Diode bridge - simplest scheme, which converts alternating current to direct current. It is used in almost all modern electronics, so a competent master must understandand be able to repair it. In Russian sockets, the current frequency is 50 Hertz, and to equalize it for the operation of the equipment, this simple device is used.
Let's see how it works this device. It is assembled from diodes - elements that pass current in one direction. Modern diodes are semiconductor devices small size- in this article we will not analyze their features and markings, but will only talk about How does a diode bridge work?
Composition and principle of operation of the diode
The diode has two contacts - anode and cathode. Current flows from the anode to the cathode with virtually zero resistance. But if the situation changes and current is supplied to the cathode, then the opposite resistance prevents it from breaking through the element (the current is practically zero and in most cases can be neglected). You can see the operation diagram in the figure above.
Simplified diagram
You already know what a diode bridge is , so let’s look at the simplest principle of its operation. When alternating current enters the anode Uin, it passes through the positive half-cycles, while the negative ones are completely removed. In this case, the output voltage, designated with right side under the abbreviation Uout, is not rectified, although it runs in one direction. Its frequency is the same 50 Hertz, or 50 peaks per second.
To smooth out these peaks, a high-capacity capacitor is connected to the circuit. It turns out rectifier diode bridge - At the peak, the capacitor charges, and at the fall it releases charge into the network. This allows you to partially smooth out the frequency graph and level it, bringing it to a constant value.
Such a circuit for connecting a diode and a capacitor is called half-wave and is not sufficient to equalize the current in modern devices. It has serious disadvantages:
- It is impossible to normalize the pulsations to a real straight line.
- The circuit has a rather low efficiency.
- Irrational use of the transformer, excessive weight of the device.
These systems are practically not used today or are used for low-power devices. More logical and reliable circuits are called full-wave. Their main advantage is the ability to invert the lower waves into the upper ones. It is precisely such systems that are called a diode bridge.
Classic diode bridge
Standard contains, instead of one diode and capacitor, four diodes, combined in the manner shown in the figure. It can be roughly divided into two half-cycles. In each half-cycle there are two diodes operating in one direction, and two that prohibit the passage of current. Positive voltage comes to the anode VD1, negative voltage to the cathode VD3. These diodes open, and VD2 and VD4 close.
When a positive half-cycle is replaced by a negative one, a change in performance occurs. Positive voltage comes to the anode VD2, negative - to the cathode output VD4. There is a change in direction, but the current flows in the right direction. It turns out that in such a circuit the frequency doubles, due to which it is possible to achieve better anti-aliasing, using a capacitor identical to the first circuit. Thanks to this, the efficiency of the device increases and possible losses decrease.
Operating principle of a classic bridge Studying do not forget that it is not necessary to solder it from four microelements and select the appropriate capacitor. In most cases you can purchase ready-made solution in the store, with selected parameters and known characteristics. The advantages of such an assembly are its small size, uniform thermal conditions and low weight. The main disadvantage is that if one element fails, the entire assembly has to be replaced.
Three-phase bridge
Now that you know what is a diode bridge for? and what it is, consider a more complex three-phase circuit that produces a pulsating current. It is as close to constant as possible and is suitable for use in devices that require a stable supply. The input of this system is connected to a source that supplies three-phase power (of course we are talking about alternating current). This could be a transformer or a generator. The output of the system is an almost ideal direct current, which can be easily smoothed out.
Rectifier circuit To make a quality full-wave rectifier from connection diagrams for a diode bridge with a capacitor, study our drawing. In this case, the current is rectified, which is removed from the step-down transformer winding. Equalization occurs due to an electrolytic capacitor of 5-10 thousand microfarads, which charges and releases the charge into the network. An additional resistor is also introduced into the circuit, which rectifies the current during idle operation. The higher the load, the lower the output voltage, so a stabilizer based on classic transistors is connected to it X.
Diode bridge - electrical diagram, designed to convert alternating current into pulsed direct current. The invention of the circuit in 1897 is attributed to the German physicist Leo Graetz, although English-language sources claim that back in 1895 the diode bridge was created by the “Polish Edison” - electrical engineer Karol Pollak. The scheme became most widespread after the widespread introduction of semiconductor diodes.
