How it know? - Page 1
Forum section: electronics / general

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 25, 2017, 12:40:55 pm
content: Hi, I have a DT-838 multimeter that gives the ambient temperature without the thermister attachment. How does the chip inside determine this without the sensor? I guess the chip has built in thermistor.P.S. can't read the chip on board.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 25, 2017, 12:55:54 pm
content: I don't think it is inside the chip, there will be a seperate thermistor on the board.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 25, 2017, 02:57:08 pm
content: This is a $8 dollar chinese multimeter. It´s sold with an without the temp function. Some versions measure Hz. Its got to be on the blob on chip which is covered by goop. Does anyone know what chip this meter uses?

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 25, 2017, 03:21:23 pm
content: Quote from: joseph nicholas on January 25, 2017, 02:57:08 pmSee here:https://www.eevblog.com/forum/testgear/dt-838-digital-multimeter-just-curious/

author: RoGeorge
author_rank: Super Contributor
timestamp: January 25, 2017, 03:31:26 pm
content: For 0.0V coming from the sensor, a multimeter will usually display the compensation temperature, AKA cold junction temperature (thermocouples can not measure absolute temperature, they measure the difference between the tip temperature and the cold junction temperature).For cheap devices, cold junction temperature is considered to be the temperature of a normal room, so these will always display 23-24, no matter the room temperature (without the thermocouple inserted).More elaborate ones are suppose to have another temperature detector at the cold junction point (usually at the probe connector). These are suppose to show the real room temperature (without the thermocouple inserted).

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 25, 2017, 05:12:47 pm
content: I put this meter inside my freezer for several hours. It recorded the temp inside the freezer without any probe attached. I'm starting to doubt my own sanity.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 25, 2017, 05:15:41 pm
content: Then the cold junction compensation sensor is inside the chip, nothing unusual.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 25, 2017, 06:12:51 pm
content: Quote from: RoGeorge on January 25, 2017, 05:15:41 pmAre you saying that the chip stays at room temperature when powered on?

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: January 25, 2017, 06:34:39 pm
content: Well I read the excellent English manuel and it says "set the select switch to temp, it will display the room temperature, plug in the k type thermistor and take a measurement". This is whats happening. I'm pretty sure the chip on board has a built in thermistor.Interestingly, it will probabily do Hz measurement with a new selector switch and paper sticker for the front cover.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 25, 2017, 06:35:35 pm
content: Quote from: PA0PBZ on January 25, 2017, 06:12:51 pmYes , many microcontrollers these days are very low power, so they don't heat themselves too much. E.g. microcontrollers from TI series MSP430 have a PN junction that is internally polarized, and the voltage drop is read by the ADC convertor included on the MSP chip. Once calibrated, is a good enough sensor to make a room thermomether. It can even sense the raise of the temperature when keeping the finger on the chip.If you can open the multimeter, try it. Keep your finger on the chip.- Is it hot? Probaly not.- Does the temperature displayed by the multimeter increases when you keep a finger on the chip? Probably yes.Anyway, it's nice to have a cold joint compensated thermocouple on your DMM, congrats!

author: RoGeorge
author_rank: Super Contributor
timestamp: January 25, 2017, 06:41:03 pm
content: Quote from: joseph nicholas on January 25, 2017, 06:34:39 pmI bet it's a PN junction (a diode) acting as a temperature sensor. The voltage drop on the diode is proportional with the temperature.Thermistors are not common to be build on a silicon chip die.