Diode bridge microcircuit. Diode bridge - how does it work? Diode bridge operating principles
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A diode bridge is an elementary electronic circuit used to convert alternating current into direct current. It is the most common radio component, without which no rectifier power supply can do.
Structural types of semiconductor bridges
The diode bridge can be assembled from individual semiconductor elements or made as a monolithic assembly. The convenience of the latter is the ease of installation on a printed circuit board and small overall dimensions. The parameters of the elements in it are carefully selected at the factory, which eliminates their scatter and skewed operating temperature conditions, however, if one element of such a circuit fails, the entire assembly must be replaced. If you are not satisfied with ready-made diode assemblies, you can assemble this simple circuit yourself. The elements can be mounted on a printed circuit board, but most often it is mounted directly on the transformer. If a high-power diode bridge is required, one should not forget that the diodes can get very hot; in this case, they are mounted on an aluminum radiator to remove excess heat. Diodes for the bridge must be selected in accordance with the required power of the circuit. The load value can be calculated using Ohm's law; for this, the maximum current must be multiplied by the maximum voltage. The result should be multiplied by two so that the circuit has a margin of safety. When assembling a diode bridge, you should remember that only 70 percent of the rated current flows through each diode.
Principle of operation
An alternating voltage is supplied to the input of the circuit; in the first half-cycle, the electric current passes through two diodes, the second pair of diodes is closed. In the second half-cycle, the current passes through the second pair of diodes, and the first one is closed. Thus, the output of the diode bridge produces a pulsating voltage, the frequency of which is twice as high as the input. To smooth out the ripple of the output voltage, a capacitor is placed at the bridge output.
Application area
Diode bridges are widely used in industrial equipment (power supplies, chargers, motor control circuits, power regulators), in power supplies for household appliances (TVs, refrigerators, vacuum cleaners, computers, power tools, etc.), in lighting devices (fluorescent lamps, in solar battery modules), in electricity meters.
Diode bridge for welding machine
Such a rectifier must be assembled on the basis of powerful diodes (for example, type B200 with a maximum current of 200 amperes is suitable). They have substantial overall dimensions; their body must be placed on an aluminum radiator to remove heat energy. The housing of such diodes is energized, and so is the radiator, so installation must take these features into account. As a result, the design of the welding machine increases in size. However, there are ready-made assemblies on sale, integrated into one case. The dimensions of such a bridge are comparable to a matchbox or one B200 type diode without a radiator. The maximum current is 30-50 amperes, and the price is significantly lower than the diodes described above.
Generator diode bridge
This rectifier unit, consisting of three parallel half-bridges, is assembled on six diodes (circuit of the Soviet scientist A. N. Larionov). This circuit converts three-phase alternating voltage into direct voltage.
A diode is a semiconductor unit with different conductivities determined by the applied voltage. It has two terminals: cathode and anode. If direct voltage is applied, that is, the potential at the anode is positive compared to the cathode, the unit is open.
If the voltage is negative, it closes. This feature has found application in electrical engineering: the diode bridge is actively used in welding to rectify alternating current and improve the quality of welding operations.
How to make a straightener with your own hands?
If the craftsman has the necessary components, it is quite possible to make a homemade welding rectifier. Provided that all recommendations of specialists are followed, it is guaranteed to provide the process of manual arc welding with direct current, but it will be necessary to use a coated electrode.
It is also permissible to use wire without coating, but only if you have extensive experience in welding matters. For an inexperienced welder, it will be almost impossible to cope with it.
Diode bridge for welding machine.
Coating when melting the electrode prevents the penetration of air components into the molten metal of the welded joint. Without it, contact of molten metal with nitrogen and oxygen will reduce the strength properties of the seam, making it brittle and porous.
First you will need to select or wind up a step-down transformer with your own hands with the required parameters. Assemble the transformer before connecting the diode bridge.
If you choose to manufacture the device yourself, it is important to correctly calculate its elements, including:
- magnetic circuit parameters;
- current number of turns;
- cross-sectional dimensions of busbars and wires.
On a note! Calculations for the manufacture of transformers are carried out using a unified methodology, so this task does not present any difficulties even for an inexperienced welder with school knowledge of electricity.
The work cannot be done without LEDs: they are needed as current conductors in one single direction. The simplest diode, created using a bridge circuit, is mounted on a radiator for the purpose of heat exchange and cooling.
