Trying to determine dc current - 12v lead acid battery charger

[tasmod]

pedja089, thanks for info I will check that Max chip out.

Meanwhile,

Hmmm interesting one http://http://electronics-diy.com/70v_pic_voltmeter_amperemeter.php

This uses a .047 in ground line. I've just been out and bought a .047 3W resistor anyway, so will try a mod to my board and see if I can re-configure things around.

Rob

[fratello]

...and this is the original code from this schematic :

'****************************************************************
'*  Name    : VOLTMETRE.BAS                                      *
'*  Author  : [Erol Tahir Erdal]                                *
'*  Notice  : Copyright (c) 2005 [ETE]                          *
'*          : All Rights Reserved                               *
'*  Date    : 01.03.2005                                        *
'*  Version : 1.0                                               *
'*  Notes   :                                                   *
'*          :                                                   *
'****************************************************************
DEFINE LCD_DREG		PORTB	'LCD data bacaklarý hangi porta baðlý?
DEFINE LCD_DBIT		4		'LCD data bacaklarý hangi bitten baþlýyor?
DEFINE LCD_RSREG	PORTB	'LCD RS Bacaðý Hangi Porta baðlý ?
DEFINE LCD_RSBIT	1		'LCD RS bacaðý Hangi Bite baðlý ?
DEFINE LCD_EREG		PORTB	'LCD Enable Bacaðý Hangi Porta baðlý?
DEFINE LCD_EBIT		3		'LCD Enable Bacaðý Hangi bite baðlý ?
define lcd_RWREG    PORTB
define LC_RWBIT     2
DEFINE LCD_BITS		4		'LCD 4 bit mi yoksa 8 bit olarak baðlý?
DEFINE LCD_LINES	2		'LCD Kaç sýra yazabiliyor
	 
DEFINE	ADC_BITS	10	    ' Set number of bits in result
DEFINE	ADC_CLOCK	3	    ' Set clock source (3=rc)
DEFINE	ADC_SAMPLEUS 50	    ' Set sampling time in uS
     
@ DEVICE pic16F877
@ DEVICE pic16F877, WDT_off
@ DEVICE pic16F877, PWRT_ON
@ DEVICE pic16F877, PROTECT_OFF
@ DEVICE pic16F877, XT_OSC

ADCON1=%10001110
TRISA=%00010011
TRISB=0
TRISC=0

VOLT    VAR  WORD
HAM     VAR  WORD
HVD     var  word
AMPER   var  word


MV      VAR  BYTE
MA      VAR BYTE

PORTB=0
PORTA=0
VOLT=0
PAUSE 100
LCDOUT $FE,1


BASLA:
       ADCIN 0,HAM
BAK:   IF ADCON0.2=1 THEN BAK
       PAUSE 5
       GOSUB HESAPV
       
       ADCIN 1,HAM
       PAUSE 5
       GOSUB HESAPA
       
       GOSUB EKRAN
       PAUSE 1500
       GOTO BASLA       
       
       
EKRAN:
'       LCDOUT$FE,2,"HAM=",#HAM  'gerekirse okanan ham deðerini görmek için
       LCDOUT $FE,$02,"VOLT =",DEC VOLT,",",DEC1 MV 'dec2 yazdýk çünki virgülden sonra 2 hane hassasiyetimiz var.
       LCDOUT $FE,$C0,"AMPER=",DEC2 AMPER,",",DEC2 MA 
'       LCDOUT $FE,$C0,"AMPER=",DEC HVD
       RETURN
       
HESAPV: '0-25V deðerlerini oukuyup göstermek içindir
       HVD=((HAM+1)*/875)/10 'bu ifade aslýnda (HAM+1)*625/256 nýn karþýlýðýdýr
'CARPAN=625  yani ((25x10)/1024)*256 sonuç 62,5 çýktýðý için 625 almak için 
' 10 yerine 100 ile çarpýyoruz. yani CARPAN=((25*100)/1024)*256
       VOLT=HVD/10  'baþlangýçta 100 ile çarpýlmýþ halini hesaplamýþ idik ohalde 100 e böleceðiz
       MV=(HVD)//10 'seaplanan sayýný 100 bölünmesinden kalan sayý ondalýk hanesidir
      RETURN
'      Formül;
'      Okunacak Maksimum Volt=OMV olsun
'      CARPAN= ((OMV*10)/1024) * 256   veya ((OMV*100)/1024)*256
'      CARPAN deðerini 3 haneli rakam olarak kullanmaya gayret edin.
'      þayet  62,5 gibi 2 haneli bir rakam çýkar ise bunu 625 kabul edin ve
'      (OMV * 10) deðerini (OMV * 100) olarak kullanýn. Sonuçta hesaplanan volt deðerini 
'      10 yerine 100'e bölmeniz gerekecek tabiiki. OV hesabýnda da Yine 100 bölünmüþ
'      deðerden kalaný hesaplamak gerekecek. Yukarýdaki örnekte olduðu gibi. 
'      Hesaplanacak Volt Deðeri =HVD olsun 
'      ADC den okunan Deðer HAM olsun
'      HVD=(HAM+1)*/CARPAN    bulunacak deðer gerçek volt deðerinin 10 ile çarpýlmýþ halidir.
'      Tamsayý VOLT (TV)= HVD/10      veya TV=HVD/100  
'      Ondalýk VOLT (OV)= HVD//10     veya OV=HVD//100
'Ekrana yazdýrýrken;
'      LCDOUT $FE,2,"VOLT:", DEC TV,",",DEC1 OV   veya DEC2 OV olacaktýr
HESAPA:
       HVD=(HAM+1)*/250
       AMPER=HVD/100
       MA=HVD//100
       RETURN

Credits to 320volts.com !