The operating principle of this type of rectifier device is based on the property of a semiconductor diode to pass electric current in one direction and not in the other. So, if we connect the plus and minus correctly, current will flow through the device. If we swap the plus and minus positions, there will be no movement.
Alternating current differs in that during one half-cycle it moves in one direction, and during the second - in the opposite direction. And if you simply connect one diode to the circuit, then it will work “beneficially” only for one half-cycle. What if you connect the diodes so as to use both half-cycles? Thanks to this idea, bridge rectifiers appeared.
The diode bridge-rectifier circuit is quite simple and can be assembled with your own hands. It consists of four diodes connected in the form of a square. Alternating current is supplied to two opposite corners from a generator. A constant is removed from the other two opposite angles. During the first half-cycle, two diodes open, rectifying the half-wave of alternating current. During the second half-cycle, two other diodes open, converting the second half-wave. The result is a direct current output with a pulse frequency twice as high as the alternating current frequency.
Advantages and disadvantages of the scheme
- To use rectified current, the pulse component must be smoothed using a filter capacitor. The higher the frequency, the better the smoothing process. Therefore, doubling the frequency in a bridge circuit is an advantage.
- Full-wave rectification makes it possible to better utilize the power of the supply transformer and thereby reduce its size.
Flaws.
- Double the voltage drop compared to a half wave rectifier.
- Power loss due to heat dissipation is doubled. Low-dropout Schottky diodes are used to reduce losses in high-power low-voltage circuits.
- If one of the bridge diodes fails, the rectifier device will work, but its parameters will differ from normal. This, in turn, can negatively affect the operation of systems powered by a rectifier.
Use and application
Today, bridges are widely used in all cases where direct current is used - from mobile phones, to cars. The industry produces a large number of rectifier devices made using a bridge circuit. Therefore, choosing the right bridge is not difficult, provided you have a clear understanding of why it is being purchased and what functions it will perform.
Structurally, rectifiers can be made on separate diodes or as a single unit. In the first case, if one of the diodes is damaged, you can replace it. To do this, you need to know how to ring a diode bridge. The test is carried out in the form of a sequential search of all diodes to pass current in the forward and reverse directions. As an indicator, you can use either a regular light bulb or a device that measures current or resistance.
Despite the availability of factory rectifiers, many are interested in how to make a 12-volt diode bridge on their own. The fact is that 12 volts is the most common voltage for powering many devices, for example, personal computers. And the desire to assemble a rectifier yourself is often quite justified. After all, most inexpensive power supplies that can be purchased do not meet the declared parameters for current and power.
Of course, a homemade unit is unlikely to look like a factory one, but it will allow you to connect devices in full accordance with the required parameters.
Despite the fact that the rectifier bridge is not complex circuit, its assembly requires not only the ability to solder parts, but also correctly calculate their parameters. First of all you will need power transformer, lowering the voltage to 10 volts. The fact is that the output voltage of the bridge is approximately 18 percent higher than the input voltage. Therefore, if we supply 12 volts of alternating current to the rectifier, we will get 14-15 volts of direct current, and this can be dangerous for devices designed for 12 volts.
Next, you need to select diodes designed for a double current reserve. So, if it is assumed that the rectifier must provide a current of 5 amperes, then the diodes must withstand at least 10 amperes. The capacitor should also have a double reserve, but in terms of voltage. And in order to better smooth out the rectified current, it must have a large capacity. Therefore, the optimal solution is an electrolytic capacitor, designed for a voltage of 25 volts, with a capacity of 2000 microfarads. All these parts just need to be connected correctly and the output parameters checked using instruments.
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 was left only top 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 charger for cell phone. 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 the middle point of the capacitors is called 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 identical rectifiers.
It gives us the same tension, only with double 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
Ro - 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
In any devices and electronics that are powered by AC electricity, there is 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 very good effect when using them 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, 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 more high degree 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 electrolytic capacitor C, having 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. On 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.
