Soft Start PSU

Two soft start power supplies. The output voltage slowly increases to the desired output.

The output voltage rises slowly and reaches 15 V in 5 seconds.

In order to perform this soft-start function, the LM317 voltage regulator IC requires an external universal PNP transistor and the L200 uses its internal comparator (pin 2).

After switch-on, the rising voltage on the positive side of the charging electrolytic capacitor slowly turns the (initially conducting) transistor off, thus raising the voltage (relative to ground) on the adjustment pin of the LM317. In the L200 circuit, the corresponding electrolytic capacitor's rising voltage gradually relaxes the current-regulation loop inside the L200.