electrical logic of current limiting

[yefj]

Hello,given the schematics below the zener diode gets reversed biased at 4.7 V.
so the current is I=(15-4.7)/R .
we automatickly assume we have 4.7V on the zenner, and we can calculate the current by the resistor.
under what conditions the zenner will not enforce the 4.7V threw it?
Thanks.

https://www.mouser.co.il/datasheet/2/268/1N746A_1_1N759A_1_2c1N4370A_1_1N4372A_1-3442192.pdf

--- Updated Sunday at 7:06 PM ---

UPDATE:
Hello ,I have made a plot shown below, I see that in 9mA current I have good 4.7V output as shown below.
In the lab I have a voltave source of 12V , how this plot will help me deside what current limiting resistor to choose?
Thanks.

 

[crutschow]

You decide the current you want for the desired voltage, and then calculate R1 from the equation in your first post.

 

[BradtheRad]

5mA is commonly recommended to operate a zener diode. Or 9mA is reasonable. If the load requires greater Amperes then you can reduce the resistor Ω.

However if the zener overheats (hot enough to burn fingers) then consider installing a higher W zener. Or make the zener part of a transistor network which performs the shunt role.

 

[yefj]

Hello, There is a similar case in NPN,I swept the base current.
I want to decide what base limiting resistor to use.
I made plots shown below.
I have 12V voltage source i want base current 37uA.
that meens my resistor is 12/37uA=R which is very very large.
Is there a way to reduce the need for such a large limiting resistor?
Thanks.

--- Updated Sunday at 9:05 PM ---

UPDATE:
I mamaged to lower the needed base limiting resistor value needed, by changing the resistor values in collector and emitter.
Thanks.