RTD Circuit in Open state

[garg29]

Hi friends,
I'm measuring temperature with a RTD Sensor. The circuit is working fine ( I have attached the circuit). But in case if there is no sensor present the output voltage becomes -10.47 volts, instead of this I need +5volt or above to detect the "SENSOR OPEN" state. Sorry I'm very poor in analog circuitry. Can you please help me out for a solution.

RW1, RW2 & RW3 are wire resistance (wire from circuit to sensor)

Thanks.

 

[noether]

I guess the problem is because R7 is unconnected when no sensor is present.

What about a pull down resistor before R7? about 1MegaOhm (I'm not an expert too xD)

 

[FvM]

What about a pull down resistor before R7

You actually need to pull it up. You'll cause the smallest possible error by connecting a high ohmic (e.g. 5 or 10 Mohm)
resistor between both positive (source and sense) RTD terminals.

 

[garg29]

Thanks FvM for replying. I completely agree with your statement. But can't this eror be compensated back in software. I tried a solution in simulator will this work?

 

[erikl]

... I tried a solution in simulator will this work?

Yes, it will work. But a few comments:

• I'd think FvM's solution (10 MΩ resistor between both positive (source and sense) RTD terminals) is simpler & better
• you missed the original 50kΩ (org. R7=49.9kΩ) resistor, which is good for input offset current cancellation (at normal operation conditions)
• R8 (voltage limit) is only necessary for simulation, not in reality.

 

[garg29]

Thanks a lot FvM indeed your solution was much better than mine. G-o-d knows when I'll learn electronics atleast the basics !!!!!!!!!!
Thanks erikl to you too.....
I clicked helped me for both of you... G-o-d Bless you