Hi everyone,
Ive designed a board in eagle and would like somebody to go over it and have a look at my maths and the Eagle layout. Its the first time i have used Eagle and by far the most ambitious component ive ever built.
Thanks in advance,
Design came from http://openenergymonitor.org/emon/buildingblocks/rtd-temperature-sensing
The intention is to use a 2 wire 'EBAY' PT100 to provide a 0-0.98v analog range over a -20°C-200°C temp range.
The only variables would be the sensor itself, and the trim pot, which will be 5160ohm.
PT100 Eagle.zip (360 KB)
Hi,
I think you need to present you board pattern first as a graphic export, jpg, and post it.
Not everybody has eagle.
Have you breadboarded your circuit and proved it will work and is scaled properly.
Tom..... 
Why not use a proper chip designed for reading PT100?
Runs on 3.3V, so you need to buffer SCK, CS, and MOSI going to the part.
Can be wired in 2-wire, 3-wire (shown here) or 4-wire configuration depending on your probe.
Ill put up a schematic when i get a minute TomGeorge.
Crossroads, i looked everywhere for a chip or board that would do exactly that. Seems i didn't look hard enough. I will probably end up going that way, but i am still interested to see if the original design works. I have built it on a breadboard, but without an acurate method of checking the temp, all i know is i get a voltage something close to what i think it should be. Ill put up a pic of schematic like i said above, my thought was that to calibrate all i would need to do is adjust the ref voltage using the trim pot.
Here is the schematic in a .PNG format
Also now ive read the fact sheet on the MAX31865 chip, the temp range isnt going to cover the application. The ic has a range of -40 to 125degC and i need a top of 200degC or something close to it.
Update,
Circuit worked perfectly on breadboard, but PCB i had made did not. Took me a while, but i discovered the problem. The 30k resistors on the feedback and gnd in the above schematic are back to front.
Anyway, learning experience........................
