shifting ADC VRef- for PIC16F876A

[be80be]

Tell use something ok? The Op has a 250 ohm resistor gnd to RA0 then he feeds 4mA to RA0 it would read 4 mA

The 250 ohms is to keep RA0 from going over 20mA right?

Ok I put 4mA on RA0 and it reads 4 mA right

I put 20mA on RA0 whats it read ? 0

 

[Ian Rogers]

The Idea of the shunt resistor is a current to voltage converter , @ 4mA there will be 1 volt Gnd - RA0 and @ 20mA there will be 5 volts gnd - RA0.

A 4 - 20mA sensor will only provide this current

 

[be80be]

Ian Rogers I know how it works But I don't see this working I read from gnd to top of a stunt that gives you a mV reading 20 mV which is 20mA.

I just got done making with one that can read Amps and give you watts using a .5 ohm stunt and a opamp. to scale to 0 to 5 volts

This what I see happening You put 20mA on RA0 the resistor will clamp that to 0

 

[Ian Rogers]

Burt.. I know that you know that I know.. blah de blah... I am trying to understand.... "feed 20mA into RA0", "the resistor clamps it to 0" I don't get your drift.

 

[be80be]

Well I figured I didn't have any thing better to This works find if you set the hardware right. Nice 20 ma reading and the 4 mA is a little off

 

[ericgibbs]

Well I figured I didn't have any thing better to This works find if you set the hardware right. Nice 20 ma reading and the 4 mA is a little off

hi burt,
That circuit has been on the table for the past few posts. ref post #17

Like the rest of us, I am waiting to hear from 'aljamri' what is the problem really is.??

If he has followed the advice in the posts, he should have +1V on -Vref , +5V on +Vref.

A 250R resistor from ADC0 to 0V, with the 4mA thru 20mA from the signal current loop, this will give +1V thru +5V at the input of the ADC0 pin.
The PIC program has ADCON0 set up correctly, for the Vrefs etc..... so it should work.

So, 1V at the ADC0 pin gives 0 adc counts, for 5V at the pin gives 1023 adc counts.

Its method is not rocket science, its in common usage.

So aljamri what is the problem you are seeing.???

 

[be80be]

Hi ericgibbs this is my point aljamri using MiKro C he using the built in library he can set ADCON0 = any thing he wants and it's not going work


MiKroC mask off the ADCON0 bit as a matter of fact Adc_Read(2) will change it to a preset value to use RA2 the rest stay default

 

[ericgibbs]

hi burt,
I dont use MikroC, so will take your word for that.

I hope he realises that ADC0 is not available.??

 

[aljamri]

Sorry Sirs,

During my experiments, all of a sudden, my circuit stopped working except for 00.00 indications. Since then, I was trying to get it work to try all your valuable suggestions, but until today, No progress.

I'll check all my hardware, hoping that I did not made any harm to my only PIC16F876A .

Thanks For All.

 

[be80be]

He will have to wire his own code to read the ADC. If aljamri want I'll post him something that should work.

 

[aljamri]

As long as my circuit refuses to operate, I turned my attention for the software trying to achive the zero shift and I think I've made some progress, here is the code and the OshonSoft shot is attached:

 char *value = "00.00 %";
 unsigned int ADC_Value;
 unsigned int adc_val ;
void main() {
  ADCON0 = 0b11000000;
  ADCON1 = 0b10001111;
  TRISA  = 0xFF;              // PORTA is input
  TRISB  = 0;                 // PORTB is output
  Lcd_Init(&PORTB);           // Initialize LCD
  do {
       adc_val   = ADC_Read(0);
 
       adc_value = adc_val - 204 ;          // To shift the 0 1V
 
       value[0] =  adc_value/82 + 48;      // (1024 - 204)/10 = 82
       value[1] =  (adc_value/8)%10 + 48;  // (1024 - 204)/100 = 8.2
 
       value[2] = '.';
       value[3] = (adc_value/1)%10 + 48;    // (1024 - 204)/1000 = 0.82
       value[4] = adc_value%10 + 48;
   Lcd_Out(1,5,value);
   Delay_ms(2000);
  } while(1);
       }

EDIT: I have to tell you that Nigle in post#13 here, and Eric in another thread, told me to do software-zero-shift. Thanks for both.