This 5 V circuit was found in December 1978 Radio Electronics magazine. The power transistor must be mounted on a good heat sink.
This circuit appears elsewhere on this site, but from different sources. Using the LM3900, we find this...
This interesting source or isolation transformer also allows to vary the output voltage by...
In this circuit the CA3140, an operational amplifier, is used as a comparator with the zener diode...