NTC TCR Tolerance mismatch from datasheet - Page 1
Forum section: electronics / projects

author: thexeno
author_rank: Regular Contributor
timestamp: October 25, 2017, 09:38:15 pm
content: I dig into NTC parts and I wanted to find their error without calibration. So I started from datasheet of theNTCLE100and verified what I learned.What I've learned: I have understood how to find the resistance error due to B factor, the temperature coefficient and the final temperature error, so that I can survive even with very minimal datasheets. To verify this, I thought to get fancy datasheets with very well explained tables (like the NTCLE100).Correct me if I am wrong.Basically, the \$\frac{\delta R}{R} = \frac{\delta R_{25}}{R_{25}}+B\cdot B_{toll}\cdot(\frac{1}{T}-\frac{1}{T_{nom}})\$and the \$ TCR = -\frac{B}{T^2}\$ and \$ \delta T = \frac{\frac{\delta R}{R}}{TCR}\$But from the datasheet of a slightly different NTC,NTCLE413, with this table I can pickup values, and try if things are like the NTC100:While I try to verify the formulas, I don't mismatch that much from datasheet value and calculations, but for example, at 0°C I get 0.67K of temperature absolute error instead 0.71K.While if I apply the values from the datasheet to verify the other parameters with the previous formulas, I tried with the provided TCR = -5.10% to find (and verify) if the B used is the one stated in datasheet, according to values on the table (like such TCR), instead 3984K, and I find 3805K instead. Which is more than 0.5% from the centered value of B.So I think that the values in the table are actually calculated (or measured?) from a slight different B characteristics (maybe some kind of worst case?). I'd like to know why I get a mismatch in those values, but is not clear to me yet. Rounding maybe, or implied assumptions from the manufacturer?EDIT: there is a datasheet of thisNTCthat actually provides two types of B, one "nominal" with a given error and one slight different between round brackets. May this be relative to what I find in calculation mismatches?

author: thm_w
author_rank: Super Contributor
timestamp: October 26, 2017, 01:12:34 am
content: Did you try this?:http://www.vishay.com/thermistors/ntc-curve-list/They are probably not using B values to get the numbers in that table, but some higher order steinhart-hart calculations.https://www.thermistor.com/sites/default/files/specsheets/Beta-vs-Steinhart-Hart-Equations.pdfQuote

author: thexeno
author_rank: Regular Contributor
timestamp: October 29, 2017, 06:37:14 pm
content: Quote from: thm_w on October 26, 2017, 01:12:34 amYes I know it. But for own knowledge I use the auto-generated curves for exercise or the like, while I learn to be independent from this engineering services, since many other manufacturers do not provide such table generation.QuoteAnd this also made me think about a thing. The Beta error provided in every datasheet, does take into account the Beta deviation from the real curve? Since we know it is less accurate than the Steinhart-Hart equations, I wonder if this is taken into account while calculating the \$ \delta T \$.

author: thm_w
author_rank: Super Contributor
timestamp: November 01, 2017, 11:19:47 pm
content: Quote from: thexeno on October 29, 2017, 06:37:14 pmIf they don't provide the table generation, then ask them for a data table or steinhart constants, etc. I suspect any decent manufacturer would be able to provide that.Maybe in some cases you can work backwards from the data they provide.QuoteIts a good question.Although I personally wouldn't think much about it, as you wouldn't want to be using the beta equation if you care this much about accuracy.

author: thexeno
author_rank: Regular Contributor
timestamp: November 02, 2017, 09:42:33 pm
content: Quote from: thm_w on November 01, 2017, 11:19:47 pmI see, that is one way to see things. I have Beta tolerances which I can apply, but if I want to see how the ideal one deviates, maybe is not a good idea to use the Beta. If this is a good way to proceed, maybe the influence should not be that much, like let's say on 0.5°C of temperature deviation, it shall be << than 0.5°C, otherwise manufacturers are "lying" and my design which shall be precise to 0.5°C (maybe even with overestimated error) has a chance to not be compliant not on designer's fault.Would be nice to see an opinion from someone in this industry.