measuring own power supply

hi all, as you well know with i2c it is very tricky sometimes.



if i meassure external power source, let's say car battery or simply one 18650 which are not used to power rpi, i get a nice accurate readings

1x 18650 multimeter: 4.127V

adc_pi readings: 4.1167



but since i meassure the battery,which is powering rpi_zero [they have common GND], then i am not able to get accurate reading. this is the setup:

input 12v car battery -> dcdc stepdown --> output 5v --> usb power rpi_zero



then i have a voltage divider with R1 = 51k and R2 = 6k8. this is factor 7.5 so for 12V

U1 = 10.588 V

U2 = 1.41 V



simple schema:

+12 V --> R1 --> [ +A1 ] --> R2 --> -12 V / [ -A1 GND]



all verified with multimeter that math was done correctly. so for 12V betwen +A1 and -A1 i get nice 1.411V with multimeter when rpi is off [i am using same setup to measure 12v and also 5v]

i have to have order like 51k first and 6k8 second so i get +A1 in between both resistors [otherwise the voltage will be not 1.4V but +11V due current travel through i2c bus, so it will kill ADC pi]

running a simple code:

###

from ADCPi import ADCPi

adc = ADCPi(0x68, 0x69, 12) #default i2c settings

adc.set_conversion_mode(1)

samples = 1000



def test(pin):
value = []
for x in range(0, samples):
value.append(adc.read_voltage(pin))
averagevalue = float(sum(value))/max(len(value), 1)
return averagevalue

###

>>>a_result = test(1)
>>> a_result
1.0171278460000053 # this should be close to 1,41V

>>> 12 / a_result

11.797926924517547 #this should be close to 7.5 factor



the same error goes for 5V readings.

when i verify the ADC accuracy and meassure 3.3V from RPI output, i get nice

3.300895234000026V



please, do you have any suggestion on how to improve my circuit for voltage divider to get accurate readings?

thank you, pavel

Hi Pavel

The ADC Pi contains a voltage divider on each input using 10K and 6K8 resistors so to increase the voltage range you can use a single resistor in series with the input to increase the resistance of the 10K resistor.

We have a calculator you can use to find the correct resistor value for your voltage range. You can find it at ADC Pi Input Calculator

For a 12V battery, a 15V voltage range would give a good margin for overvoltage, so a 33.2K resistor connected to the ADC input would increase the voltage range of the ADC to 15.059V. This should be more accurate than adding a second voltage divider in front of the one built into the ADC Pi.

hallo andrew, thank you for replay. maybe i wrote it wrongly. will try again:



- i know adc_pi got voltage divider

- i measure 12v and 5V not 15v. but this is not important, what ever i need to measure, i will do the resistor math from voltage input and ouput

- each circuit has it's own voltage divider (one for 5v and second for 12v)

- for 12V i need to use another voltage divider with two resistors, using just one will burn adc_pi,as explained in original post

-also i believe that using another voltage divider will make some offset, but it should not get error like 33%

did you try to measure with adc_pi own power source for rpi like you describe?

thank you,pavel

i have give it a try and it looks better. i will need to study this more in deep. thank you , pavel

Hello Pavel

On the ADC Pi, the A- pins are connected to ground so your voltage measurements will be the input voltage relative to ground. If your battery, DC/DC converter and Raspberry Pi all share a common ground you can use a single resistor to drop the voltage on the ADC input through the built-in voltage divider. You will not damage the I2C bus as the built-in voltage divider combined with the external resistor will drop the voltage to a safe range.

The image below shows how you need to connect the 12V battery and 5V wires to the ADC inputs using a resistor on each input.



When selecting a resistor for the ADC input you need to use a voltage higher than the expected voltage to allow for a safety margin. A 12V car battery will be higher than 12V when fully charged and can be up to 14.5V when connected to or recently disconnected from a battery charger which is why I suggested a 15V maximum voltage for the ADC input. A 33.2K resistor in series with the 12V battery and the ADC input gives a maximum voltage of 15.059V so you need to multiply the reading from the ADC input by 2.9762 to get the correct voltage.

On the 5V wire, I selected a 3.16K resistor which will give a voltage range of 0 to 6V to allow for any drift in the output from the DC/DC converter. Multiplying the ADC reading by 1.1881 will give you the correct voltage.