I've seen the discussion in an old question here

https://community.st.com/s/question/0D50X00009XkbAd/how-to-protect-analog-inputs-

Accordingly, STM32 ADC inputs are not 5v tolerant in ADC mode.

I started studying some Zener diodes, in particular 1n4727 Vz 3v

https://www.epitran.it/ebayDrive/datasheet/Zener_1N47xx.pdf

What I noticed in the graphs is that instead of what we commonly assumed of a reverse Zener voltage

https://en.wikipedia.org/wiki/Zener_diode

It looks instead like a conventional forward biased diode curve

https://en.wikipedia.org/wiki/Shockley_diode_equation

with a turn on voltage around 1.5v

This is quite curious and interesting as it implies that if I use this diode as a shunt or voltage limiter to GND, below 1.5v I get a 'linear' response as the diode don't conduct. Above 1.5v, it becomes a non-linear 'square root' style response. If I'm willing to make do with the fuss about that 'non-linear-ness', it would seem I can achieve much higher 'Vin' ?

Such diodes apparently can be found on those 'eBay/AliExpress' type sites

https://www.aliexpress.com/wholesale?catId=0&SearchText=1n4727

edit:

another question though

I can't seem to find a 'Shockley' equation to represent the zener breakdown region.

Any hints about an equation representing that part? i.e. the reverse zener breakdown region.

most cases seem to assume 'vertically down', but this 'weird' case doesn't seem to fit anywhere

https://community.st.com/s/question/0D50X00009XkbAd/how-to-protect-analog-inputs-

Accordingly, STM32 ADC inputs are not 5v tolerant in ADC mode.

I started studying some Zener diodes, in particular 1n4727 Vz 3v

https://www.epitran.it/ebayDrive/datasheet/Zener_1N47xx.pdf

What I noticed in the graphs is that instead of what we commonly assumed of a reverse Zener voltage

https://en.wikipedia.org/wiki/Zener_diode

It looks instead like a conventional forward biased diode curve

https://en.wikipedia.org/wiki/Shockley_diode_equation

with a turn on voltage around 1.5v

This is quite curious and interesting as it implies that if I use this diode as a shunt or voltage limiter to GND, below 1.5v I get a 'linear' response as the diode don't conduct. Above 1.5v, it becomes a non-linear 'square root' style response. If I'm willing to make do with the fuss about that 'non-linear-ness', it would seem I can achieve much higher 'Vin' ?

Such diodes apparently can be found on those 'eBay/AliExpress' type sites

https://www.aliexpress.com/wholesale?catId=0&SearchText=1n4727

edit:

another question though

I can't seem to find a 'Shockley' equation to represent the zener breakdown region.

Any hints about an equation representing that part? i.e. the reverse zener breakdown region.

most cases seem to assume 'vertically down', but this 'weird' case doesn't seem to fit anywhere

Last edited: