Did some research on Zeners ...

When using the following schematic ...

First I would need to find out the equivalent load resistance of the circuit.

I am feeding it with a onespot which puts out 9.4V. Because R = V/I = 9.4V /

**x** (current draw of the circuit) =

**Y** Ohms.

To calculate the series resistance, I need to know which input voltages I would allow. Let's say 9-10V.

My zener is 500mW.

500mW/9.1V = 54.9mA and so,

R1_min = (10V-9.1V)/(

**x** mA + 54.9mA) =

**z** Ohms.

For real zeners a more practical rule of thumb is to ensure the zener current doesn't drop below about 1/10th it's maximum current.

In this case the minimum zener current would be 5.5mA and

R1_practical_minimum = (10V-9.1V)/(

**x** mA + 5.5mA) =

**z** Ohms

Rs_max = (9.2V-9.1V)/x mA =

**z** Ohms

If this is correct, then I still don't have a solution for the Rs_max if I feed the circuit with 9V. I guess the Zener just does not conduct anything and the circuit functions as before.

Could some mastermind perhaps shed some extra light?