3 to 9V adjustable 100mA regulated power supply circuit

3-9V adjustable 100mA stabilized power supply. The regulated power supply shown in Figure 12-30 uses a series voltage stabilizing circuit with amplifier tubes. The output voltage is continuously adjustable from 3 to 9V, and the output current can reach 100mA. It is suitable for power supplies. A variety of transistor radios with different voltages are used.

Circuit principle: The low-voltage alternating current on the secondary side of the transformer is rectified by the rectifier diodes VD1-VD4 and filtered by the capacitor c1 to obtain direct current and sent to the voltage stabilizing part. The voltage stabilizing part consists of regulating tubes VT1 and . It is composed of the amplifier tube VTZ and the silicon diode tubes VD5 and VD6 that stabilize the voltage.

The voltage drop between the collector plate and emitter of a transistor is referred to as tube drop. The tube voltage on the adjustment tube VT1 is variable. When the output voltage tends to decrease, the tube voltage drop will automatically become smaller, maintaining the output voltage unchanged. When the output voltage tends to increase, the tube voltage drop will decrease again. will automatically become larger while maintaining the same output voltage. It can be seen that VT1 is equivalent to a variable resistor. Due to its adjustment effect, the output voltage remains basically unchanged.

The adjustment function of the adjustment tube is controlled by the amplifier tube VT2. The output voltage is divided by the potentiometer, and part of the output voltage is added between the base of VT2 and ground. Since the voltage between the emitter of VT2 and the ground is stabilized by the transistors VD5 and VD6, it can be considered that the voltage between the emitter of VT2 and the ground is constant. This voltage is called the reference voltage. so. :The change in the base voltage of r2 reflects the change in the output voltage. If the output voltage tends to decrease (referring to the absolute value, the same below), the voltage between the base and emitter of VT2 will also decrease, which will cause the collector current of VT2 to decrease and the collector voltage to increase. Since VT2 and VT1 are directly coupled, the collector voltage of VT2 increases, that is, the voltage between the base and emitter of VT1 increases, which strengthens the conduction of VT1, reduces the tube voltage drop, and maintains the output voltage unchanged. . Similarly, if the output voltage tends to increase, the tube voltage drop of VT1 will increase through the action of VT2, keeping the output voltage unchanged.

VD5 and VD6 stabilize the voltage by utilizing the characteristic that their forward voltage drop basically does not change with the current when they are in forward conduction. The forward voltage drop of the silicon tube is about 0. 7V. Two silicon diodes connected in series can obtain a stable voltage of about 1.4V. Using forward characteristics for voltage stabilization is not as effective as using reverse breakdown characteristics. Its advantage is that it can provide a lower stable voltage. Horse is the current limiting resistor that provides the forward current of VD5 and VD6. Rz is the load resistance of VT2 and the bias resistance of VT1. Ri is set to reduce the voltage drop of the VT1 tube when the output current is large. C2 is set in consideration of reducing the AC component of the output voltage when the mains voltage decreases . The function of G is to reduce the AC internal resistance and ripple of the regulated power supply.