zener diode inuition machanism for regulation

[yefj]

Hello,I have the following circuit of a shunt regulator.I kow that the zenner diode enforces certain voltage threw it.
In case of sudden increase in R_L the V_l wil rise so Vbe will be smaller.
So how the zenner diode willinteract with this process and make the V_L voltage smaller beack to the original value?
Thanks.

 

[Tony Stewart]

another perspective 0 to 2.8A active NPN load from a DC-biased triangle generator.

The 1N4742A is a 1W, 12V Zener diode, is a a nominal Zener voltage of 12V 5% at a test current IZT=21 mA, in the datasheet has a 9Ω Zener impedance, 5 μA maximum reverse leakage current, etc.). A 1A diode was added to correct for the Vbe error in a 50W power transistor to obtain 12V out.

As the output goes towards no-load, Vbe drops rapidly with Ib as Iz rises but it does not raise Vz as much as the drop in Vbe so any design must ensure the error budget does not exceed the output tolerance for all ranges in bias current.

There is no attempt to say this is an efficient design, nor optimized for low Vo error, but to demonstrate the weakness of Zener linear regulators and starving Vbe of current for learning how to avoid pitfalls in regulation errors.