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RTD PT100 current sensor + 4-20ma output + display on 7 segment using microcontroller

Rakesh Sharma
Prodigy 40 points
Other Parts Discussed in Thread: XTR105, TMP112, ADS1118

Dear Sir,

I am designing a system where I want to measure the temperature and display the same on 7 segment using microcontroller. So I can easily make standalone temperature device but I want to produce 4-20ma also output corresponding to the temperature. I have selected XTR105 out of your products which will produce 4-20ma output. But this output will be used for data transmission. If I measure temperature using this 4-20ma output (putting 250 Ohm resistor) this will change my readings when actual load is connected to the 4-20ma output ( because the overall load resistance will change ).

Is there any solution using XTR105 or any other cheap product which can give temperature information as well as 4-20ma output ?

My preference will be PT100 sensor but any other thermocouple can also be connected.

Thanks and Regards:

Rakesh

  • TI__Guru**** 174800 points

    Hi Rakesh!

    Welcome to the e2e forum! Do you have a particular data converter in mind to connect with your PT100?  Do you have a preferred micro controller from the MSP430 line or one of the other DSP/DSC controller families?

  • in reply to Tom Hendrick
    Prodigy 40 points

    Hi Tom,

    Thanks for your reply.

    Currently I am using 16F833 ( it has 10 bit ADC). Because my initial development was on this controller so I have board and everything ready. However I wanted to use MSP430 but I don't have much expertise with it and also the availability and price in India is a big concern. May be I can move to msp430 in future because of it's better performance but again depends what price I get from vendors. This product will be in production in 2-3 months. I don't have any experience on DSP/DSP controller so I will avoid to use them because of shortage of time.

    So leaving behind the controller part, I have to feed the input to controller and also in parallel to generate 4-20ma output w.r.t. temperature. This is my primary requirement is to get the 0-5V output for the range of 0-100 degree C. I can feed this to any controller for processing. 

    Also just FYI I have only 24V DC and I want to avoid any -ve voltage which will increase my cost.

    Will it be easier to use perform this task if I use DSP/DSC controller ?

    Thanks and Regards:

    Rakesh

  • in reply to Rakesh Sharma
    TI__Guru**** 174800 points

    Hi Rakesh,

    You could take a look at the ADS1118 or perhaps a device like the TMP112 - the TMP series devices would be the simplest to use.  I don't believe going to a DSP/DSC will make your task any simpler at this point.

  • in reply to Tom Hendrick
    Prodigy 40 points

    Hi Tom,

    Thanks for your post.

    It seems you didn't understand my requirement fully. 

    I wanted to interface PT100 which 100Ohm RTD.  TMP112 is the on chip temperature monitoring system which is not my requirement. Again ADS1118 is the evaluation kit from TI which uses thermocouple. Both the products are far away from my requirement.

    I thought TI should be having some one chip solution like XTR105.

    Well thanks for your time. I am designing using OP-AMPs which will meet my requirement and cost also.

    Thanks and Regards:

    Rakesh Sharma