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Problem with current less than 4ma in a data logger

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amredries

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i am desgin data logger usin pic 16f877 it simply take from current loop ( first i cahnge current level 4-20ma to 0-5 volt by ic called rcv420 ) to its A/d but there is small error when cureent is less than 4 ma the ic give me -ve voltage which the microcontroller read it as zero (i can not make any different betwen 4ma or current less 4ma)
i know that i can't measure -ve voltage but i need to know that the voltage is negative or not so i can display error message
can any suggest a curcuit or code can make this
thanks a lot
 

Re: data logger

You can make hardware circuit that will give certain logic when current is less than 4 mA
use output of this H/W as input to the PIC IC and monitor it.

Thanks
 

Re: data logger

Hi amredries

Change the RCV420 range to 0-20mA => 0-5V converter ref: RCV420 data sheet.

You can then auto-zero in software ≈ 4/20 * 1024 ≈ 205
ADC Range ≈ 205<=>1023 ...... trim hardware to 200<=>1000 counts

So 4mA ≡ 200 , 20mA ≡ 1000 ....... (50 counts/mA => Resolution=>20µA/count)

Any count < 200 =>> ERROR-Under Range
Any count >1000 =>> ERROR-Over Range

This will simplify the maths in the software for logging/recording/display/tx.

Hope this helps ... Polymath
 

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