Powerful diodes for a welding machine, like the VD-200, emit a fairly large amount of thermal energy during operation. To ensure a falling current characteristic, a choke will need to be connected in series to the circuit.
Active variable resistance in such a circuit will provide the welder with the ability to smoothly regulate the welding current. Next, one pole needs to be connected to the welded wire, and the second to the work object.
An electrolytic capacitor in the circuit is needed as a smoothing filter to reduce ripple.
It is not difficult to wind a rheostat on your own, but for such a task you will need a ceramic core and nickel or nichrome wire. The actual wire diameter will be determined by the amount of adjustable current in the welding operation.
The calculation of the rheostat resistance must be carried out taking into account the specific resistance of the electrode, its cross-section and total length.
Electrical circuit for welding with a diode bridge.
The current adjustment step for welding depends on the diameter of the turns. If you correctly assemble the listed parts into a single unit, the welding process will be accompanied by direct current. It would not be superfluous to install a resistor that prevents short circuits during operation.
It can occur when the wire touches metal without igniting the arc. If at this time there is no resistance on the capacitor, it will instantly discharge, a click will occur, the electrode will collapse or stick to the metal.
If you have a resistor, you can smooth out the discharges on the capacitor and make ignition of the electrode easier and softer. Making a device for rectifying weld current with your own hands will allow you to create the most accurate and durable welds. .
Results
A diode bridge for a welding machine converts alternating current into direct current, which improves the quality of welded joints. Such a device can be purchased ready-made or created with your own hands, following the advice outlined in the article.
Everyone knows that household networks operate with alternating electrical voltage with an amplitude of 220 Volts. However, certain examples of modern electronic devices (your mobile, for example) require constant or rectified voltage. A transformer will help to lower it to the required value, and to rectify the variable component you definitely need a diode bridge (photo below).
The rectifier devices discussed here are part of most electronic devices that require direct current for normal operation (from welding units to miniature power supplies).
This review provides a detailed description of the circuit and operating principle of a classic rectifier diode bridge. It will also discuss the issue of how to make a diode bridge with your own hands.
Composition of the rectifier module
We advise anyone who would like to become more familiar with what a rectifier is to take a short historical excursion. Let's start with the fact that the progenitor of the rectifier bridge is considered to be a circuit invented by the German scientist L. Graetz, assembled on the basis of 4 elements (diode assemblies).
Note! These devices are better known professionally as Graetz bridges or full-wave rectifiers.
Such assemblies of four diodes eventually became known as bridge circuits, which began to be used as universal rectifier modules.
The classic diode bridge, the circuit of which is presented below, contains rectifier diodes connected in a certain way.
From the above figure it can be seen that the bridge circuit includes four semiconductor elements (diodes), the connection order of which corresponds to the back-to-back principle. One pair of these devices is connected in the conducting direction, and the other has a reverse connection.
Operating principle
To understand how a diode bridge works, let's first get acquainted with the very essence of the effect of rectifying alternating voltages.
The operating principle of a classic rectifier bridge based on four diodes is as follows:
- When a positive wave of mains voltage arrives at the positive terminal of a diode connected to the load, a current signal of the same polarity passes through it;
- At the same time, no current passes through another diode from the pair in the bridge, the connection of which is reversed to the first one, since its junction is closed by a potential of opposite sign;
- But a half-wave of reverse polarity passes through it in due course, forming a current pulse at the output in the same direction as in the first case.
We can say that for each half-wave of the input voltage there is a diode that generates (after connecting it to the load) a current in the same direction.
According to the theory of electrical engineering, the effect observed in this case means its straightening.
The operating principle of the diode bridge discussed above allows us to draw the following conclusions:
- As a result of the described process, current half-waves are formed at the output of the rectifier, having the same positive polarity (figure below);
- If you look at the signal at the bridge load with an oscilloscope, you can see a pulsating direct current in the form of half-waves of the same polarity repeating at a frequency of 100 Hz;
- This value (100 Hz) is obtained by doubling the mains frequency of 50 Hz at the output of the diode rectifier;
- The doubling of the frequency is explained by the fact that each half-wave of the input signal is processed by its own diode (more precisely, a pair of them).
Additional Information. After filtering the resulting ripples after rectification (this is done using electrolytic capacitors), a rectified voltage is obtained at the load.
Sometimes, in order to record its presence at the output of the circuit, the latter is supplemented with LED indication. When the LED connected through the limiting resistor lights up, you can be sure that a constant potential has appeared at the output.
