low cost K type thermocouple interfacing - Page 1
Forum section: electronics / projects

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 06, 2017, 01:34:53 pm
content: We have some ASIC out there like MAX6675, the problem is that I need something more cheaper, my MCU cost only 0.5USD and it's a cortex M0 from ST (STM32F030K6T6)The price of the thermocouple interfacing circuit using MAX6675 is an overkill for our project. Do you suggest a cheaper solution? something like 1USD

author: trevwhite
author_rank: Frequent Contributor
timestamp: January 06, 2017, 01:43:12 pm
content: I think Microchip do a single channel 18bit adc that might be cheaper option

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 06, 2017, 01:55:27 pm
content: One option is to get a precision op amp and amplify the TC voltage and then feed into A/D.You will need a reference temperature also from a part like an LM60.Software will have to do the scaling and reference subtraction.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2017, 02:27:20 pm
content: For the TC interface you need a reference temperature and the measurement of a small voltage. For the reference temperature one could use something like a diode as a cheap sensor to read out by the µC. For the low voltage, a low offset amplification and than going to the µC internal ADC is good enough. Cheap AZ OPs like the MCP6V... series start at below $1. Accuracy of TC is usually not very high - so more or less normal 1% resistors could be good enough, depending on the requirements.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 06, 2017, 02:39:18 pm
content: Thanks for the suggestions, I think using the temp-sensor inside the MCU is a free option too. which op-amp amplifier topology or op-amp model do you suggest?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: January 06, 2017, 02:41:16 pm
content: AD8495 is not very expensive if you can use an internal ADC on the micro.CheersAlex

author: Marco
author_rank: Super Contributor
timestamp: January 06, 2017, 02:42:00 pm
content: You don't absolutely need auto-zero for the opamp, just pull the input low ... hey presto, you know the offset.Build something around HX711 if you want the absolute cheapest solution I guess, or are there any other ultra-cheap high bit ADCs out there now for cut throat prices?All these integrated solutions seem to charge a lot for very few transistors.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 06, 2017, 02:55:57 pm
content: HX711 sounds so nice! do you suggest connecting the thermo directly to the inputs?

author: Kleinstein
author_rank: Super Contributor
timestamp: January 06, 2017, 03:09:45 pm
content: Like some other SD ADCs the HX711 is sensitive enough to directly read the small TC voltage. Depending on the application one might want ESD protection or EMI filtering between the TC and the ADC.If an amplifier and µC internal ADC is used, one generally needs amplification, though a few µC already have quite sensitive ADC input (or internal amplification). It does not has to be an AZ OP, other OPs with low Offset ( <0.5-1 mV) are also Ok, but may not be cheaper.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 06, 2017, 03:22:00 pm
content: The TC voltages are with in the input range of the input B of the HX711,http://www.ni.com/white-paper/4231/en/#toc2The K type TC almost fit's the entire 80mv range, and we can get about 16bit ENOB from this so damn cheap part!https://forum.arduino.cc/index.php?topic=365107.0We got a winner in here, any Ideas to improve it future?

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 06, 2017, 08:13:19 pm
content: Does biasing the TC with a resistor divider to connect it to Port A of HX711 to have a higher gain a good design practice?

author: snarkysparky
author_rank: Frequent Contributor
timestamp: January 06, 2017, 09:24:42 pm
content: I don't think resistor biasing will increase the gain.Attached is the TC section of a meter I made for my grill. It has the distinct disadvantage of not reading temps less than the reference though. It does seem accurate.

author: Marco
author_rank: Super Contributor
timestamp: January 06, 2017, 09:53:45 pm
content: Quote from: ali_asadzadeh on January 06, 2017, 08:13:19 pmDoesn't increase the gain, but the common mode for the input has to be 1.3V removed from the rails so it's in fact necessary.

author: LaserSteve
author_rank: Super Contributor
timestamp: January 06, 2017, 10:16:22 pm
content: Microchip has a very nice APP-Note set on TC interfacing...http://ww1.microchip.com/downloads/en/AppNotes/00844a.pdfhttp://ww1.microchip.com/downloads/en/AppNotes/00684a.pdfSteve

author: Bootalito
author_rank: Unknown
timestamp: January 06, 2017, 11:16:50 pm
content: I've used an ad623AN(gain of like...333 or something, up to ~15mV as I only need around 300C range) instrumentation amplifier into an ADS1115 (gain of 4x, 0..1.024V) at 8 samples per second and was pleased with the results. I think I may have added extra software filtering. I like to use this page:http://www.schwietering.com/jayduino/filtuino/to calculate a butterworth low pass filter. Its WONDERFUL at really smoothing out the code.

author: ali_asadzadeh
author_rank: Super Contributor
timestamp: January 07, 2017, 06:02:18 am
content: The resistor Bias is only used so that I can interface the TC to the Port A of HX711, Please note that only port A has a 128 PGA and port B is only limited to 32