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Take as an example this circuit: this question is in fact related to a previous one.

When considering a variable, decreasing load \$ R_L \$,

$$ R_L I_L = R_L (I_S - I_Z) = V_Z $$

can be constant as far as the Zener diode balances the reduction of \$ R_L \$ by decreasing \$ I_Z \$, so: the Zener diode regulates the voltage. When \$ I_Z = 0 \$, the diode is useless and the resistances \$ R_S, R_L \$ determine the voltage across it, so they "regulate" this voltage. It seems that the Zener diode needs an external circuit (like the one composed by \$V_S, R_S, R_L \$) which can put it in reverse bias, and it can become a regulator only after this condition is reached. Otherwise, it is useless and its voltage is determined by the remaining part of the circuit.

Then, who is the actual regulator in this circuit? The diode or the resistances?

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The concept of different components "controlling" the behavior of a circuit under different operating conditions is not unusual in EE. These conditions have been explained in the previous question.

The answer to your question is that the zener+the resistors do the regulation when there is sufficient voltage across the zener to push it into conduction the resistors do the regulation the rest of the time.

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    \$\begingroup\$ ...meaning that you have to operate the zener diode within certain conditions for the circuit to work as you expect it to. There are always conditions where the circuit will operate differently and not have the behavior you want. The trick is to design the whole thing such that the proper conditions are always met under the circumstances you're using it. In this case that means that there needs to be some current flowing through the zener diode. How much ? Consult the zener's datasheet. \$\endgroup\$ – Bimpelrekkie Feb 2 '17 at 9:38

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