For a three-phase supply line, special types of bridge circuits must be used, selected and included taking into account the characteristics of the power supply of power plants. We send everyone who wants to get acquainted with how a three-phase rectifier bridge works to the following address http://hardelectronics.ru/shema-diodnogo-mosta.html.
Making your own bridge
Before soldering the diode bridge, be sure to check the serviceability of each of the diodes included in its composition. We also draw attention to the fact that it can be assembled from individual (discrete) elements or taken in the form of a solid housing assembly with four output contacts.
Each of these bridge options has its own pros and cons.
Important! If one diode in a monolithic assembly fails, the entire assembly will have to be replaced (despite the fact that the three remaining elements may be serviceable).
But such a module is very convenient when soldering a rectifier circuit, when you need to connect a diode bridge to an alternating voltage source on one side and to a load on the other.
In a situation where we assemble a diode bridge with our own hands from discrete elements, it is always possible to replace each of them independently of the others. But with this approach, the manufacturing process itself becomes more complicated, for which all four of its components will have to be soldered.
After completing the self-assembly of the rectifier product, all that remains is to connect the diode bridge to a transformer or other source from which alternating voltage is supplied.
In the final part of the review, devoted to how the diode bridge circuit works, we draw attention to the fact that when assembling it yourself, you should study the parameters of the elements included in its composition. Knowledge of this data will allow you to correctly calculate the permissible load currents, and also be sure that the diode assembly will not fail.
Video
There is a bridge across a river, across a ravine, and also across a road. But have you ever heard the phrase “diode bridge”? What kind of bridge is this? But we will try to find an answer to this question.
The phrase "diode bridge" is derived from the word "diode". It turns out that the diode bridge must consist of diodes. But if there are diodes in the diode bridge, it means that the diode will pass in one direction, but not in the other. We used this property of diodes to determine their performance. If you don't remember how we did it, then this is the place for you. Therefore, a bridge of diodes is used to obtain a constant voltage from an alternating voltage.
And here is the diagram of the diode bridge:
Sometimes in diagrams it is designated as follows:
As we can see, the circuit consists of four diodes. But in order for the diode bridge circuit to work, we must connect the diodes correctly and apply alternating voltage to them correctly. On the left we see two "~" icons. We apply alternating voltage to these two terminals, and remove constant voltage from the other two terminals: plus and minus.
In order to turn alternating voltage into direct voltage, you can use one diode for rectification, but it is not advisable. Let's look at the picture:
AC voltage changes over time. The diode passes voltage through itself only when the voltage is above zero, and when it drops below zero, the diode turns off. I think everything is elementary and simple. The diode cuts off the negative half-wave, leaving only the positive half-wave, which is what we see in the figure above. And the beauty of this simple circuit is that we get constant voltage from alternating voltage. The whole problem is that we lose half the AC power. The diode stupidly cuts it off.
To correct this situation, a diode bridge circuit was developed. The diode bridge “flips” the negative half-wave, turning it into a positive half-wave. This way we save power. Wonderful isn't it?
At the output of the diode bridge we have a constant pulsating voltage with a frequency twice as high as the mains frequency: 100 Hz.
I think there is no need to write how the circuit works, you won’t need it anyway, the main thing is to remember where the alternating voltage goes and where the constant pulsating voltage comes from.
Let's take a practical look at how a diode and diode bridge work.
First, let's take a diode.
I unsoldered it from the computer's power supply. The cathode can be easily identified by its stripe. Almost all manufacturers show the cathode with a stripe or dot.
To make our experiments safe, I took a step-down transformer, which transforms 220 Volts into 12 Volts. For those who don’t know how he does this, you can read the article transformer design.
We connect 220 Volts to the primary winding, and remove 12 Volts from the secondary winding. The cartoon shows a little more, since no load is connected to the secondary winding. The transformer operates at so-called "idle speed".
Let's look at the oscillogram that comes from the secondary winding of the trance. The maximum voltage amplitude is easy to calculate. If you don’t remember how to calculate, you can look at the article Oscilloscope. Operating Basics. 3.3x5= 16.5V is the maximum voltage value. And if we divide the maximum amplitude value by the root of two, we get somewhere around 11.8 Volts. This is the effective voltage value. Oscill is not lying, everything is OK.
