PIC 16f877 A/D conversion Allegro ACS712

[dhouston]

If I could get a reading form the ACS712 at the top/bottom of each wave then it could be done but then that is what the RMS to DC is going to do for me give me a nice straight DC line.

You are still making an unwarranted assumption that you have a sine wave. Darrell is suggesting the RMS chip and I suggested looking at the output of the Allegro chip with an oscilloscope (to see the waveform) because we don't think you have a sine wave. 0.707 x the peak will work with a sine wave and averaging ALL of the readings will work with any waveform (if the readings can be taken fast enough to get a good sampling of the wave form).

There are at least three ways to do this with non-sinusoidal waveforms. See the web page I referenced earlier (via email)...

• http://www.pge.com/includes/docs/pdf...idal_power.pdf

I vaguely recalled a somewhat similar thread - it took me a while to find it.

• http://www.picbasic.co.uk/forum/show...=motor+current

[DavyJones]

Dave,
I can see were I made it sound like I thought I still have a pure sine wave thats not what I meant. What I was actually trying to say was what you just stated that because it was not a pure sine wave if I took thousands of readings and averaged those it would work.

I'm looking at the document I saw that link in the email but it came acrossed somewhat broken I did not get the entire link as a complete link in the email.

Thanks
David

[dhouston]

I can see were I made it sound like I thought I still have a pure sine wave thats not what I meant. What I was actually trying to say was what you just stated that because it was not a pure sine wave if I took thousands of readings and averaged those it would work.

It's a bit more complicated - it requires some complex math. See Digital True RMS Converters on page 3 of the referenced pdf.

If you look at the waveform and see it is more or less sinusoidal, then 0.707 x the peak output of the Allegro should get you close enough for what you're trying to do.

[DavyJones]

Dave, Thanks I appreciate all you and Darrel are teaching me and not getting to stressed with me while I try to learn this. I'm going to work on capturing the wave this evening and take a look at what I see. I do have the components that Darrel suggested on order as well I want to hook those up in any event to help me understand. Things are so much easier when the device is DC like they were with the pressure transducer.

thanks again
David

[ukemigrant]

I work with someone called Darrin and Darryl and I always get them mixed up. Sorry for getting your name wrong Darrel - lord knows I've seen it often enough .

[thronborg]

Is this the way to get negative reading? i am new to this, so sorry if i mixed it up.

I use Allegro ACS714 but with a PIF16F690 with A/D set to 8bit, should not be any difference.

It works and can read +30Amp to -30Amp.

The problem is that i use 0V and 5V as reference. I cant figure out to get 0.52V to read 0 and 4.48V to read 255? Is it a math problem or must i add an external ref?

The ACS gives 66mV/A:
Amp Vin Bit
+30A 4.480 255
+10A 3.160
+1A 2.566
0A 2.500 127
-1A 2,434
-10A 1.840
-30A 0.520 0

See CODE below and attached DRAWING

'Define OSC and ADC
DEFINE OSC 4             ' Set internal Oschillator to 4Mhz
DEFINE    ADC_BITS 8       ' Set number of bits in result
DEFINE    ADC_CLOCK 2      ' Set clock source (3=rc)
DEFINE    ADC_SAMPLEUS 50  ' Set sampling time in uS
' Define LCD pins
Define LCD_DREG  PORTC   'LCD data port
Define LCD_DBIT  0       'LCD data starting bit 0 or 4
Define LCD_RSREG PORTC   'LCD register select port
Define LCD_RSBIT 4       'LCD register select bit
Define LCD_EREG  PORTC   'LCD enable port
Define LCD_EBIT  5       'LCD enable bit
 
 
TRISA = %00001001                         ' RA0 = A/D input
ADCON1.7 = 0                                  ' RA.1 = +Vref, Set PORTA analog and left justify result
PORTb.6  =0                                     ' Prepare RB0 for high-going pulseout

ANSEL = %00000101                         ' Set PORTA.2 analog, rest digital
ANSELH = %00000000

' Variables
outpuls VAR WORD                           ' Variable for the calculated puls out in mS
POT_POS VAR BYTE                          ' Potentiometer position CC=0, CCW=255
amp var word                                   ' Ampere 30A-0-30A (0-127-255)
                                 
Pause 500                                 ' Wait for LCD to start

MainLoop:                                 ' The Loop start here!
ADCIN 0,POT_POS                           ' Read A/D channel 0 to variable SVO_POS

'Check if negative or positive Ampere
IF POT_POS=>127 then
amp=(235 * (POT_POS-127))
else
amp=(235 * (128-POT_POS))
endif

Lcdout $fe, 1, "POT_POS= ", #POT_POS      ' Display POT Valu between 0-255 on line 1

' Check if negtive or positive Ampere, only for - minus sign on LCD display
IF POT_POS=>127 then
LCDOut $fe,$C0, "AMP= ",DEC (amp/1000),".", DEC1 amp' Display +Amp on line 2
else
LCDOut $fe,$C0, "AMP= -",DEC (amp/1000),".", DEC1 amp' Display -Amp on line 2
endif 

PAUSE 20                                  ' Constant 20mS pulses(low) between outpuls
GOTO MainLoop                             ' Forever
End