Temperature AI with S7-300 SM331

Hello all,

Here is what I am trying to figure out:

(Note: The Thermistor itself is fixed there is no way I can change it out so I have to design around it. Also it is not an RTD.)

I have an SM331 Module on a Siemens S7-300 314C-2 DP

1. What is the best way to convert a 2.7kohm thermistor signal to an analog input for use with the SM331.

2. I have two Type K Thermocouples that I need to read in as well.

For the thermistor, I have searched online quite a bit and have seen suggestions from a simple voltage divider to some more complex bridge circuits. Some of these invole constant current sources etc.

I searched for an off the shelf signal converter and have come up with nothing.

I have access to anything I need to build a conversion circuit or if there is something I can buy for the S7-300 that just plugs in and I don't have to do anything that's even better.

For the two thermocouples, are there module(s) for the S7-300 that I can just plug the thermocouples into? Maybe I just missed them in the catalog.

What would you do? Thanks in advance.

I would find a 10 volts DC supply, wire it to one side of the thermister, and wire the other side of the thermister to my 0-10 vdc analog input. Now I would physically test to find the measured voltage levels for Maximum Expected Temperature and Minimum Expected temperature. I would apply heat to the thermister and measure the temperature and the voltage output to the analog input. Then I would set up a scale function to scale the analog input to convert the measured voltage values to the actual temperatures.

I do not know if Siemens has a Thermocouple Input Module. Allen Bradley does, so probably Siemens does also.

S7-300 Analog Input Module SM 331; AI 8​

All the standard Siemens AI modules take voltage, current, thermocouple and RTD - all at the same time. It's just a matter of setting up each channel in the Hardware Config and following the wiring for that channel in the manual. There are also some specialised RTD or TC modules where you need particular ranges or specs for which you're prepared to pay. I would guess that 90%+ of applications will be dealt with using standard modules.

Ken

Thanks for your replies.