Once again, I could have used 220 Volts, but 220 Volts is no joke, so I lowered the alternating voltage.
Solder our diode to one end of the secondary winding of the trans.
We cling again with oscillation probes
Let's look at the oscillations
Where is the bottom of the image? The diode cut it off. The diode left only the upper part, that is, the one that is positive. And since he cut off the lower part, he consequently cut off the power.
We find three more such diodes and solder the diode bridge.
We cling to the secondary winding of the trans according to the diode bridge circuit.
From the other two ends we remove the constant pulsating voltage with oscillator probes and look at the oscillators.
Well, now everything is in order, and we haven’t lost any power :-).
In order not to mess with diodes, the developers placed all four diodes in one housing. The result is a very compact and convenient diode bridge. I think you can guess which is imported and which is Soviet))).
And here is the Soviet one:
How did you guess? :-) For example, on a Soviet diode bridge, the contacts to which an alternating voltage must be applied are shown (with the "~" symbol), and the contacts from which a constant pulsating voltage must be removed ("+" and "-") are shown.
Let's check the imported diode bridge. To do this, we connect two of its contacts to the variable, and from the other two contacts we take readings on the oscillator.
And here is the oscillogram:
This means that the imported diode bridge works just fine.
In conclusion, I would like to add that the diode bridge is used in almost all radio equipment that consumes voltage from the network, be it a simple TV or even a cell phone charger. The diode bridge is checked for serviceability of all its diodes.
So, my dears, we have assembled our scheme and it’s time to check it, test it and enjoy this happiness. Next up is connecting the circuit to the power source. Let's get started. We won’t dwell on batteries, accumulators and other power supplies; we’ll move straight to mains power supplies. Here we will look at existing rectification schemes, how they work and what they can do. For experiments we will need single-phase (at home from an outlet) voltage and the corresponding parts. Three-phase rectifiers are used in industry, we will not consider them either. If you grow up to be an electrician, then you're welcome.
The power supply consists of several most important parts: Mains transformer - indicated in the diagram as similar to the one in the figure,
Rectifier - its designation may vary. The rectifier consists of one, two or four diodes, depending on which rectifier. Now we'll figure it out.
a) - a simple diode.
b) - diode bridge. Consists of four diodes connected as in the figure.
c) - the same diode bridge, only drawn simpler for brevity. The contact assignments are the same as for the bridge under letter b).
Filter capacitor. This thing is unchanged in both time and space, and is designated as follows:
There are many designations for a capacitor, as many as there are designation systems in the world. But in general they are all similar. Let's not get confused. And for clarity, let’s draw a load, denote it as Rl - load resistance. This is our scheme. We will also outline the contacts of the power source to which we will connect this load.
Next - a couple of postulates.
- The output voltage is defined as Uconst = U*1.41. That is, if we have 10 volts of alternating voltage on the winding, then on the capacitor and on the load we will get 14.1 V. Like that.
- Under load, the voltage sags a little, and how much depends on the design of the transformer, its power and the capacitance of the capacitor.
- Rectifier diodes should be 1.5-2 times more current than required. For stock. If the diode is intended for installation on a radiator (with a nut or bolt hole), then at a current of more than 2-3A it must be installed on the radiator.
Let me also remind you what bipolar voltage is. If someone has forgotten. We take two batteries and connect them in series. The middle point, that is, the point where the batteries are connected, will be called the common point. It is popularly known as ground, ground, body, common wire. The bourgeoisie call it GND (ground), often referred to as 0V (zero volts). Voltmeters and oscilloscopes are connected to this wire; relative to it, input signals are supplied to the circuits and output signals are taken. That's why its name is common wire. So, if we connect the tester with the black wire to this point and measure the voltage on the batteries, then the tester will show plus 1.5 volts on one battery, and minus 1.5 volts on the other. This voltage +/-1.5V is called bipolar. Both polarities, that is, plus and minus, must be equal. That is, +/-12, +/-36V, +/-50, etc. A sign of bipolar voltage is if three wires go from the circuit to the power supply (plus, common, minus). But this is not always the case - if we see that the circuit is powered by a voltage of +12 and -5, then such power is called two-level, but there will still be three wires to the power supply. Well, if as many as four voltages are supplied to the circuit, for example +/-15 and +/-36, then we will simply call this power supply - bipolar two-level.
Well, now to the point.
1. Bridge rectification circuit.
The most common scheme. Allows you to obtain unipolar voltage from one winding of the transformer. The circuit has minimal voltage ripple and is simple in design.
