Many systems require one or more relatively low-current voltage sources which can be programmed tna few predetermined levels. The circuit shown above produces positive output levels, but could be modified for negative or bipolar outputs. Q1 is the series regulator transistor, selected for the required current and power capability.
R1, Q2, and Q3 form an optional short circuit protection circuit, with R1 chosen to drop about 0. 7 V at the maximum output current. The compensation capacitor, C, should be chosen to keep the overshoot, when switching, to an acceptable level.