Simple power supply design based on 555 timer IC. The bottom-right voltage source is the reference voltage. In real life, I recommend replacing this with a voltage divider on the +30V input. For more precise adjustment you can use a two potentiometer design (http://everycircuit.com/circuit/5806487055630336).
If you adjust the reference voltage and the output voltage doesn't change fast, but slowly, that's because the low simulation speed. With these part values it takes about 140 milliseconds for the output to fall to 1V from 29V.
The top-right resistor simulates the load. You can replace the 100uF capacitor with a better low-pass filter. The frequency of the 555 timer is 90,91 kHz.
