Thermocouple mux selection - Page 1
Forum section: electronics / projects

author: lincoln
author_rank: Regular Contributor
timestamp: July 21, 2015, 09:24:38 pm
content: Hello,I find my self in need of a multi channel thermocouple meter that has the ability to log data over the course of a few days.Fist though was is guyhttp://paxinstruments.com/products/sku927/I don't really need a graphic display but for 150$, it seems to fit the bill. But they are out of stock. (web site says they will be in stock this month, I may still buy one)Ok, dug through my bins, found aMAX6675, (K type to SPI ADC) and a few AVRs and a ftdi chip (yes I know, boo this man) So I have everything I need for one channel.I haven't used analog muxs before so on that basis I will do this project. Having read a few paper form Analog Devices and Maxim ic on cold junction temperature comp and analog mux'ing, as long at the ACD and the mux are iso-thermic that should not cause a problem.looking at the criteria for picking a mux chip:* Signal level is low, so a 0~5v or 5~ -5v mux will work.* Input impedance to the ADC is 60k, it shouldn't matter too much what the RDSon of the mux switches are. ie I don't need a mux with the lowest .4 ohm RDSon.* Is should be an smd chip so that is can be mounted next to the ADC.* I really only *need* 3 channels, My thoughts are that a dual 1:4 chip would fit the bill. Internally if there is only one die, both muxs would be well matched. if the were two separate dies they should sill be closer than just two randomly selected chips. Though for this application, if the is ±50 ohms diff I don't see that it would matter.Is this reasonable? What am I not taking into account?Specifically looking at the MAX6675, Its not what I would consider a "Lab grade" thermometer chip, more so when you get into the higher temperatures. It also doesn't go below 0c. I think this will be good enough for now.Looking at the data sheet, the inverting input must be tied to ground. At that point do I even need a two channel mux? Instead the Non-inverting side would go to a mux and a star point ground for the inverting input and all the ground returns for the thermocouples. There is the issue of input bias currents....What do you think?Link

author: Frant
author_rank: Regular Contributor
timestamp: July 21, 2015, 10:41:25 pm
content: You may want to consider using the LTC2983.

author: lincoln
author_rank: Regular Contributor
timestamp: July 22, 2015, 05:07:29 am
content: Quote from: Frant on July 21, 2015, 10:41:25 pmjeslus, 35$, 20 channel just about any sensor... that guy may just warent its own project....

author: Kleinstein
author_rank: Super Contributor
timestamp: July 22, 2015, 09:11:53 am
content: If the thermocoules are isolated (not external connection) a single channel MUX would be enough.If you don't mind doing the calculation yourself one could also consider a simple amplifier ADC combination instead of the special thermocoulple sensor. The cold junction temperature is just one more channel with a simple (NTC or diode) sensor. However this would need a 0 point calibration.The MUX is not that critical. Even a HC4051 should work - with still the option to upgrade to a max4051. Consider 1 channel for a fixed 0 Volts. As the MUX is directly a the input consider a soket, even if this means a minimal extra source of errors. Thermal design ist likely more important than the chip quality.With only 3 channels one might even consider having amplification beforre the MUX, if highest accuracy is needed.

author: lincoln
author_rank: Regular Contributor
timestamp: July 28, 2015, 05:42:10 pm
content: Hello,Here is a giant photo dump of current progress:http://imgur.com/a/eBEm7still need to drill holed for the buttons and find a DC/DC converter module to power the DAC.

author: charlespax
author_rank: Regular Contributor
timestamp: July 28, 2015, 07:12:06 pm
content: Quote from: lincoln on July 21, 2015, 09:24:38 pmHi, lincoln :-) Thanks for considering theT400. I'm in Shenzhen right now working to get it manufactured, ship the Kickstarter rewards, and stock some units.If I had known how long this project would take, I would have waited much longer to launch it. The worst part is the long wait times between steps.All, but a handful of days have been waiting on samples. I wish there were work I could to do make things move along more quickly. I work on other projects in the mean time, but it doesn't stop my heart from pounding in my chest when I think about the delays. A healthy mix of anxiety and excitement.The custom thermocouple connectors have been holding things up. We received samples that werealmost good enough, but we had to make modifications to the tooling. A little extra money. A little extra time :-(As they say, "you can have it fast, cheap, or good. Pick two." I'm going with cheap and good. The goal here is to deliver a solid product without making any huge costly mistakes. This actually worked really well in the custom connector situation. The connector changes require changes to the PCB, which require a slight change to the enclosure. If we weren't doing things serially, this could have been very costly or we would have had to compromise on quality.If you would like to be notified when the T400 is stocked, please sign up for themailing list.