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I'm currently studying the above circuit and trying gain a better understanding of it, I found the following forumlas to calculate Rs Max and Rs Min:

$$R_Smax = R_L(\frac{V_{in}}{V_z}-1)$$

Which I think comes from:

$$V_{in} = IR_{total}$$ $$V_{in} = I(R_S+R_L)$$ $$\frac{V_{in}}{(R_S+R_L)} = I$$ $$\frac{V_{in}}{(R_S+R_L)} = \frac{V_z}{R_L}$$ $$V_z = \frac{V_{in}R_L}{(R_S+R_L)}$$

Hence:

$$R_Smax = R_L(\frac{V_{in}}{V_z}-1)$$

If this is where it came from it doesn't seem right to me, Isn't the Zener Diode and Resistor L in a paralell circuit? because if the formula came from there it's like saying the zener diode is in series with Rs, but from what I see in the circuit drawing is that Resistor L is in paralell with the zener diode, so why not use the loaded potential divider formula? as opposed to the potential divider? plus it's like saying that the current Is is gonna be the same as the current in IL....

$$I_S = I_Z + I_L$$

The above is true right?

if someone can explain this to me I'd be grateful, it's been bothering me for days.

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  • \$\begingroup\$ From my practics I have found that there is a minimal zenner current, this current is bigger when diode has higher power rating. If the current trough zenner is below this minumum current, then it will not stabilze the voltage. The max current is calculated from max power rating and zenner voltage, everthing else is as you described Is=Iz+Il \$\endgroup\$ – Marko Buršič Sep 13 '15 at 22:01
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All of your assumptions are entirely correct. The piece that you are missing is when calculating Rsmax it is assumed that Iz is zero. The idea is that Rl is using all of the available current and the Zener diode is not conducting at all. If the Zener current is zero then it follows that the circuit is a simple voltage divider. The first equation that you have is the voltage divider equation that is solved for Rsmax because the other three values are known.

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I found this page help me a lot.

Full explain about it here

(sorry for my english but Im native speaking español but I love electronic and I find all page that can help with improve my knowledge in electronic and english too)

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