Most power plants produce alternating current. This is due to the design features of the generators. The only exception is solar panels, from which direct current is removed.
In general, the choice between direct and alternating current from the point of view of production, transportation and consumption is a struggle of contradictions.
It is more convenient and easier to produce (generate at power plants) alternating current.
It is economically profitable to transport direct current. Changing half-cycles of alternating voltage leads to losses.
From the point of view of transformation (reducing the voltage), it is more convenient to work with alternating current. The operating principle of transformers is based on pulsating or alternating voltage.
Most electricity consumers (we are talking about devices) operate on DC. Electrical circuits cannot operate on alternating voltage.
As a result, we have the following picture:
The outlet receives alternating current with a voltage of 220 volts. And all household electrical appliances (except those that contain powerful electric motors and heating elements) are powered by direct current.
Within the majority home equipment there are power supplies. After reducing (transforming) the voltage value, it is necessary to convert the current from alternating to direct. The basis of such a circuit is a diode bridge.
What is a diode bridge for?
Based on the definition, alternating current with a certain frequency (in a household electrical network 50Hz) changes its direction, while maintaining a constant value.
Important! Since we know that polar voltage is needed to power most electrical circuits, in the power supplies of devices, alternating current is replaced with direct current.
This happens in two or three stages:
With the help of a diode assembly, alternating current turns into pulsating current. This is already a straightened graph, however, this quality of power supply is not enough for the normal functioning of the circuit.
To smooth out pulsations, a filter is installed after the bridge. In the simplest case, this is an ordinary polar capacitor. If it is necessary to increase the quality, a choke is added.
After conversion and smoothing, it is necessary to ensure a constant operating voltage.
For this, at the third stage, voltage stabilizers are installed.
And yet, the first element of any power supply is a diode bridge.
It can be made either from individual parts or in a mono body.
The first option takes up a lot of space and is more difficult to install.
There are also advantages:
This design is inexpensive, easier to diagnose, and if one element fails, only that element is replaced.
The second design is compact and installation errors are eliminated. However, the cost is slightly higher than that of individual diodes and it is impossible to repair one element; the entire module has to be replaced.
Operating principle of a diode bridge
Let us recall the characteristics and purpose of the diode. Without going into technical details, it allows electric current to flow in one direction and blocks its path in the opposite direction.
This property is already enough to assemble a simple rectifier using one diode.
The element is simply connected to the circuit in series, and every second current pulse going in the opposite direction is cut off.
This method is called half-wave, and it has many disadvantages:
Very strong ripple; between half-cycles there is a pause in the current supply, equal to the length of half a sine wave.
As a result of cutting off the lower waves of the sine wave, the voltage is halved. At precise measurement The decrease turns out to be greater, since there are losses in the diodes.
The ability to reduce the voltage by half when rectifying it has found application in housing and communal services.
Residents of apartment buildings, tired of changing constantly burning out light bulbs, equip them with diodes.
When switched on in series, the brightness of the glow decreases and the lamp “lives” much longer.
True, strong flickering tires the eyes, and such a lamp is only suitable for emergency lighting.
To reduce losses, a combination of four elements is used.
Full-wave diode bridge, operating diagram:
Regardless of the direction in which the alternating current flows at the input contacts, the output of the diode bridge ensures constant polarity at its output contacts.
The ripple frequency of such a connection is exactly twice as high as the frequency of the alternating current at the input.
Since the bridge arms cannot simultaneously pass current in both directions, stable protection of the circuit is ensured.
Even if the diode bridge in your device is burned out, there will be no short circuit or voltage surge.
The reliability of the bridge circuit has been proven for decades. Input overvoltage protection is guaranteed by the transformer.
The output stabilizer protects from overload. It breaks through the diode bridge only if defective parts are used, or in a car where the circuit is subjected to constant loads.
How does a diode bridge work at minimum voltage?
The voltage drop in the diode bridge is up to 0.7 volts. When using conventional components in low-voltage circuits, sometimes the voltage drop is up to 50% of the power supply rating. Such an error is unacceptable.
