Need help with ABElectronics ADC board


3 posts
by bgirardot » Wed Nov 07, 2012 5:10 pm
I know very little about electronics, but I am trying to make what seems like a simple voltage monitor using an ADC board and the RPi.

I want to monitor the voltage of a 20Ah AGM 12V battery (not to scale in the diagrams).

I am using this board which uses I2C and a couple of ADC chips:

ADC Board Product Page

ADC Board Schematic PDF

The board works fine, I have tested it with a simple 3-pole thermistor and it worked as expected when the thermistor was powered from the ADC board's 5V and GND pins.

However, now I need to actually try and do the voltage monitoring of the separate 12V battery and I am a bit clueless in the proper way to do that.

What I have done is create a simple voltage divider circuit with 2 resistors, R1 100K Ohm and R2 33K Ohm which divides the 12V by about 4.

This simple circuit works fine, my digital multi meter shows that if the battery is at 12V the Vout of the circuit is 3V when measured on the breadboard. So far so good.

It is when I try and feed that 3V from the breadboard to the ADC board attached to the RPi I run into trouble and have no idea the right way to do it.

The ADC board is connected with a stacking header to P1 on the RPi and is powered via the RPi 5V pin from a separate power supply that is powering the RPi.

If I just take the Vout from my voltage dividing circuit and put it into one of the analog pins of the ADC, the ADC reads nothing, no current, 0V.

To get the ADC board to read anything, I connected the GND pin from the ADC board to the GND rail on my breadboard, but if I do that, the voltage the ADC reads in only 1.27V and my DMM also shows 1.27 as the Vout voltage when measured on the breadboard.

What am I doing wrong here? What is the correct way to connect this circuit to my ADC board so the ADC reads the correct 3V from my voltage dividing circuit.

The attached images are exactly as I have my breadboard set up, right down to which holes I have my wires inserted into.

Any help is greatly appreciated.

ADC-VPlus.jpg
Only the Vout of the circuit connected to ADC board
ADC-VPlus.jpg (31.04 KiB) Viewed 1478 times

ADC-VPlus-GND.jpg
Vout and GND connected to ADC board
ADC-VPlus-GND.jpg (32.04 KiB) Viewed 1478 times
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by bri999 » Thu Nov 08, 2012 10:08 am
The ADC Pi already has a voltage divider in the input stage which brings a 5V input down to the 2.048 which the ADC chips read. The input resistor is a 10k and the base resistor is a 6.8K. If you add a 33K resistor in series with the input and connect this to your supply it will give you a maximum input voltage of 15V. The ground needs to be connected to the ground on the ADC Pi. A 12v battery can go up to 14V so having a 15V upper limit will avoid damaging the ADC Pi board.
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by bgirardot » Thu Nov 08, 2012 1:57 pm
Sounds good, I thought there was something like that going on, but I knew it was beyond me to actually figure out. Thank you very much for helping me out.
Posts: 517
Joined: Wed Oct 10, 2012 6:20 am
Location: Switzerland