The functions of various parts of the voltage stabilizing circuit
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The functions of various parts of the voltage stabilizing circuit

Posted Date: 2024-02-05

Assume that the terminal voltage Vo in the voltage stabilizing circuit shown in the figure drops a little. At this time, the actions of each part are as follows:

I=Vo/(R2+R3)

VR= I* R3=Vo/(R2+R3)*R3

1. The role of the sampling part

When the terminal voltage Vo drops, the voltage between ac in the above figure drops at the same time. Because the resistors R2 and R3 are fixed resistors, the current flowing through these two resistors also decreases as the voltage decreases.

Therefore, VR =(Vo/(R2+R3))R3 will also decrease. As long as this VR is used as the sampling voltage, the information is notified to the input terminal.

The shaded part in the picture is the sampling part. In order not to cause the terminal voltage to drop, connect them in parallel. The resistance values ​​of resistors R2 and R3 must be made very large.

Since the sampling voltage VR is added to the base of the transistor Tr2, it can control the collector current.

2. The role of the benchmark part

In order to keep the output voltage constant, just set a certain reference value and then correct the reference value.

The setting of the reference voltage takes advantage of the characteristics of the Zener diode ZD. The figure shows the characteristic curve of the Zener diode. The reverse voltage of the Zener diode has nothing to do with the current flowing through it and is a fixed value. This voltage is called the Zener voltage. In order to obtain a diode voltage stabilizing element that has a stable voltage drop regardless of the size of the circulating current, it is called a Zener diode (also called a Zener diode).

The characteristics of several Zener diodes are shown in the table.

Model Maximum rated value Zener voltage Vz/V Power P/mW Current Iz/mA RD12K 400 - 12 05Z12 500 38 12 1S227 1000 74 12 The picture is the circuit of the reference part. The emitter of the transistor Tr2 is connected in reverse to the ground. of.

Because the Zener diode is connected, the potential of the emitter can be kept constant at the Zener voltage through the emitter current (almost equal to the collector current IC) according to the change of the voltage VR applied to the base. This is the reference voltage.

3. The role of comparison part

In the figure above, since the terminal voltage of the Zener diode is fixed, the voltage VBE between the base and the collector of the transistor Tr2 decreases as the output voltage Vo decreases.

Therefore, the collector current IC1 of Tr2 changes in proportion to the terminal voltage VR of the resistor R3 of the sampling part.

4. The role of the control part

In the figure, if the collector current IC1 of the transistor Tr2 is reduced, the voltage drop caused by R1 will also be reduced, and the base potential of the transistor Tr2 (point d) will increase.

As a result, it increases the base current of Tr1, and at the same time, the collector current IC2 also increases, so the output voltage Vo rises, functioning to offset the fluctuation.

This is exactly the same as the effect shown in the figure of keeping the output voltage Vo unchanged by changing the internal resistance RCE between the collector and emitter of Tr1. The regulated power supply automatically completes this adjustment.

The working flow of the voltage stabilizing working circuit is shown in the figure.

Review Editor: Huang Fei


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