AD595, multiple thermo couples and digital switches

Projects General Guidance
1

So i recently purchased a Dual extruder for the 3d printer and typically I didn't notice that it used thermocouples not thermisters. Thus was not directly compatible with my RAMPS 1.4 board.

I found solutions online which use either the AD595 or MAX31855 to connect to RAMPS 1.4 (both of which are supported by the Marlin firmware) but both these chips are quite expensive and as such the costs build up pretty quickly for a dual extruder setup. Also none of the breakouts have been designed such that they fit straight onto the AUX parts of the RAMPS board. Looking at the two solutions I wasn't a fan of the MAX chip with SPI because you would need to connect it up under the SD card connection I use for the full graphics display which has an SD card slot included.

So the AD595 only needs 5v, gnd and one analog port meaning it could be used on AUX1 which wouldn't interfere with the SD card. But the problem remains for a setup with multiple thermocouples it is going to get expensive if we use one chip for each thermocouple. So it got me thinking could I not use something like this quad bilateral switch (50p) to simply change between the thermocouples and use a single AD595 chip (£7 or so per chip). You could use 3 of the aux pins (D1, D2, D58) and then A3 for the input. then 5v and Gnd for the power. leaving the other 5v and gnd pins unconnected.

Anyay this would mean a possible 3 thermocouples could be interfaced with using a breakout that would probably cost <£10 rather than having to use 3 seperate ones which are £7-8 each

Anyway i'm not an Electronics expert in any way so I need some advice to whether this would be feasible or not?

2

Thermocouples provide very low voltages, which do not get well through multiplexers.

3

Why is it difficult to pull the thermocouples out and replace with appropriate thermistors?

4

It is all neatly packaged in the hot end at the moment and i'd rather keep it that way (i've ordered two ad595 breakouts).

This was just me wondering if there was a cheaper and neater solution.

DrDiettrich I was worried about that - I'm guessing that is probably why no one has commercialised this idea already!

5

There's commercial products that do this. Industrial furnaces may have a hundred thermocouples and a PLC that doesn't have 100 thermocouple inputs.

Those boxes cost thousands though.

6

Yeah i've been trawling through a bit more on google and come across such things. I must admit the £10 for two breakouts that i managed to find on amazon doesn't seem so bad! I wonder why all the Hot ends from china come with thermocouples instead of thermisters.

MorganS:
There's commercial products that do this. Industrial furnaces may have a hundred thermocouples and a PLC that doesn't have 100 thermocouple inputs.

Those boxes cost thousands though.

7

MorganS:
There's commercial products that do this. Industrial furnaces may have a hundred thermocouples and a PLC that doesn't have 100 thermocouple inputs.

As soon as every thermocouple has its own amplifier, that amplified signal can be multiplexed, of course. Remember that the Arduino already uses an multiplexer for its analog inputs. The cold junction can be shared by a couple of inputs as well.

For really many thermocouples a multiplexer, ADC and digital interface can be integrated into modules for e.g. 8 or 16 thermocouples each. Such modules also reduce the number and length of the thermocouple wires.