Diode bridge

Diode Bridge - a design that allows you to rectify the current efficiently. The diode bridge is considered a full-wave rectifier.

Diode, bridges and difficulties of rectifying the current

Initially, diodes were called electron tubes with two electrodes. The heated cathode emitted electrons capable of flying in a single direction - on the anode. And in the opposite direction, the current is not flowing. This made it possible to cut off part of the AC voltage period. As a result, the current became rectified.

The design flaw is obvious - part of the time, half the interval, the scheme is inactive. For this reason, creating high performance is difficult. We are not talking about efficiency, but rather affect the total power. The voltage in the network is limited to nominal, it is required to effectively use the existing one. If you increase the consumption through a single diode, it will overheat and burn. Here a diode bridge comes to the rescue.

Scheme The constructions discussed in this article are aimed at improving certain properties. Otherwise a diode bridge of a single configuration would have been used for a long time. The well-known diode bridge on four valves is far from the only one for the simple reason - it is designed to work with a single voltage phase. This is a flawed option, delivered to our homes from the purpose of saving wires, and is not applied in industry.

Let's start with Nikola Tesla. This man first came up with a rotating magnetic field. Previously, alternating current was used, but with the help of a single phase, the announced phenomenon cannot be created. Inside the engine you need to rotate the field. The only phase to physically provide is not able to. Nikola Tesla invented the asynchronous motor, with many poles. Note that the collector types of motors are able to operate on AC and DC, but it is recommended to avoid designs with permanent magnets. The rotor and stator are assembled from copper windings. We believe that in the 19th century there were no similar types of engines.

Back to the phases. Having invented an asynchronous( induction) AC motor, Nikola Tesla, in passing, noted in the patent the possibility of further increasing the phases, but did not go further. Later Dolivo-Dobrovolsky proved that it is much more efficient to use three phases. Today, industrial structures use this option. Note that any motor can run to consume and generate current; readers will understand that a single-phase diode bridge will not be the ideal solution. This is a flawed, trimmed version for household appliances. No more.

The on-board systems carry a three-phase generator, this is the most efficient design possible today. Already used scheme Larionov. This achieves the best ratio of savings and efficiency. Good characteristics have Mitkevich rectifier circuits. School and university physics courses have a simplified structure due to the too strong development of science: it is impossible to fit all the information into the students' heads for a semester.

Gretz Diode Bridge for home appliances is not considered the only one possible. There are three variants of the phase, much more common than it seems initially. Diodes in design and characteristics are very different from each other. This determines the specifics of the application. For example, power varieties are powerful, but suffer great losses. Therefore, Schottky diodes with a small voltage drop at the p – n junction are used in the output circuits of the switching power supplies.

Designs of diode bridges

The only design of a diode bridge is not able to provide for all needs. Therefore, in cars used Larionov scheme. We now discuss the design, first clarify why the diode bridge is so called. In 1833, a scheme was proposed for measuring resistance, based on the equalization of the potential of the middle pins of two branches:

1. Four resistances are connected in a square( one for each side of a geometric figure).
2. Two corners are supplied with power from a battery or other source.
3. With two other angles, readings of any voltage or current are taken.

The meaning of the work is to turn the indicator readings to zero with a potentiometer. Then they say - the balance of the bridge has come. At that time( before the publication of the Kirchhoff laws), they already knew that the voltage drop across two resistors is proportional to their value, which means that it is true that: R1 / R2 = R3 / Rx, where R2 is a potentiometer, R1 and R3 are constant resistances of a known nominal, Rx is the element under study. Then from the simple proportion is the desired value.

The bridge circuit in the English-language literature is called for the reason that between the two branches of the electric circuit consisting of the resistances R1, R2 and R3, Rx, respectively, there is a jumper - measuring device. It reminded people of the bridge, the scheme was named accordingly.

Gretz Diode Bridge

In 1897, the Elektronische Zeitung magazine( part 25) published a note by Leo Gretz on the study of a diode bridge. Separate readers decided that the specified person became the inventor of the device. To this day( in 2016), the Russian domain of Wikipedia continues to assert an indisputable fact. In fact, the inventor of the Gretz diode bridge was the Polish electrical engineer Karol Pollack. The review authors could not find a biography of a scholar husband in Russian. It is not surprising that little is known about patent number 96564 of January 14, 1896.

