Un - load voltage.

• We select diodes, in which the value of the rectified current and reverse voltage is higher than the calculated ones.
• Find the value of the capacitance of the capacitor:

  SF \u003d 3200 In / Un Kp,

  Cf - capacitance of the filter capacitor,

  In - maximum load current.;

  Un - voltage on the load,

  Kp - pulsation coefficient (10 -5 -10-2).

Welding rectifier

The VD welding rectifier is used as a power source when welding with any electrodes. It is used to eliminate intercurrent interruptions during welding, which results in a high-quality welding seam.

• The rectifier is universal and can be used in the most difficult working conditions.
• Insensitive to temperature fluctuations, changes in humidity, voltage drop in the network, dustiness.
• Reliable
• durable
• It has a low cost and is able to replace expensive installations.

Now you know everything about who wants to know how to make a straightener at home. This will allow you to solve problems due to its absence on your own and at the lowest cost.