To ensure operation of power supplies with voltages from 1.5 volts to 12 volts, Schottky diodes are used.
When current flows directly, the voltage drop across one crystal is no more than 0.3 volts. We multiply by four elements in the bridge - we get a completely acceptable loss value.
In addition, if the Schottky diode bridge is at a noise level, you will get a value that is unattainable for silicon pn diodes.
Another advantage due to absence of p-n transition - the ability to operate at high frequencies.
Therefore, high-frequency voltage rectifiers are made exclusively using diodes of this type.
However, Schottky diodes also have disadvantages.. When exposed to reverse voltage, even short-term, the element fails.
Checking the diode bridge with a multimeter shows that this particular reason has irreversible consequences.
An ordinary germanium or silicon cell with p-n junction recover independently after polarity reversal.
Therefore, Schottky diode bridges are used only in low-voltage power supplies and with reverse voltage protection.
What to do if there is a suspicion of a bridge failure?
The rectifier is assembled on a conventional element base, so we will tell you how to check the diode bridge at home with a multimeter.
The illustration shows how current flows through the bridge. The testing principle is the same as when testing single diodes.
We look in the reference book to see which terminals of the module correspond to the variable input or polar output - and carry out the dialing.
How to ring a diode bridge without desoldering it from the circuit?
Since current does not flow in the opposite direction through the diode, incorrect test results indicate a breakdown of the bridge.
There is no need to remove the bridge; the remaining elements of the power supply do not affect the measurement.
Bottom line: Any of you will be able to assemble the diode bridge yourself and repair it in case of breakdown. It is enough to have basic skills in electrical engineering.
Watch the video: how to use a multimeter to check the diode bridge of your car’s generator.
A detailed story about how to check a diode bridge with a multimeter in this video story
One of the most important parts of electronic devices powered by a 220 volt AC network is the so-called diode bridge. A diode bridge is one of the circuit solutions on the basis of which the AC rectification function is performed.
As you know, most devices require direct current rather than alternating current to operate. Therefore, there is a need for rectification of alternating current.
I think it’s clear that in the case of individual diodes, you just need to replace one faulty diode, which, accordingly, will cost less.
In reality, the diode bridge assembly may look like this.
Diode assembly KBL02 on the printed circuit board
Or like this.
Diode assembly RS607 on the computer power supply board
And this is what the DB107S surface mount diode assembly (SMD) looks like. Despite its small size, the DB107S assembly can withstand a forward current of 1 A and a reverse voltage of 1000 V.
More powerful rectifier diode bridges require cooling, as they get very hot during operation. Therefore, their housing is structurally designed to be mounted on a radiator. In the photo - diode bridge KBPC2504, designed for direct current 25 amperes.
Naturally, any bridge assembly can be replaced with 4 separate diodes that meet the required parameters. This is necessary when the required assembly is not at hand.
Sometimes this confuses newbies. How to connect diodes correctly if you plan to make a diode bridge from individual diodes? The answer is shown in the following figure.
Conventional image of a diode bridge and diode assembly
As you can see, everything is quite simple. To understand how to connect diodes, you need to inscribe the image of a diode on the sides of the rhombus.
On circuit diagrams and printed circuit boards diode bridge can be designated differently. If separate diodes are used, then the abbreviation is simply indicated next to them - VD, and its serial number in the diagram is placed next to it. For example, like this: VD1 – VD4. Sometimes the notation is used VDS. This designation is usually indicated next to the symbol of the rectifier bridge. Letter S in this case implies that this is an assembly. You can also find the designation BD.
Where is the diode bridge circuit used?
The bridge circuit is actively used in almost any electronics that is powered by a single-phase AC power supply (220 V): music centers, DVD players, CRT and LCD TVs... . Yes, where is he not! In addition, it has found application not only in transformer power supplies, but also in switching ones. Example pulse block power supply in which this circuit is used can serve as an ordinary computer unit nutrition. On its board it is easy to find either a rectifier bridge made of individual powerful diodes, or one diode assembly.
In welding machines you can find very powerful diode bridges that are attached to the heat sink. These are just a few examples of where this circuit solution can be used.
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