Resistor R SER must provide current, I ZEN, that can keep the zener diode in reverse breakdown ... Figure 3 Two LED drivers operating in series or parallel let you boost zener voltage or current.

The circuit is simple enough: it uses a capacitor, a resistor, and a pair of diodes (one of them a zener diode). He uses this basic circuit to drive simple regulators with input and output filter ...

[James Lewis] does an experiment with a regulator circuit ... You might also add a capacitor in parallel with the Zener to smooth out some of its noise. Zener diodes are wonderful little devices ...