Here is the circuit diagram of a simple soft start power supply using the well known voltage regulator IC LM317.Here the IC is wired in soft start mode in which the output of the IC gradually increases to the desired output voltage .When the input voltage is given to the IC , the capacitor C1 starts to charge and the current from the adjust pin (pin2) of the IC increases.The output voltage of the IC at any time is proportional to this adjust current(I adj).So the output of the IC slowly increases from 0 to 15V which is set output voltage here.

Circuit diagram with Parts list.



  • The circuit must be assembled on a good quality PCB.
  • LM 317 can deliver only up to 1A 0f current.
  • The input voltage must be few volts higher than the desired output voltage.
  • Here the input voltage can be 18 or 20V DC.
  • Capacitor C2 must be rated at least 30V.

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