Three forms of series voltage regulated power supply

1. As shown in Figure 1, this is the simplest series voltage regulator circuit (some books call it a parallel voltage regulator circuit, I personally always think it should be a series voltage regulator circuit), the resistor RL is the load resistor, R is the voltage regulator adjustment resistor, also called the current limiting resistor, and D is the voltage regulator tube. The voltage regulator value output by this circuit is equal to the nominal voltage regulator value of D, and its working principle is to use the characteristics of the voltage regulator tube working in reverse breakdown to achieve. Figure 2 is the volt-ampere characteristic curve of the voltage regulator tube. From this curve, we can see that when the reverse current changes greatly within a certain range, the voltage at its end point remains basically unchanged. When RL becomes smaller, the current flowing through RL increases, but the current flowing through D decreases. When RL becomes larger, the current flowing through RL decreases, but the current flowing through D increases. Therefore, due to the existence of D, the current flowing through R is basically constant, and the voltage drop on R is basically unchanged, so the output voltage is also basically unchanged.

Figure 1 Simple series voltage regulator circuit

Figure 2 Voltage-ampere characteristic curve of the Zener diode

When the load requires a larger output current, this circuit will not work. This is because the resistance of R must be reduced at this time. The reduction of R requires D to have a larger power consumption. However, since the power consumption of general voltage regulator tubes is relatively small, this circuit can only provide tens of milliamperes of current to the load. The 30V tuning voltage of color TVs is usually obtained using this circuit.

2. As shown in Figure 3, this circuit is an improved circuit for the shortcomings of the above-mentioned circuit. The difference from the first circuit is that R in the circuit is replaced by a transistor BG, the purpose is to expand the output current of the voltage regulator circuit. We know that the collector current IC of BG = β * Ib, β is the DC amplification factor of BG, Ib is the base current of the transistor, for example, now to provide a current of 500MA to the load, BG β = 100, then the circuit only needs to provide a current of 5MA to the base of BG. Therefore, this voltage regulator circuit is actually equivalent to expanding the first voltage regulator circuit by β times due to the addition of BG. In addition, since the base of BG is embedded by D at its nominal voltage regulation value, the output voltage of this voltage regulator circuit is V0 = VD-0.7v, 0.7V is the positive bias voltage drop of BG's B and E poles.

Figure 3 Improved circuit

In practical applications, we often provide different power supply voltages for different circuits, that is, the output voltage of the voltage-stabilized power supply is required to be adjustable. For this reason, a third form of series voltage-stabilized circuit has emerged.

3. Although the second voltage stabilizing circuit can provide a larger output current, its output voltage is restricted by the voltage stabilizing tube D. For this reason, people slightly modify the second circuit to make it a series voltage stabilizing power supply with continuously adjustable output voltage. The basic circuit is shown in Figure 4. From the circuit, we can see that this circuit adds a triode and several resistors compared with the second circuit. R2 and D form the reference voltage of BG2, and R3, R4, and R5 form the output voltage sampling branch. The potential of point A is compared with the potential of point B (due to the existence of D, the potential of point B is constant). The result of the comparison is that the collector output of BG2 causes the potential of point C to change, thereby controlling the conduction degree of BG1 (at this time, BG1 plays the role of a variable resistor in the circuit) to stabilize the output voltage. R4 is a variable resistor. Adjusting it can change the potential of point A (that is, change the sampling value). Due to the change of point A, the potential of point C will also change, so that the output voltage will also change. This circuit has a flexible and variable output voltage, so it is widely used in various circuits.