How to drop input voltage to an L7805 using a zener diode
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Voltage dropper for L7805.


  I want to use a zener diode on my adjustable pin of my regulator so as to decrease the amount of voltage going in my regulator.


The GND pin can be used as an adjust but this increases the output voltage.

You are trying to reduce the input voltage to reduce the power dissipation in the L7805. To do this you add a zener in series with the input.

You need to calculate the power dissipated in each. For the zener the power dissipated in it will be \$ P = V_Z \cdot I \$. For the L7805 the power dissipated will be \$ P = (V_{IN} - V_{OUT}) I \$.

This will not be efficient but if the current is low it may not matter to you.