2. Half-wave circuit.
Just like the pavement, it prepares us a unipolar voltage from one winding of the transformer. The only difference is that this circuit has double the ripple compared to a bridge circuit, but one diode instead of four greatly simplifies the circuit. It is used for small load currents, and only with a transformer that is much larger than the load power, because such a rectifier causes one-sided magnetization reversal of the transformer.
3. Full-wave with midpoint.
Two diodes and two windings (or one winding with a midpoint) will supply us with a low-ripple voltage, plus we will get lower losses compared to a bridge circuit, because we have 2 diodes instead of four.
4. Bridge circuit of a bipolar rectifier.
For many, this is a sore subject. We have two windings (or one with a midpoint), we remove two identical voltages from them. They will be equal, the ripples will be small, since the circuit is a bridge circuit, the voltage on each capacitor is calculated as the voltage on each winding multiplied by the root of two - everything is as usual. A wire from the midpoint of the windings equalizes the voltage on the capacitors if the positive and negative loads are different.
5. Voltage doubling circuit.
These are two half-wave circuits, but with diodes connected in different ways. It is used if we need to get double the voltage. The voltage on each capacitor will be determined by our formula, and the total voltage on them will be doubled. Like the half-wave circuit, this one also has large ripples. You can see a bipolar output in it - if you call the middle point of the capacitors ground, then it turns out like in the case of batteries, take a closer look. But you can’t get a lot of power out of such a circuit.
6. Obtaining different polarity voltage from two rectifiers.
It is not at all necessary that these are the same power supplies - they can be either different in voltage or different in power. For example, if our circuit consumes 1A at +12 volts, and 0.5A at -5 volts, then we need two power supplies - +12V 1A and -5V 0.5A. You can also connect two identical rectifiers to obtain a bipolar voltage, for example, to power an amplifier.
7. Parallel connection of identical rectifiers.
It gives us the same voltage, only with double the current. If we connect two rectifiers, then we will have a double increase in current, three - triple, etc.
Well, if everything is clear to you, my dears, then I’ll probably give you some homework. The formula for calculating the filter capacitance for a full-wave rectifier is: 
For a half-wave rectifier, the formula is slightly different: 
The two in the denominator is the number of rectification “cycles”. For a three-phase rectifier, the denominator will be three.
In all formulas, variables are named like this:
Cf - filter capacitor capacity, µF
Po - output power, W
U - output rectified voltage, V
f - frequency of alternating voltage, Hz
dU - pulsation range, V
For reference, permissible ripples:
Microphone amplifiers - 0.001...0.01%
Digital technology - ripple 0.1...1%
Power amplifiers - ripple of a loaded power supply 1...10% depending on the quality of the amplifier.
These two formulas are valid for voltage rectifiers with a frequency of up to 30 kHz. At higher frequencies, electrolytic capacitors lose their efficiency, and the rectifier is designed a little differently. But that is another topic.
In many electronic devices operating at alternating current of 220 volts, diode bridges are installed. The 12 volt diode bridge circuit allows you to effectively perform the function of rectifying alternating current. This is due to the fact that most devices use direct current to operate.
How does a diode bridge work?
An alternating current having a certain varying frequency is supplied to the input contacts of the bridge. At the outputs with positive and negative values, a unipolar current is generated, which has increased ripple, significantly exceeding the frequency of the current supplied to the input.
The pulsations that appear must be removed, otherwise the electronic circuit will not be able to work normally. Therefore, the circuit contains special filters, which are electrolytic filters with a large capacity.
The bridge assembly itself consists of four diodes with the same parameters. They are connected into a common circuit and are housed in a common housing.
The diode bridge has four terminals. Two of them are connected to alternating voltage, and the other two are the positive and negative terminals of the pulsating rectified voltage.
A rectifier bridge in the form of a diode assembly has significant technological advantages. Thus, one monolithic part is installed on the printed circuit board at once. During operation, all diodes are provided with the same thermal conditions. The cost of the overall assembly is lower than four diodes separately. However, this part has a serious drawback. If at least one diode fails, the entire assembly must be replaced. If desired, any general diagram can be replaced by four separate parts.
Application of diode bridges
Any devices and electronics that are powered by alternating electric current have a 12-volt diode bridge circuit. It is used not only in transformers, but also in pulse rectifiers. The most typical switching unit is the computer power supply.
