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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Converting a NTC voltage into temperature in Celcius
Date: Sat, 12 Apr 2025 00:57:03 +1000
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On 12/04/2025 12:22 am, Don wrote:
> Bill Sloman wrote:
>> Don wrote:
>>> brian wrote:
>>>
>>> <snip>
>>>
>>>> I ended up using a diode as a temperature sensor as I
>>>> couldn't get it linear enough.
>>>
>>> Interesting. Thank you for sharing. When operated in Zener mode (as
>>> opposed to avalanche mode) a Zener diode's NTC apparently remains
>>> approximately the same, regardless of orientation (forward or reverse
>>> biased).
>>
>> Whether a reverse biased diode breaks down in Zener mode or avalanche
>> mode depends entirely on the diode you have selected.
>>
>> if it breaks down at less than 5V reverse voltage it is breaking down by
>> the Zener mechanism (with a negative temperature coefficient) , and if
>> it the breaks down at an 8V or a higher reverse voltage the avalanche
>> mechanism is dominant (with a positive temperature coefficient.
>>
>> https://toshiba.semicon-storage.com/us/semiconductor/knowledge/faq/diode/are-there-any-special-considerations-for-the-temperature-coeffic.html
>>
>> doesn't exactly conform to your claim.
> 
> My statement
> 
>      When operated in Zener mode (as opposed to avalanche mode)
> 
> pertains Zener diodes in Zener mode. Why do you want to talk about the
> positive temperature coefficient of Zeners in avalanche mode?

The problem is that you choose the mode when you choose the diode. Your 
formulation suggests that you think somebody could to operate one diode 
in one mode or the other. Only a hopeless newbie could think that, but 
some of the lurkers aren't all that sophisticated, and a few of our 
posters aren't much better.

Gerhard Hoffmann has pointed out that even a 4.7V zener diode has enough 
avalanche component in it's breakdown mechanism to introduce lots of 
noise though the temperature coefficient of the reverse voltage is still 
marginally negative.

-- 
Bill Sloman, Sydney