In this circuit, a 317 variable voltage regulator has been provided with a facility whereby, after the input voltage has been switched on its output voltage rises only slowly to the value set with P1. For instance, if P1 was set for an output of 15 V, that value is reached only after 5 seconds. If the output voltage was set at 7.5 V, that value would have been reached after 2.5 seconds. In other words, the delay time is directly proportional to the set output voltage. The delay may be lengthened by increasing the value of R3 and C3. The delay may be disabled by removing the link at JP1 (which is useful when P1 is being adjusted).
Diode D2 protects the base-emitter junction of T1 against to high a reverse bias. Without the diode, if the output of the regulator were short-circuited, charged capacitor C3 could cause a UBE that exceeds then permissible maximum 6 V, Diode D1 protects the regulator if the input voltage were (accidentally) to drop faster than the output voltage.
The circuit draws a current- of only a few mA, although the peak current may amount to 1.5 A. To prevent the temperature protection of the LM317, mounted on a 14 K W-1 heatsink, coming into operation, the constant output current should not exceed 500 mA.