This switch-mode power supply is built around a 555 timer IC. It provides a maximum output voltage of 40 V with a 12-V input voltage. The voltage can easily be set using a Zener diode, and it must be higher than the input voltage (the minimum output voltage is always 12 V). The NE555 is used in an unconventional way here. In the normal configuration, the output of the oscillator IC is low longer than it is high. With the configuration used here, the output can be high for a shorter time than it is low. The NE555 switches FET T1 on and off. When T1 is conducting, energy is stored in L1. When T1 stops conducting, this energy is transferred to CI and C2 via Schottky diode DI, so the voltage on these capacitors rises. The voltage is limited by Zener diode D2. If the voltage rises above the Zener voltage, the current through the Zener diode causes T3 to conduct. This reduces the voltage on pin 5 of the NE555, which in turn decreases the relative duration of the high level on pin 3. T1 thus conducts for a shorter interval, so less energy is stored in L1 and the output voltage is stabilized.
Current limiting is provided by R6, R5, and T2. If the voltage across R6 is more than 0.6 V, T2 starts to conduct. This drives T3 into conduction, causing the voltage to decrease in order to limit the current.
C5 and R7 provide a soft-start effect. The value of R1 can range from 22 kr2 for an output voltage of 15 V to 10 kr/ for an output voltage of 40 V. For the sake of safety, limit the Zener voltage to a maximum of 40 V. T1 and T2 can be rated for a maximum of 50 V. The FET is not critical; you may already have one in your spare parts bin that can switch enough current. If the coil becomes warm, the core is too small or the wire is too thin. The Schottky diode is the only component that is actually critical. Do not use an ordinary diode, since it will become much too hot. You’re bound to find a Schottky diode in an old computer power supply (just check for a forward voltage of 0.2 V on the diode range of your multimeter). The supply shown here can deliver approximately 200 W. The input supply voltage can range from 7 V to 15 V. Don’t forget that the maximum voltage the NE555 can handle is 15 V. Finally, this power supply is not short-circuited proof. A slow-blow fuse on the 12-V side is recommended.