Briefly describe the working principle of a 3~12V adjustable discrete component DC regulated power supply

This article introduces in detail the circuit diagram and working principle of a 3~12V adjustable discrete component DC regulated power supply . The circuit diagram is shown in Figure 1, and the printed board diagram is shown in Figure 2.

Figure 1 Discrete component voltage regulator circuit diagram

Figure 2 Discrete component voltage regulator power supply printed circuit board diagram

The following briefly describes the basic working principle of this voltage-stabilized power supply :

220V AC is converted into 12V low-voltage AC by step-down transformer B, and then rectified by VD1~VD4 bridge and filtered by C1 to obtain a DC voltage of about 16V (12V×). This voltage is unstable and will change with the input AC voltage and load current. Crystal transistors VT1 and VT2 form a composite adjustment tube, VT3 is a comparison amplifier, R3 and RP are both current-limiting resistors of LED and form a sampling and reference voltage circuit together with LED.

A DC voltage Ui of about 16V is applied to the input end of the adjustment tube. R1 is the base bias resistor of the composite adjustment tube, which provides the conduction voltage for the adjustment tube. After VT1 is turned on, the emitter has a voltage Uo output. The output voltage Uo is taken out by the sampling circuit and sent to the base of the comparison amplifier tube VT3. After being compared with the reference voltage, the error control voltage is output from the collector to control the conduction depth of the adjustment tube, so that the voltage Uo output by the emitter of the adjustment tube VT1 is stable within the specified value range. If Ui increases due to some reason, resulting in an increase in the output voltage Uo, the voltage stabilization process is expressed as:

The voltage-stabilized power supply cleverly utilizes the forward conduction voltage (1.8V to 2V) of the LED to replace the voltage-stabilized tube with a low voltage value, and on the other hand, it can also serve as a power supply indicator.

Capacitor C2 is mainly used to eliminate the modulated AC noise when powering the radio. If the radio still has modulated AC noise during debugging, just swap the secondary of the power transformer and connect it to the circuit.

Transformer B should be selected with a power of more than 15W to ensure a larger current output. When the load current is ≥300mA, VT1 should use a medium-power tube such as C2073 and install an appropriate heat sink.

The rated working voltage of the electrolytic capacitor is 25V. There are no special requirements for other components. When the maximum output current is required to be 500mA, replace 8050 with C2073. If conditions permit, it is best to add a heat sink. There are no special requirements for the other components. They can be installed according to the component parameters shown in Figure 1.