The figure shows an explanation of the name of the circuit - the diode bridge, there are all signs:

1. Two branches of the diodes in the center are shorted by a load circuit.
2. AC power is supplied to the two sides of the square.
3. At the output there is a constant voltage.

The fact is a drawback of the circuit: the voltage drop at the pn-junction doubles. At any time, the current passes through a pair of diodes, and not one, as in the case of a full-wave rectifier. At high voltages, it is possible to neglect the losses so that the circuit does not burn, it is supplied with large rugged metal radiators. Motorists have already understood what they are talking about, mere mortals note that for home appliances this is not always true( there is no radiator).The reason is not the power in the passenger car chain. Rather, at a constant voltage of 12 V on the onboard network, the current is high, this fact leads to such a strong heat release.

We explain. According to the Joule-Lenz law, the heat from the flow of electric current is proportional to the square of the magnitude of the current. In low-voltage circuits, for this reason, copper wires have to be made thick. This is the reason why industrial voltage is higher than 12 V. Kilovolts run in power lines, which helps to reduce the cable cross-section and save on materials. A transformer is used for conversion between the lines; as a rule, it is located at the input of any household appliance.

This is needed to quickly create voltage ratings that are close to the required ones. The statement is especially vividly traced by the example of a cathode ray tube television. The input transformer carries a lot of output windings according to the number of circuits. It remains only to rectify the current if necessary, which reduces the complexity of the equipment. To do this, after the output winding of the transformer put the diode bridge Gretz( we are talking about single-phase networks of 220 V).

In modern pulsed power supplies in a different way. The diode bridge is placed directly after the input filter, then the rectified voltage is cut on a thyristor( transistor) key to the high-frequency pulses applied to the transformer. This allows you to repeatedly reduce the size of the core and windings. Look at the adapter for a cell phone: there is a pulse transformer inside. The size does not compare with the power supply of the TV.We recommend to pay attention to the system unit of a personal computer, where the source produces at least 350 watts. That’s enough for a CRT TV.

After the pulse transformer, the rectifier again stands. Sometimes it is a diode bridge based on Schottky diodes with low voltage drops at a pn junction. Recall the above disadvantages. For low output voltages of the switching power supply, the use of diode bridges is unprofitable, the number of valves doubles. As a result, the loss is higher, which naturally reduces the efficiency. An additional factor is heat generation: at low voltages, radiators have to be used with a high resistance of the pn junction.

Resistance pn-junction

Gretz diode bridges are the de facto dominant today in home appliances. We make a small digression about the resistance of the pn-junction.

As is known, the characteristic of a diode resembles a parabola in the positive part of the x-axis. No matter the form, the important fact is that at any point in the graph it becomes possible to find resistance. You just need to divide the voltage by current. It turns out that the resistance of the diode depends on the applied voltage and in a typical case is constantly changing. We find, similar to the current voltage value( 220 V), the average figure for this parameter. Losses depend on it. The lower the resistance of the pn-junction, the better. Therefore, it is advantageous to use Schottky diodes.

Single-phase rectifiers according to Mitkevich's scheme

The circuit does not look like a bridge, except for some similarities. It can be seen from the figure that the load short-circuits the branches of the transformer winding and the diodes. This is a stretch. So any chain can be called a bridge. At any given time, half of the construction works for the Mitkevich circuit. The second is locked.

The same is said about the Gretz diode bridge, but here the statement applies to the transformer winding, which cannot be noted in the previous case.

Three-phase rectifiers

The Larionov rectifier( see picture) is not considered a bridge, although drivers call it so persistently. There are two types of designs, in the terminology of three-phase lines called star and triangle. Motorists are more often in contact with the first option, where the voltage is slightly higher and the loss is less. This is due to considerations of economy.

The scheme giving the mentioned one hundred points of the handicap is known. This is a true diode bridge, parallel or series connection of three full diode bridges.