[HankMcSpank]

pedja089, thanks for info I will check that Max chip out.

This uses a .047 in ground line. I've just been out and bought a .047 3W resistor anyway, so will try a mod to my board and see if I can re-configure things around.

Rob

Whilst that higher resistor will give you 4x the ADC reading vs a 0.1R resistor, that's still only an ADC variance of about 60 steps between 1000mA & 300mA for a PIC supply of 5V....like I say, if you use a PIC where the ADC positive reference can be tied to it's internal 1.024V fixed reference (eg 16f1824), you'll then get a variance of about 240 steps between 1000mA & 300mA current draw ...which is starting to become useable.

[tasmod]

fratello. Yes I saw that one but it isn't this one, this one had various calibration routines etc in the code. Notice the defines for xtal osc. I do remember the author for mine came from India.

Hank,
I'm getting old I just don't get this. I've modded the board to have the .47 (not.047 as i put above) in series to the ground line and put a 100k to the adc input as per the schematic from the link above. This uses a 16f876 and I've duplicated the adc input circuit. They are claiming up to 10amp and with 10mA resolution.

[HankMcSpank]

Hank,
I'm getting old I just don't get this. I've modded the board to have the .47 (not.047 as i put above) in series to the ground line and put a 100k to the adc input as per the schematic from the link above. This uses a 16f876 and I've duplicated the adc input circuit. They are claiming up to 10amp and with 10mA resolution.

It's pretty simple...let me try & explain.

If your PIC is running at 5V, then you're ADC resolution becomes 5V/1024 (the 1024 being 10 bits)...this results in a resolution of 0.0048828125V (4.9mV) per ADC bit, which sounds great, but not if your sense resitor is very small & therefore the maximum voltage drop will be very small. So at 1000mA & a .47 ohm resistor, your maximum voltage drop will be .47V across it (V=IR). ) .....so if you PIC has the aformentioned resolution of 0.0048828125V, then the maximum ADC reading you'll get is .47V/0.0048828125V ...or a max reading of 96 ...that's with 1A flowing (that's an awful lot of wasted bits, bearing in mind you've got 1024 bits available in your PIC).

To get more resolution, if the PIC is modern & has a P-ref register for the ADC module (16f1828 etc)....you tell the PIC that rather than use 5V as the upper limit for the ADC, to use an internally generated fixed 1.024V instead (again the PIC needs to be up to date & have one onboard) - there are no more components involved, you just need to set a register in the PIC & you're off to the races.

Now your ADC resolution becomes 1.204V/1024 bits ....or 1mV per ADC bit, therefore with 1A flowing through the sense resistor, your maximum ADC reading will be 470 which is a lot healthier amount of 'reading' to play with.

Of course if you want up to 10A, then this is not the way to go (but I thought you were talking of 1A max?)

[tasmod]

Thanks Hank,

That all makes perfect sense and is what I believed and kinda saw all along.

I'm therefore puzzled by the circuit and claims for the above of 10A and 10ma resolution. This using a 16f876 which is the smaller brother of the 16f877 I have in the circuit.

I've read the datasheets. Looked at the circuit, there's nothing different to what I'm doing. I see no offset input, it has no internal 1.024v ref, I'm also now in the ground line with the .47.

This isn't the first circuit I've seen setup this way which claims 10A measurement with 10ma resolution but also using a 16f877. With 1023 bits to play with "it does not compute Captain"

[HankMcSpank]

Thanks Hank,
This isn't the first circuit I've seen setup this way which claims 10A measurement with 10ma resolution but also using a 16f877. With 1023 bits to play with "it does not compute Captain"

It is kind of correct..... 10mA through a 0.47R resistor = 0.0047V drop across it (which, if you look back above the above posts, with a 5V ADC reference & 1024 bits, you can resolve 0.0048828125V per ADC bit)

the spec seems to have changed a little though ...I'd thought earlier on you were speaking of 1000mA being your maximum anticipated current ...if that were the case, then 10mA resolution didn't strike me as being that granular....however if 10A is to be your max current, then 10mA of resolution is just dandy! So I guess you need to bottom out your requirements & take it from there.....

[Aussie Barry]

Hi Rob,

If you plan to increase the maximum current from 1 Amp to 10 Amps then I would suggest that you change the value of the shunt resistor due to power handling ratings. 10 amps flowing through a 0R47 shunt resistor will need a hi power resistor (P=I^2 x R = 47 Watts).

Your circuit shows the original 0R1 shunt resistor power rating as only 3 watts which has a theoretical limitation of just under 5.5 amps.

Cheers
Barry
VK2XBP