Correct temperature coefficient of silicon diodes - Page 1
Forum section: electronics / projects

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:32:34 pm
content: In connection with the development of my “poor man’s voltage reference”I am using one diode of the BD 139 silicon transistor to maintain the LM 317being used as the voltage reference at a constant temperature. Thistemperature regulator itself is not the problem. But I would like to displaythe temperature as measured with the BD 139 with some precision.For this I need the actual temperature coefficient for silicon. Mostcommonly a value of -2 mV/K is stated in many textbooks. But I havealso found other values ranging from -1.4 mV/K to -2.2 mV/K. Beforegetting a precision Pt 100 temperature sensor and running thismeasurement myself, I would like to ask the community here,if anyone has run or is willing to run his own precision silicon temperaturecoefficient measurements.The conditions in my circuit are:Supply voltage: 5 V.Dropping resistor: 1 kOhmTransistor: BD 139 (base-emitter diode with the collector connected to base).Whereby any medium power silicon transistor will do.Temperature range: about 20 to 50 deg. C.I would certainly welcome any results.Yours – Messtechniker

author: Kleinstein
author_rank: Super Contributor
timestamp: November 03, 2015, 09:44:04 pm
content: The TC depends on the current through the diode. With silicon diodes the voltage will be rather linear, extra polating to about 1.205 V at 0 K. So you can calculate the expected TC from the volatage at one known temperature. If its 605 mV at 300 K you should get 2 mV/K.Not all the BD139 will be the same.

author: ralphd
author_rank: Frequent Contributor
timestamp: November 03, 2015, 09:55:45 pm
content: Jack has written everything you need to know.http://www.cliftonlaboratories.com/1n400x_diode_family_forward_voltage.htm

author: nuno
author_rank: Frequent Contributor
timestamp: November 04, 2015, 12:44:05 am
content: Don't remember where I got this info from, but what I have, for a typical PN diode, is -2.3mV / ºC @ I = 1mA "pretty linear" in the range 0ºC .. 100ºC.

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 04, 2015, 07:28:49 am
content: Quote from: ralphd on November 03, 2015, 09:55:45 pmIndeed. Thanks for the link. Based on the well researched information provided there,I will use a value of -2.0 mV/K.Yours - Messtechniker

author: Alex Nikitin
author_rank: Super Contributor
timestamp: November 04, 2015, 09:28:38 am
content: Quote from: Messtechniker on November 03, 2015, 09:32:34 pmI would just use a calibrated temperature sensor, for example the LM35. It outputs 10mV per degree Centigrade (i.e. for 50C the output is 500mV) and has <1C (0.4C typical) error at 25C for the cheapest "D" version (£1 each from Farnell for TO-92 case version and £1-50 for TO-220 with a metal tab).CheersAlex

author: Messtechniker
author_rank: Frequent Contributor
timestamp: November 04, 2015, 11:28:20 am
content: Excellent Alex! Did not know that LM 35s existed with a TO-220 case.And they are available from Reichelt (a large supplier of electronics parts in Germany) too.A couple are on my purchasing list now. Will make a nice pack with the LM 317 TO-220and the two BD 139s used for heating as well as the two voltage setting resistors attached to the packas per your instructions.By the way, my poor man's 10 V voltage reference is now in a metal case and I am heatingit to around 50 deg. C. Am getting something like +/-2 ppm peak to peak over one hourwithout much burn-in. Averaging over 100 samples (seconds) yields +/- 0.5 ppm re. 10 V.This probably also includes some drift from my Keysight 34465A.Graphs to follow in a week or two.Simply need time for burning-in.Yours - Messtechniker