Multiplex Thermocouple Input

Other Hardware LEDs and Multiplexing
1

I want to multiplex thermocouple inputs into a MAX6675. There must be a million multiplexers out there. I'm guessing I need an analog one but don't know where to start looking on DigiKey or elsewhere. I did a search for 4051 and found hundreds to choose from... any suggestions are welcome. Also do I need to multiplex both wires or just one and and have one side of the thermocouples all connected together?

Thanks
John

2

Don't know the answer but some questions arise:

  • How many thermocouples do you intend to use?
  • Which type of thermocouples ?
3

I think up to 4 type K thermocouples should suffice.

I might add that high speed is not needed as a measurement once a second or so on each is overkill...

Thanks
John

4

http://datasheets.maxim-ic.com/en/ds/MAX6675.pdf

This data sheet looks good to me. The device is specifically made for type K thermocouples with cold junction reference. I guess you can use several chip select lines to drive several chips.

I'm adding this device to my ICs bookmarks.

5

Here is where I got mine for a kiln control I did recently, He also has a library for the arduino.
http://ryanjmclaughlin.com/store/featured/nulzg.html

Glenn

6

Awesome! 90 cents?! $)

7

I might mention that for the multiplexer package a DIP is preferred so it will work on the Arduino proto shield.

Thanks
John

8

Can Analog Multiplexer/Demultiplexer CD4051 be used as Digital Multiplexer/Demultiplexer ?

9

idris:
Can Analog Multiplexer/Demultiplexer CD4051 be used as Digital Multiplexer/Demultiplexer ?

It'll work but you'll lose a lot of precision. The CD4051 has a lot of resistance (and its variable) in its outputs. Look at the values of RON when looking for a multiplexer; a good chip will have differential channels and matched in the <3 ohm range. The ADG609 is commonly cited but I think you can find better chips if you poke around a bit.

10

So it means for analog multiplexer, i must use analog multiplexer, not digital multiplexer... tQ!!

11

No, it means that a thermocouple - is a thermocouple.

You have to understand what a thermocouple is; you have to know what "cold junction" means. You can't just connect a thermocouple to any old random circuit.

We were talking about multiplexing thermocouples, were we not?

All 4051s are analog multiplexers; but they can have different parameters; a 74HC4051 is restricted to 5V logic levels and has lower but significant "on" resistance. Allowing for the restrictions of the "on" resistance, they can be and are used as digital de/multiplexers.

12

If accuracy is a requirement, use a converter/cold junction compensator the each thermocouple and a member of the CD405XBE family to multiplex the data to the controller 1/2 of a CD4016BE (2 gates) will handle your issue nicely... A good rule of thumb is to use voltage followers from the thermocouple converter outputs to the multiplex inputs. THis is done to shift the output resistance down as far as possible to eliminate the small temperature variance of the on channel gate or switch.. It probably wouldn't hurt to read this on the Seebeck Effect Thermoelectric effect - Wikipedia.
Multiple measurements with thermocouples can be 'difficult' to do with any great accuracy and the only solution I've found so far is to measure the temperature and convert it locally to a buffered, possibly scaled value local to the thermojunction. This in order to present two or more clean (noise free) amplified and or scaled channels of analog data to the controller.

Doc

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