12V three terminal voltage regulator is often used when 12V electrical appliances (such as instrument, radio, fan, etc.) are installed on 24 V diesel vehicles. However, due to the voltage drop of 12V, high power consumption and high temperature, the regulator is easy to be damaged. We have designed a switching power transformer, which can reduce the voltage from 24 V to 12 v. the circuit is shown in the figure. 555 acts as a pulse oscillator in this circuit. The fifth pin of 555 is connected with a voltage regulator to obtain the + 6V reference voltage, and the second pin obtains the sampling voltage from the sampling circuit composed of R7 and R8. When the voltage of pin 2 is less than + 3V, pin 3 will output high level to saturate BG3, BG1 and BG2 and supply power to the load. At the same time, the power supply charges C2 via R6, and when the potential of pin 6 and 7 reaches + 9V. If pin 2 is above + 3V, pin 3 outputs low level, the capacitor is discharged through pin 7, and the switches BG3, BG1 and BG2 are cut off. When the voltage of the capacitor connected to pin 2 is lower than + 3V, pin 3 outputs high level again, which makes the output voltage stable at the rated output voltage of 12V. The starting circuit is composed of R1, R2, C1 and D1. When the circuit is just connected, it supplies the base current to BG3, makes the switch tubes BG1 and BG2 turn on, and supplies the working voltage to 555. When the load is short circuited, 555 will lose the working voltage, so as to cut off BG3, BG1 and BG2, so as to protect the circuit and switch tubes. Inductor L and freewheeling diode D2 can reduce the burden of the switch. L. use E7 iron core and Φ 1.0 mm enameled wire to wind it completely. D2 uses fast recovery diode or common diode larger than 5A. If higher output current is needed, BG1 parameters are icm30a, vceo100v, pcm150w; BG2 parameters are icm1.5a, vceo100v, pcm5w, the output current can reach 10A. BG1 and BG2 can also be replaced by high power PNP Darlington tube.