Raspberry Pi Forums

Posted: **Thu Dec 20, 2018 8:24 am**

Hi all,

I'am trying to use one ACS712 with MCP3008 and Raspberry, but I'm having problems.
If I use my ACS712 with Arduino everything works well, I get good measurement current .

But now I want to use this with my Raspberry. I use a MPC3008,ACS712 output is analog value. But when I try to read current with my raspberry I don't have same value with my Arduino.
I use same formula, my MCP works, I have tried to read battery voltage and I read the good value.

I use voltage divider (R1 => 1.2k and R2 => 1.8k) in MCP DOUT because Vcc and Vref are 5V.
The zero current value seems to be good I get 512 (1024/2) from MCP3008. But If i plug a charge on ACS712 value doesn't change...

for read MCP channel (On my Arduino I don't use this function, I use AnalogRead function)

Code: Select all

def ReadChannel(channel):
  adc = spi.xfer2([1,(8+channel)<<4,0])
  data = ((adc[1]&3) << 8) + adc[2]
  return data

For read current (I use the same function on Arduino)

Code: Select all

def getVPP():  
  result = 0.0  
  readValue = 0         
  maxValue = 0      
  minValue = 1024
  
  for i in range(0,100):
    readValue = ReadChannel(current_1_channel) //On Arduino this line is ReadValue = AnalogRead(A0);
       
    if (readValue > maxValue):         
      maxValue = readValue
       
    if (readValue < minValue):
      minValue = readValue    
       
  result = ((maxValue - minValue) * 5)/float(1024)
   
  return result;

So, I get the same result with or without charge in my ACS712, I am a little lost because the same code works on arduino... Why? MCP is not accurate enough ?

Thanks

Posted: **Thu Dec 20, 2018 3:16 pm**

Have you grounded any unused analog pins on the MCP3008? What the voltage between the Vref (pin#15) and AGND (pin#14)?

Posted: **Fri Dec 21, 2018 8:14 am**

DougieLawson wrote: ↑
Thu Dec 20, 2018 3:16 pm
Have you grounded any unused analog pins on the MCP3008? What the voltage between the Vref (pin#15) and AGND (pin#14)?

Thanks for your reply,

Yes all unused analog pins are connected to the ground, and between Vref and AGND I have 5.06V.

I really don't understand why analog output from ACS712 are the same with or without charge. I have tested ACS712 he works with Arduino, I have tested MCP3008 it also works

Posted: **Fri Dec 21, 2018 8:35 am**

You need the 5V connected to Vref, GND connected to AGND and OUTPUT connected to CH0. This code should get a reading for you.

Code: Select all

#!/usr/bin/python3
from gpiozero import MCP3008
from time import sleep
volts = MCP3008(channel=0, device=0)

while True:
    adcValue = (volts.value * 5.06) * 100
    print('{:.1f}'.format(adcValue), 10 * ' ')
    sleep(0.1)

Posted: **Fri Dec 21, 2018 8:51 am**

I have just tried your code and I get the same result, with or without the voltage is the same.
I tried to redo a voltage measurement with the mcp3008 on channel 0 and it works. I'am sure that my ACS712 works and MCP3008 too

Posted: **Fri Dec 21, 2018 9:01 am**

Test it on your Arduino again.

Test the MCP3008 with a potentiometer wired between Vref & AGND with the wiper connected to CH0 (pin#1).

Posted: **Fri Dec 21, 2018 9:43 am**

DougieLawson wrote: ↑
Fri Dec 21, 2018 9:01 am
Test it on your Arduino again.

It works, if I plug a charge, I get good current on my Serial print (50mA)

DougieLawson wrote: ↑
Fri Dec 21, 2018 9:01 am
Test the MCP3008 with a potentiometer wired between Vref & AGND with the wiper connected to CH0 (pin#1).

It also works, I test with multimeter, I have the same voltage display in my terminal

I really don't understand what's going on, my ACS712 always out 2.52V (this voltage is good datasheet say : for zero current analog output is VCC/2)

Posted: **Fri Dec 21, 2018 9:48 am**

I don't think I can help much more. I've got MCP3008s but I don't have an ACS712 hall-effect sensor (and it would be 2019 before I can get one).

Posted: **Fri Dec 21, 2018 9:51 am**

DougieLawson wrote: ↑
Fri Dec 21, 2018 9:48 am
I don't think I can help much more. I've got MCP3008s but I don't have an ACS712 hall-effect sensor (and it would be 2019 before I can get one).

Thank you for your help

I continue my research and if I find I will say the solution here (and I really hope to find the solution

)

Posted: **Fri Dec 21, 2018 11:22 am**

I didn't read everything.

But what for type of ACS712 are your using. Because there are three types. And not all are suitable for measuring a low current.
And I would advice you to take a look at this topic. Because it is almost the same question. And it is explained in detail with schematic and code.

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