Rectifier bridge: diode bridge connection diagram, device operation principle and assembly technology

A rectifier bridge (diode) is a semiconductor diode designed to convert an alternating current into a permanent one. This is far from a complete area of ​​application of rectifier diodes: they are widely used in control and switching circuits, in circuits voltage multiplication, in all high-current circuits, where strict requirements are not imposed on time and frequency parameters electrical signal.

General characteristics

Speaking about what a diode bridge is needed for, depending on the value of the maximum permissible direct current rectifier diodes are divided into diodes of small, medium and large power:

• small power - designed for rectifying direct current up to 300mA;
• medium power - from 300mA to 10A;
• large power - more than 10A.

By the type of material used, they are divided into germanium and silicon, but today silicon rectifier diodes are most widely used, due to their physical properties.

Silicon diodes, in comparison with germanium ones, have many times lower return currents at the same voltage, which makes it possible to emit diodes with a very a high value of admissible back voltage, which can reach 1000 - 1500V, then, as in germanium diodes, it is within 100 - 400V.

The working capacity of silicon diodes is preserved at temperatures from -60 to + (125 - 150) º С, and germanium ones - only from -60 to + (70 - 85) º С. This is due to the fact that at temperatures above 85 ° C, the formation of electric-hole pairs becomes significant that there is a sharp increase in the return current, and the efficiency of the rectifier falls.

The three-phase circuit uses half-bridge diode rectifiers. The output voltage is obtained here with less ripple.

Manufacturing technology and construction

The design of rectifier diodes is a single plate of a semiconductor crystal, in the volume of which there are two areas of different conductivity, therefore such diodes are called planes.

The technology of manufacturing such diodes is as follows: on the surface of the crystal of a semiconductor with n-type electrical conductivity melt aluminum, indium or boron, and on the crystal surface with p-type electrical conductivity melt phosphorus.

Under the action of high temperature, these substances are strongly fused with the crystal of the semiconductor. The atoms of these substances penetrate (diffuse) into the thickness of the crystal, forming in it an area with a predominance of electrical or perforated electrical conductivity. This produces a semiconductor device with two areas of different types of electrical conductivity, and a p-n junction is established between them. Most of the widespread flat silicon and germanium diodes are manufactured in this way.

To protect against external influences and to ensure reliable heat dissipation, a crystal with a p-n junction is mounted in the case. Small-power diodes are manufactured in a plastic case with flexible external leads, medium-power diodes - in a metal-glass case with rigid external leads, and diodes of large power - in a metal-glass or metal-ceramic body with glass or ceramic isolator.

Crystals of silicon or germanium with a p-n junction, they are soldered to the crystal holder, which is a simultaneous base of the body. A body with a glass insulator is welded to the crystal holder, through which one leads out of the electrodes.

Small diodes, having relatively small dimensions and weight, have flexible leads, with the help of which they are mounted in circuits. For diodes of medium power and strong, designed for significant currents, the outputs are much more powerful. The lower part of such diodes is a massive heat-conducting base with a screw and a flat outer a surface designed to ensure reliable heat contact with external heat dissipation (radiator).

Electrical parameters

Each type of diode has its own working and maximum permissible parameters, in accordance with which they are chosen for operation in one or another scheme:

• Iobr - constant return current, mKA;
• Upr - constant forward voltage, V;
• Ipr max - the maximum permissible direct current, A;
• Uobr max - maximum allowable reverse voltage, V;
• P max - the maximum permissible power dissipated by the diode;
• Operating frequency, kHz;
• Working temperature, C.

Here are far from all the parameters of the diodes, but if you need to find a replacement, these parameters are enough.

Simple rectifier circuit

AC mains voltage is applied to the input of the rectifier, in which positive half periods are highlighted in red, and negative ones - in blue. Κ The output of the rectifier is connected to the load, and the function of the rectifier element will be performed by the diode.

At positive half-periods of voltage applied to the anode of the diode, the diode is opened. At these moments of time, through the diode and the load fed from the rectifier, the direct current of the diode Ipr flows.

With negative half-periods of voltage flowing to the anode of the diode, the diode closes, and a slight backward current of the diode will flow throughout the circuit. Here the diode, as it were, cuts out the negative half of the alternating current.

As a result, it turns out that through the load connected to the network through the diode, it no longer flows variable, because this only flows during positive half periods, and the pulsating current only directions. This is the straightening of the alternating current.