In addition, diode bridges are used in compact fluorescent lamps or energy-saving lamps. They give a very good effect when used in electronic ballasts. They are widely used in all models of modern devices.
How to make a diode bridge
A diode bridge will help convert alternating current into direct current - the diagram and principle of operation of this device are given below. In a conventional lighting circuit, an alternating current flows, which changes its magnitude and direction 50 times within one second. Its transformation into a permanent one is a fairly common need.
Operating principle of a semiconductor diode
Rice. 1The name of the described device clearly indicates that this design consists of diodes - semiconductor devices that conduct electricity well in one direction and practically not conduct it in the opposite direction. An image of this device (VD1) on circuit diagrams is shown in Fig. 2c. When current flows through it in the forward direction - from the anode (left) to the cathode (right), its resistance is low. When the direction of current changes to the opposite direction, the resistance of the diode increases many times. In this case, a reverse current slightly different from zero flows through it.
Therefore, when an alternating voltage Uin (left graph) is applied to a circuit containing a diode, electricity flows through the load only during positive half-cycles when a positive voltage is applied to the anode. Negative half-cycles are “cut off”, and there is practically no current in the load resistance at this time.
Strictly speaking, the output voltage U out (right graph) is not constant, although it flows in one direction, but pulsating. It is easy to understand that the number of its pulses (pulsations) per second is 50. This is not always acceptable, but the ripples can be smoothed out if you connect a capacitor with a sufficiently large capacitance in parallel with the load. Charging during voltage pulses, in the intervals between them the capacitor is discharged into the load resistance. The pulsations are smoothed out, and the voltage becomes close to constant.
A rectifier manufactured in accordance with this circuit is called a half-wave rectifier, since it uses only one half-cycle of the rectified voltage. The most significant disadvantages of such a rectifier are the following:
- increased degree of ripple of rectified voltage;
- low efficiency;
- heavy weight of the transformer and its irrational use.
Therefore, such circuits are used only to power low-power devices. To correct this undesirable situation, full-wave rectifiers have been developed that convert negative half-waves into positive ones. This can be done in different ways, but the easiest way is to use a diode bridge.
Rice. 2
Diode bridge - a full-wave rectification circuit containing 4 diodes instead of one (Fig. 2c). In each half-cycle, two of them are open and allow electricity to flow in the forward direction, while the other two are closed and no current flows through them. During the positive half-cycle, positive voltage is applied to the anode VD1, and negative voltage is applied to the cathode VD3. As a result, both of these diodes are open, and VD2 and VD4 are closed.
During the negative half-cycle, positive voltage is applied to the anode VD2, and negative voltage is applied to the cathode VD4. These two diodes open, and those open during the previous half-cycle close. The current through the load resistance flows in the same direction. Compared to a half-wave rectifier, the number of ripples doubles. The result is a higher degree of smoothing with the same capacitance of the filter capacitor, increasing the efficiency of the transformer used in the rectifier.
A diode bridge can not only be assembled from individual elements, but also manufactured as a monolithic structure (diode assembly). It is easier to install, and the diodes are usually selected according to the parameters. It is also important that they operate in the same thermal conditions. The disadvantage of a diode bridge is the need to replace the entire assembly if even one diode fails.
The pulsating rectified current will be even closer to constant, which makes it possible to obtain a three-phase diode bridge. Its input is connected to a three-phase alternating current source (generator or transformer), and the output voltage is almost the same as constant, and it is even easier to smooth it out than after full-wave rectification.
Diode bridge rectifier
The circuit of a full-wave rectifier based on a diode bridge, suitable for DIY assembly, is shown in Fig. 3a. The voltage removed from the secondary step-down winding of transformer T is subject to rectification. To do this, you need to connect a diode bridge to the transformer.
The pulsating rectified voltage is smoothed out by an electrolytic capacitor C, which has a fairly large capacitance - usually on the order of several thousand microfarads. Resistor R acts as a rectifier load at idle. In this mode, capacitor C is charged to an amplitude value that is 1.4 (root of two) times higher than the effective voltage value taken from the secondary winding of the transformer.
As the load increases, the output voltage decreases. You can get rid of this drawback by connecting a simple transistor stabilizer to the rectifier output. In circuit diagrams, the image of a diode bridge is often simplified. In Fig. 3b shows how the corresponding fragment in Fig. 3 can also be depicted. 3a.
