Basics of using zener diodes

[Externet]

This is not a hijack attempt, but to approach a different Zener configuration. If believed improper here, please delete this post.
A variable voltage at Vin; will the LED turn on only at Vin higher than Vz+Vled ?

+Vin-------------Zener-------------LED----------/\/\/\/\-------------gnd

Like on a circuit to show a minimum operating voltage is present, or battery charge status is good/above a treshold ?

 

[MikeMl]

Yes, but the turn-on threshold is not very precise, and the brightness of the LED varies with input voltage beyond the threshold...



This one has a much better defined, abrupt turn-on:



Here is one that has even a more abrupt turn-on, and is adjustable with a pot.

 

[Externet]

Thank you.
Is there Zeners rated to sharper knees ?. All I have seen/noticed is Vz and Iz, never a 'sharpness' figure . The sharpness would need a visual interpretation of the specifications plot, right ? (If there is much of a variety...)

 

[ronsimpson]

never a 'sharpness' figure

I do not know of any number for sharpness.
If you need a really good part look at TL431 or the lower current version ATL431. I think this part is rated many different ways.

 

[MikeMl]

Thank you.
Is there Zeners rated to sharper knees ?. All I have seen/noticed is Vz and Iz, never a 'sharpness' figure . The sharpness would need a visual interpretation of the specifications plot, right ? (If there is much of a variety...)

There is a "dynamic impedance" or ''Max Zener Impedance' spec which is the slope of Iz/Vz at currents greater than Izt. (e. g. 7Ω for a 1N5233)