Main Site Documentation

Analog readings unstable


#1

Hello,
I have pt100 sensor connected to fez spider analog input. The resistance of sensor changes, along with temperature. I have 110 ohm resistor connected in series, so i am meassuring voltage drops. The problem is, that my readings are inconstitent , from 20 to 30C. Like that some kind of noise is present. I am just starter in electronic, so i had no idea, what causes this voltage up and downs. Is the 3.3v voltage reference ?

Thanks


#2

Do you have a link for your sensor?


#3

Yes, description of sensor is here http://en.m.wikipedia.org/wiki/Resistance_thermometer .
Thanks


#4

How often are you measuring? There could be some noise too so maybe add a smoothing capacitor to reduce it. Got a pic of your setup??


#5

also ground the inputs of the unused analog inputs.


#6

Unfortunately, i have not spider at the moment. My readings are every second. Sensor is connected to p4 into analog input…


#7

My unused inputs are not grounded. I will ground them, and report results when i get spider back.

Thanks for tip


#8

Don’t know, if this is right way, but :

  • pt100 have variable resistance, what I need is voltage.
  • I need some known resistance, to calculate voltage drop
  • I add resitor with 110 ohms in series, and use this formula:

I = ReadedVoltage / 110 ohm
R = (3.3V - ReadedVoltage) / I
temperature = GetPTTemp (R) 

I was testing in cca 25C, but temperature oscilates from 20C - 30C. The sensor is under fez voltage. It’s 2 wire sensor. If i got everytime same ReadedVoltage in same temperature environment, but temerature was wrong, i would point at my code that calculates temperature from resistance. But the problem is, that ReadedVoltage oscilates. I’m using ReadVoltage() method of AnalogInput class.

Thanks


#9

http://en.wikipedia.org/wiki/Resistance_thermometer has some interesting schematics and equations. Do you know what accuracy your PT100 is meant to be?


#10

Thank you all for informations, but this already exceeds my current electronic skills. If I just simplify all just to function ReadVoltage() that varies - I suppose I need something like lm317?


#11

Thanks, for now I got enough information. Now I have to study solution.


#12

DS18B20’s are (relatively) cheap and easy to use as an alternative


#13

@ Andre, yes, true, but again nobody knows what circumstances this is being tried under except for @ tvinko. If this is just a home project with no specific requirements then an alternative like this is very suitable - @ tvinko already admitted the complexity discussed was above their level of electronics knowledge, so it may only have been a choice of a device that seemed easy (but is in fact not quite as easy) that arrived at the PT100… Just saying.


#14

Yes, this is just home project. More like learning electronics, hopefully with no deadlines :slight_smile:
Maybe i choose too complex sensor for starting with…