Hello,
As much as I hate to admit it, I am not very good with analog circuits. And I always find myself very frustrated when I require one.
A few years ago (I got interrupted) I started working on a project that was using the analog input on a PIC 18F4550 to read a 1000 Ohm 2 wire RTD sensor. I eventually gave up after trying many different avenues.
I am back at it finally. I have traded in my PIC for a netduino for the time being because the netduino is much easier to use, and I figure this will remove the problem of errors in the program so that I can concentrate on the circuit.
That being said, I have some statements that will lead to a couple of questions.
It appears that one option is to use a constant current circuit to drive the RTD and then read the voltage coming from the RTD. Because the current is known then I should be able to derive the resistance if I know the voltage?
I also saw some references to some circuits that would cause 0 voltage at 0 degrees and higher voltage when the temperature rises. I believe that these circuits were using a comparator to compare the RTD to a known value resistor.
I have also seen some circuits that linearize the output of the RTD so that it does not have to be done in code.
This seems like it should be a simple problem, but I just don't seem to be getting it.
I am hoping that someone can point me in the correct direction. I don't want to have to use super accurate components if I can help it (i.e. precision resistors).
I would appreciate it if someone could post a circuit or give me some information about the best combination of simple and correct. The sensor only needs to be accurate to 1 degree Fahrenheit (2 would probably do, but 1 would be great).
My original post which contains my first circuit is in this thread:
I didn't really like this circuit because it does require precision resistors to operate correctly, and I never really got a good result in the end, but that could have been due to many factors.
Hopefully I am not asking too much here. Maybe simple and good don't exist for this situation? Any info would be helpful, but please remember that I am a newbie when it comes to analog so explanations will probably be necessary.
Thanks!!
~Tim
As much as I hate to admit it, I am not very good with analog circuits. And I always find myself very frustrated when I require one.
A few years ago (I got interrupted) I started working on a project that was using the analog input on a PIC 18F4550 to read a 1000 Ohm 2 wire RTD sensor. I eventually gave up after trying many different avenues.
I am back at it finally. I have traded in my PIC for a netduino for the time being because the netduino is much easier to use, and I figure this will remove the problem of errors in the program so that I can concentrate on the circuit.
That being said, I have some statements that will lead to a couple of questions.
It appears that one option is to use a constant current circuit to drive the RTD and then read the voltage coming from the RTD. Because the current is known then I should be able to derive the resistance if I know the voltage?
I also saw some references to some circuits that would cause 0 voltage at 0 degrees and higher voltage when the temperature rises. I believe that these circuits were using a comparator to compare the RTD to a known value resistor.
I have also seen some circuits that linearize the output of the RTD so that it does not have to be done in code.
This seems like it should be a simple problem, but I just don't seem to be getting it.
I am hoping that someone can point me in the correct direction. I don't want to have to use super accurate components if I can help it (i.e. precision resistors).
I would appreciate it if someone could post a circuit or give me some information about the best combination of simple and correct. The sensor only needs to be accurate to 1 degree Fahrenheit (2 would probably do, but 1 would be great).
My original post which contains my first circuit is in this thread:
I didn't really like this circuit because it does require precision resistors to operate correctly, and I never really got a good result in the end, but that could have been due to many factors.
Hopefully I am not asking too much here. Maybe simple and good don't exist for this situation? Any info would be helpful, but please remember that I am a newbie when it comes to analog so explanations will probably be necessary.
Thanks!!
~Tim