Here is the circuit diagram of a simple soft start power supply using the well known IC L200.The L200 is a monolithic IC that can be used for voltage and current regulation. The built-in features such as thermal shut down, over current protection and input over voltage protection makes this IC nearly indestructible.
When the power supply to the circuit is switched ON, the capacitor C3 starts to charge through resistor R3.The internal comparator pin (pin2) of the IC is connected to the positive lead of C3.When capacitor C3 is charging the voltage at the internal comparator pin also increases. This slowly relaxes the current regulation loop inside the IC .As a result the output voltage of the IC also slowly increase from zero to the set value of output voltage. Here the set output voltage is 15V.

Circuit diagram with Parts list.



  • Assemble the circuit on a good quality PCB.
  • The input voltage to the IC can be anything between 18 to 40V DC.
  • The IC1 must be fixed with a heat sink.
  • The output voltage can be set by the equation Vout = 2.77[1+ (R1/R2)].
  • The time duration can be set by the equation T = (R3C3/0.45) (Vout – 0.45).
  • The maximum current capacity of this regulator is 2A.

1 Comment

  1. adi triadi

    I need automatic off power supply, when there’s no load to the output