Circuit diagram of separately excited switching stabilized power supply using single-junction transistor as pulse generator

In the picture: VD1~VD4 and C1 are rectifier and filter circuits, which convert the 220V AC grid voltage into 300V DC voltage. The pulse generator consists of a single-junction transistor VT1, resistors R3, R4, R5, a potentiometer RP and a capacitor C3. Changing the resistance of the potentiometer RP can change the pulse frequency. C4 is the coupling capacitor of the output pulse. Rl and R2 divide the 300V DC voltage to provide a working voltage of 20~30V to the single-junction transistor VTl. VT2 is an inverting amplifier tube, which inverts and amplifies the positive polarity pulse output by the pulse generator, and delivers a negative polarity pulse to the switching tube VT4, so that it cuts off when VT1 outputs a positive polarity pulse.