Temperature vs Diode Voltage in Schokley's Equation - Page 1
Forum section: electronics / beginners

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 02, 2021, 01:57:50 pm
content: I was recently tasked with designing a temperature sensor using a general purpose diode.The design uses a constant current source (1mA) for diode excitation.So far so good. The circuit works as expected, with an increase in temperature the voltage across the diode decreases.The next step is to convert the diode voltage into a temperature reading so I applied Schokley's Equation:Id = Is(exp(q * Vd / n * k * Tk) - 1)Where: Id = diode Current, Is = Reverse Saturation Current, q = magnitude of Electric Charge, Vd = Voltage across the diode n = ideal factor, K = Boltzmann's Constant, Tk = Temperature,I manipulated the equation to get:Tk = (q * Vd) / (n * k * ln ((Id / Is) + 1))This shows that Temperature is directly proportional to The voltage across the diode.At this point I'm not sure why my practical test and the theory do not match.Any help solving this puzzle would be greatly appreciated.

author: TimFox
author_rank: Super Contributor
timestamp: September 02, 2021, 02:23:29 pm
content: Did you remember to use temperature in Kelvins, not Celsius?

author: emece67
author_rank: Frequent Contributor
timestamp: September 02, 2021, 02:28:12 pm
content: .

author: iMo
author_rank: Super Contributor
timestamp: September 02, 2021, 03:00:00 pm
content: 1mA is too much. Try with 50-100uA instead..

author: pqass
author_rank: Super Contributor
timestamp: September 02, 2021, 03:20:05 pm
content: Quote from: RealGsJR0M3 on September 02, 2021, 01:57:50 pmI think this guy has your answer:https://www.electronicspoint.com/forums/threads/question-about-diode-temperature-and-forward-voltage.10578/#post-67894

author: rstofer
author_rank: Super Contributor
timestamp: September 02, 2021, 03:43:12 pm
content: This TI paper might be useful. TI recommends a BJT instead of a diode because the BJT has a more consistent temperature coefficient.https://www.ti.com/lit/an/sboa277a/sboa277a.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2021, 03:51:57 pm
content: When using a single fixed current one still need 1 calibration point. In first (often quite good) approximation the voltage minus the bandgap voltage (1.12 V) is proportional to the temperature in kelvin. It still needs 1 cal point to get the constant / starting point.With two test currents well seprated (e.g. 10 and 100 µA) one could get an direkt reading with ideally (the diodes following the Shockley equation) no etwa calibration neede: dU = kT/e * ln(I1/I2) .Transistors often behave more like an ideal diode as the material is often cleaner.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 02, 2021, 04:28:23 pm
content: Also, when you use two different currents (approx a actor 10 apart) then most terms in the equation fall away and the response is fairly lineair. This was used in an elektor project somewhere in the '90-ies (or was it '80-ies?)

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 02, 2021, 06:02:29 pm
content: Thank you for the feedback. I have not plugged in any variables as of yet but I am aware that the temperature obtained using Shockley's equation is in Kelvin.

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 02, 2021, 06:09:18 pm
content: Thanks for the feedback Mr. rstofer. I got the idea to use a constant current source from that very document.

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 02, 2021, 06:11:58 pm
content: Thanks for the tip Mr IMO but how does changing from 1mA - your recommendations affect my readings?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2021, 07:57:28 pm
content: Going to a lower current gives less heat at the diode and thus less self heating effect. Even with 10 µA the differential impedance of the diode is still not too high to normally be a problem. With a relatively large transistor (e.g. To3 or TO247 case) as a sensor 1 mA and thus some 0.5 mW of power can still be OK. For a small one like a sot-23 it already gives a noticable temperature rise. Chanes are a small transistor is closer to an ideal diode region at lower current, as resistance on the die has less effect.The voltage gets a little smaller and this leads to a slighty high slope for the voltage.

author: ejeffrey
author_rank: Super Contributor
timestamp: September 02, 2021, 08:05:45 pm
content: The variation in Is is exponential in T and completely dominates the change in kT/q for a single point measurement.That is why you measure the ratio of two currents, so that Is drops out.

author: iMo
author_rank: Super Contributor
timestamp: September 02, 2021, 08:32:45 pm
content: Quote from: RealGsJR0M3 on September 02, 2021, 06:11:58 pmI built several thermometers with TO-18 (metal case) transistors (base-emitter diode) and something like 50uA current, and it was pretty linear from -18C (fridge freezer) to +100C (boiling water).

author: magic
author_rank: Super Contributor
timestamp: September 03, 2021, 09:05:34 am
content: Also, LM334

author: iMo
author_rank: Super Contributor
timestamp: September 03, 2021, 09:42:25 am
content: The LTspice is using the similar math - you may simulate the Temperature dependencies of your circuits as well.Here is an example how to do it.There is the self-heating of the diode with larger currents, for such a simulation you have to add some math in the transistor model, imho.

author: iMo
author_rank: Super Contributor
timestamp: September 03, 2021, 10:34:52 am
content: You may also print out the voltage values at specific temperatures and put the numbers into the excel sheet, see below an example..

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 03, 2021, 05:38:28 pm
content: Thanks for the feedback Mr Kleinstein ... So much for my "simple" temperature sensor... I found a document that confirms what you were saying ... looks like 1mA was the cap for doing this experiment ... i will definitely lower the current to your recommended values ...

author: RealGsJR0M3
author_rank: Newbie
timestamp: September 03, 2021, 05:43:19 pm
content: Quote from: magic on September 03, 2021, 09:05:34 amThanks for the tip with the LM334 ... but this experiment is just a learning exercise ... precision is not really the main the objective ... looks like the lecturer wants us to learn the basics characteristics of diodes in this particular application