The trouble with zener diode driven power supply is that one needs a zener of the same voltage as the desired voltage output. We can overcome this using a voltage regulator such as LM 317
This regulator maintains a constant voltage of 1.25 volts across two of its terminals. So, connecting it in the above configuration and neglecting the , we have when , . For V, if we fix to be , we get to be .
The next thing we need is the value of required. Suppose the load is such that mA. The current thorugh and is
mA. Therefore, mA. For no ripple, the capacitor should be able to supply this current without the voltage dropping below 15 V. Now, taking the worst possible instant of time, we have,
On the other hand, for a 1mF capacitor, volts.