4-20 mA using pic 16f72

[pebe]

I thought from your #17 post that you were using some special hardware to derive a voltage from your 4 to 20mA current. But you are just using a 470ohm resistor. Why that value? If you use a 122ohm (122.55oahms to be exact) as I said before, you will get a count of 125 for 20mA and a count of 25 for 4mA. All you then need to do is subtract 25 from the measured D/A value to get the scale of 0 to 100 that you want. You should check that the value is 25 or higher before you subtract, otherwise the register will 'roll under' to give you a value of something just less than 255.

 

[djc]

Hi pebe,

Thats the task i have been given with a fixed hardware, thts why i can't change it.

Thanking you.

 

[pebe]

Then the only thing I can suggest is that you get the A/D values for 4mA and 20mA and make a look-up table that gives 0 and 100 for those values, and interpolate for all values in between.

I believe you can also do the calculations in software using floating point math, but I have never done it.

 

[djc]

Hi pebe,

Yes you are right. I think i have to derive certain formula. Because sometimes scale may change from 0-100 or 0-200 or 0-1000 anything like that.

Thank you