With such a voltage, you can only supply a small load that is powered by an alternating current network and does not impose any special requirements on the power supply: for example, an incandescent lamp. Voltage through the lamp will pass only during positive poles (impulses), therefore the lamp will flicker weakly at a frequency of 50 Hz. Due to the thermal inertness, the thread will not have time to cool down in the intervals between the pulses, and therefore the flicker will be weakly noticeable.

If you power a receiver or power amplifier with such a voltage, then a low-tone hum with a frequency of 50 Hz will be heard in the speaker or speakers, called an alternating current. This will happen because the pulsating current, passing through the load, creates a pulsating voltage in it, which is the source of the background.

This deficiency can be partially eliminated if a large-capacity filtering electric condenser is connected in parallel to the load.

Charged with current pulses during positive half periods, the condenser is discharged through the load during negative half periods. If the capacitor is sufficiently large, then during the time between the current pulses it will not have time to fully discharge. The load will be continuously maintained both during positive and negative half periods.

But in such a way it is also impossible to power a receiver or an amplifier, because they will "fool": the level of pulsations is still very felt. The rectifier uses only half of the alternating current energy, so more than half of the input voltage is lost on it. This type of rectification of alternating current is called single-loop rectifiers, and rectifiers are called single-loop rectifiers. Such deficiencies are eliminated in rectifiers using a diode bridge.

Diode bridge with your own hands

The diode bridge is one of the most widespread devices in the electronics, designed for rectifying alternating voltage. As a result of the transformation at the output of the diode bridge, a pulsating voltage is generated at twice the frequency than at the input. Without such a scheme, practically no power supply of modern electrical devices is needed. The following is an instruction on how to assemble a diode bridge:

• Select the type of diode bridge. It can be made of separate diodes or in the form of a monolithic diode assembly. It has the advantage of being easy to mount on the board, but if the diode goes out of the system, it cannot be replaced by another. We'll have to change the whole scheme.
• In the absence of a ready diode bridge, you can collect it from four diodes. Diodes designed for a current of 1 A and a voltage of 1000 V. It is necessary to calculate the required power of the bridge by multiplying the limit current by the limit voltage, with a two-fold margin of power.

• Calculation example: there is a diode bridge for 1000 V and 4 A. The power of the load will amount to 1000x4 = 4000 W, taking into account the double "safety margin" - 4000/2 = 2000 W (2 KW). Similar power is considered for other models of rectifier drives. When composing a diode bridge, it is necessary to take into account that 70% of the total current will flow through each of the diodes. In other words, if the load is current 4 A, then in a separate diode of the bridge it will be 3 A.
• To cool down the bridge, it is better to use an aluminum radiator with an area of ​​800 kV. cm. The surface of the radiator is prepared: holes are drilled, a thread is cut to secure the assembly. To increase heat transfer, it is recommended to use heat transfer paste EPT-8.
• Fix the diode assembly on the surface of the radiator using M6 bolts, using a tubular key.
• You need to unsolder the circuit with a copper bus. Tire size 10 sq. mm to solder to the pins of the assembly, and the bus size 20 kV. mm should be used for the current I / O circuit. The bus must be soldered to the terminals of the diode bridges. If you connect the bridges without soldering (terminals), the ends of the leads will be very hot.

The diode bridge connection diagram is shown in the figure above.

Connection to transformer

Devices that consume a lot of current are usually powered from a 220 V network. It is impossible to connect the devices directly, since the voltage for the electrical circuits requires little, and then it is constant. Then use the mains adapter.

The voltage is reduced with the help of a transformer, which creates a galvanic isolation between the primary and secondary supply circuits. Due to this, the risk of electric shock is reduced and the equipment is protected when a short circuit appears in the circuit.

Modern adapters in most cases operate on a simplified transformerless circuit without galvanic isolation, where excess voltage is absorbed on the capacitor.

The power supply consists of two modules, where the first is a downward transformer, and the second is a diode bridge, which converts one type of voltage into another. A suitable transformer is selected. The primary winding is located with the help of a tester. Its resistance should be the greatest. By calling the multimeter in the resistance measurement mode, the required ends are found. Then other pairs are found and the mark is made.

The primary winding is 220 V. The tester switches to the alternating voltage measurement mode, then U is measured on the rest of the windings. You should select or wind one at 10 V. It is important that the voltage is not 12 V, because after a capacitive filter it increases by 18%.

The transformer is selected for the required power, after which the stock is taken by 25%. 4 diodes are twisted into a diode bridge, and the ends are soldered. Then the circuit is connected, the output is connected to a capacitor of 25 V and 2200 mkf (electrolith). The operation of the device is checked.

You can make a diode bridge yourself if you carefully study the principle of operation of the device. If all the rules for connection and manufacture are observed, then the bridge will work.