- The alternating current generated by the alternator is converted into direct current to supply power to the electric equipment and charge the battery;
- Limit the battery current back to the generator, and protect the generator from being burned by the reverse current.
U2 > 0: D1,D3 on, D2,D4 off, current path: a - > D1 - > R - > D3 - > b
U2 < 0: D2,D4 on, D1,D3 off, current path: B - > D1 - > R - > D4 - > A
The bridge rectifying circuit overcomes the shortcomings of the full wave rectifying circuit, which requires the transformer secondary to have a central tap and the diode to bear a large back pressure, but uses two more diodes. With the rapid development of semiconductor devices and low cost, this disadvantage is not outstanding, so bridge rectifier circuit is widely used in practice.
It should be specially pointed out that diodes, as rectifying elements, should be selected according to different rectifying modes and load sizes. If it is not selected properly, it may not work safely, or even burn the pipe; or overqualified, causing waste.
The bridge rectifier circuit can also be considered as one of the full wave rectifier circuits. The transformer winding is connected with four diodes according to the method above. D 1-D 4 are four same rectifying diodes, which are connected in the form of electric bridge, so they are called bridge rectifying circuits. With the guidance of diode, the secondary output can also be led to the load in the negative half cycle. The specific connection method is shown in the figure. It can be seen from the figure that in the positive half cycle, the D1 and D3 lead currents pass through RL from top to bottom, and in the negative half cycle, the D2 and D4 lead currents also pass through RL from top to bottom, thus realizing full wave rectification. In this structure, if the same DC voltage is output, the secondary winding of the transformer only needs half of the winding compared with the full wave rectifier, but if the same current is output, the wire diameter of the winding should be correspondingly thickened. As for the pulse, it is exactly the same as the full wave rectifier circuit mentioned above.
The advantages of the bridge rectifier circuit are high output voltage, small ripple voltage, and low maximum reverse voltage. At the same time, the power transformer is fully utilized and has high efficiency because of the current supply load in the positive and negative half cycle of the power transformer.
Because the output voltage of the rectifier circuit contains large pulsating components. In order to reduce the pulsating component as much as possible, on the other hand, the DC component should be kept as much as possible to make the output voltage close to the ideal DC. This measure is filtering. Filtering is usually realized by using the energy storage function of capacitance or inductance.
The inductor filter circuit uses the characteristics that the current at both ends of the inductor can't be changed suddenly, and connects the inductor and the load in series, so as to achieve the purpose of smoothing the output current. From the energy point of view, when the current provided by the power supply increases (caused by the increase of the power supply voltage), the inductor L stores the energy; when the current decreases, the energy is released to make the load current smooth, so the inductor L has a smoothing effect.
Advantages of inductive filter: the rectifier diode has large conductive angle, small peak current and flat output characteristics.
Disadvantages of inductive filter: iron core, bulky, bulky, easy to cause electromagnetic interference, generally only suitable for low voltage, high current occasions.
Capacitance filter is a large capacity capacitor connected with load in parallel after rectifying circuit. Due to the charging and discharging function of the capacitor and the existence of the voltage at both ends of the capacitor, the fluctuation of the output voltage UL of the rectifier circuit is greatly weakened, and the waveform is nearly smooth, which plays the role of filtering. The output voltage waveform of the bridge rectifier capacitor filter is shown in Figure 2-2 (actually the filtered output waveform). In this kind of capacitor filter circuit, the larger the capacity of the capacitor or the load resistance, the slower the discharge of the capacitor and the smoother the output voltage.
Compound filter is a kind of filter composed of inductance and capacitance or resistance and capacitance. Its working principle is the same as that of capacitance filter and inductance filter. It only passes through more than two times of filtering, which makes the output waveform smoother, and the effect of load is close to that of dry cell power supply voltage.