How to choose Zener diode current limiting resistor

How to choose Zener diode current limiting resistor
Overview
Zener diode, English name Zener diode, also known as Zener diode. Zener diodes can be connected in series for use at higher voltages, and more stable voltages can be obtained through series connection.
The Zener diode plays the role of voltage regulation. When the load current decreases, the voltage drop across the current limiting resistor will drop, and the output voltage will rise, that is, the reverse voltage of the Zener diode will be relatively increased, and the voltage regulator current IZ will rise, so that the IRS will also increase. As the voltage rises, the tube pressure drop of the current limiting resistor RS rises, and the output voltage drops, so the output voltage remains unchanged. The disadvantage is that a larger output current cannot be obtained.

Voltage adjustment percentage:
the stability of the voltage, the lower the ratio, the better. When the DC voltage input VS or the load current IL changes, the output Vo can remain unchanged within a certain range.

VNL: Voltage output at no load VFL: Voltage output at full load
For example: the voltage stabilizer shown in the above figure has an output voltage of 7.5V at no load, and at rated current
output, its output voltage is 7.4V, find The voltage regulation of this regulator.

Calculation of current limiting resistor:
Current limiting resistor (RS):
(1) Provide the working current of VZ.
(2) Protect VZ from overcurrent damage.
Two extreme cases: a
(input voltage VS) VS = VS(Min), IL = IL(Max) (IL is the operating current of the load)
when VS = VS(Max), IL = IL(Min),

RS = RS (minimum value), IZ(Max) = maximum operating current;

the Zener diode needs to maintain a minimum stable current Izmin when it is working, and the Zener diode will not work when it is lower than the minimum stable current. See the attached picture, in order to ensure For this basic current, the current limiting resistor should be designed according to the formula below the figure.

Example 1: If the input voltage changes from 15V to 18V, the load current changes from 5mA to 15mA, and the voltage regulation value of the Zener diode VZ = 12V, the voltage regulation current IZ(Min)=0.25mA, IZ(Max)=50mA , Find the value of the current limiting resistor RS.
Solution: VS(Max) = 18V, VS(Min) = 15V, IZ(Min) = 0.25mA, IZ(Max) = 50mA

RS = 100Ω or 200Ω
2. VS = VS(Max), IL = IL(Min) VRS = VS - VZ IS = IL + IZ

When VS = VS(Min), IL = IL(Max),

RS = RS(Max), IZ(Min) = Minimum operating current
Example 2:
Known Zener diode Input voltage U1=12V, Zener diode stable voltage Uz=6V, minimum stable current Izmin=2mA, load current IL=10mA, try to calculate the current limiting resistor R.
Solution: R=(12V-6V)/(2mA+10mA)=6V/12mA=500 ohms, actually take R=470 ohms.