It should be noted that although the forward resistance of the diodes is small, it is nevertheless different from zero. For this reason, they heat up in accordance with the Joule-Lenz law, the more strongly, the greater the current flowing through the circuit. To prevent overheating, high-power diodes are often installed on heat sinks (radiators).
A diode bridge is an almost obligatory element of any electronic device powered by the network, be it a computer or a rectifier for charging a mobile phone.
The phrase “diode bridge” is derived from the word “diode”. Therefore, a diode bridge must consist of diodes, but they must be connected to each other in a certain sequence. We will discuss why this is important in this article.
Designation on the diagram
The diode bridge in the diagrams looks like this:
Sometimes in diagrams it is also designated as follows:
As we can see, the circuit consists of four diodes. In order for it to work correctly, we must connect the diodes correctly and correctly apply alternating voltage to them. On the left we see two “~” icons. We apply alternating voltage to these two terminals, and remove direct voltage from the other two terminals indicated by the “+” and “-“ signs. A diode bridge is also called a diode rectifier.
Principle of operation
To rectify AC voltage into DC, you can use one diode for rectification, but it is not advisable. Let's look at a picture of how it all will look:
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The diode cuts off the negative half-wave of the alternating voltage, leaving only the positive, which is what we see in the figure above. The beauty of this simple circuit is that we get constant voltage from alternating voltage. The problem lies in the fact that we are losing half the AC power. The diode cuts it off.
To correct this situation, great minds came up with a diode bridge circuit. The diode bridge “turns over” the negative half-wave, turning it into a positive half-wave, thereby saving power.
At the output of the diode bridge a constant pulsating voltage appears with a frequency of 100 Hertz. This is twice the network frequency.
Practical experiences
Let's start with a simple diode.
The cathode can be easily recognized by its silver stripe. Almost all manufacturers show the cathode with a stripe or dot.
To make our experiments safe, I took a step-down device, which makes 12V from 220V.
We connect 220 Volts to the primary winding, and remove 12 Volts from the secondary winding. showed a little more, since there is no load on the secondary winding. The transformer operates at so-called “idle speed”.
3.3x5=16.5V is the maximum voltage value. And if we divide the maximum amplitude value by the root of two, we get somewhere around 11.8 Volts. That's what it is . The oscilloscope does not lie, everything is OK.
Once again, I could have used 220 Volts, but 220 Volts is no joke, so I lowered the AC voltage.
Solder our diode to one end of the secondary winding of the transformer.
Let's grab the oscilloscope again
Let's look at the oscillogram
Where is the bottom of the image? The diode cut it off. He left only the upper part, that is, the one that is positive.
We find three more such diodes and solder them diode bridge.
We cling to the secondary winding of the transformer using a diode bridge circuit.
From the other two ends we remove the constant pulsating voltage with an oscilloscope probe and look at the oscillogram
Here, now it's order.
Types of diode bridges
In order not to bother with diodes, the developers placed all four diodes in one housing. The result is a very compact and convenient radio element - a diode bridge. I think you can guess which is imported and which is Soviet))).
For example, on the Soviet diode bridge the contacts to which you need to apply alternating voltage are shown with the “~” icon, and the contacts from which you need to remove the constant pulsating voltage are indicated by the “+” and “-“ icons.
There are many types of diode bridges in different housings
There is even a car diode bridge
There is also a diode bridge for three-phase voltage. It is assembled according to the so-called Larionov circuit and consists of 6 diodes:
Three-phase diode bridges are mainly used in power electronics.
As you may have noticed, such a three-phase rectifier has five terminals. Three outputs per phase and from the other two outputs we will remove a constant pulsating voltage.
How to check a diode bridge
1) The first method is the simplest. The diode bridge is checked by the integrity of all its diodes. To do this, we test each diode with a multimeter and look at the integrity of each diode. How to do this, read
2) The second method is 100% correct. But this will require an oscilloscope or a step-down transformer. Let's check the imported diode bridge. To do this, we connect two of its contacts to alternating voltage with the “~” symbols, and from the other two contacts, with “+” and “-” we take readings using an oscilloscope.
Let's look at the oscillogram
This means that the imported diode bridge is working.
Summary
A diode bridge (rectifier) is used to convert alternating current to direct current.
A diode bridge is used in almost all radio equipment that “eats” voltage from an alternating network, be it a simple TV or even a cell phone charger.