Measure DC Power with an INA219 and RPi Pico

Specs

PGA Gain & Range

The PGA configuration sets the gain and Full-Scale Range (FSR) of voltage across the shunt resistor. The PGA gain settings in the table below can be found in Table 4 of the INA219 Datasheet.

Resolution & Sampling

The ADC resolution settings adjust the number of ADC bits or number of samples used when averaging results for the Bus Voltage Register and Shunt Voltage Register. Higher ADC resolution or sampling requires longer conversion times. The ADC resolution settings in the table below can be found in Table 5 of the INA219 Datasheet.

Chip Versions

The INA219 IC comes in a SOIC-8 (3.91mm x 4.90mm) or SOT-23 (1.63mm x 2.90mm) package. There are two versions of the INA219 chip: A and B. The A version has a ±1% total max current measurement error over the entire operating temperature range and the B version has a ±0.5% total max current measurement error over the entire operating temperature range, but the overall functionality is otherwise the same. The markings on the chip indicate whether its the A or B version.

Adafruit INA219 Module

The Adafruit board comes with a 6-pin header and a 3.5mm terminal plug for the load. On each side of the board are STEMMA QTSTEMMA QT ('cutie') connectors are Adafruit's name for 4-pin JST SH connectors with 1.0mm pitch. These connectors were implemented on Adafruit boards to make it easy to plug-n-play various sensors and devices without soldering and wiring. The STEMMA QT (JST SH) are a smaller connector than the STEMMA (JST PH) that do not fit on smaller boards. STEMMA QT is cross-compatible with SparkFun Qwiic connectors. connectors for supplying power VCC to the INA219 chip and the two I2C lines (SCLSerial Clock (SCL) is the output clock signal line from the master device. Also referred to as SCK, SCLK, or CLK. and SDASerial Data Line ). Adafruit provides an Overview of the board and an CircuitPython library on GitHub for communicating with the device over I2C.

On the back of the board are A0 and A1 solder jumpers that can be bridged with solder to change the I2C address to the list below.

• Default = 0x40
• A0 soldered = 0x41
• A1 soldered = 0x44
• A0 and A1 soldered = 0x45

There is also an LED jumper that can be cut to disable the board's power LED if you want to reduce power consumption.

Generic INA219 Module

The Generic INA219 modules are similar to the Adafruit module, with the same functionality, except they do not include the STEMMA QT connectors for I2C and the onboard power LED. A couple other minor differences are the power pin is labeled VCC instead of VIN and the I2C solder jumpers are on the front of the board instead of the back of the board.

Adafruit CircuitPython INA219 Library

The Adafruit_CircuitPython_INA219 library was created by Adafruit for their INA219 module, although the library also works with generic modules. This library is available on GitHub, along with its dependencies Adafruit CircuitPython, Bus Device, and Registers. The Bus Device and Register libraries can be downloaded separately on GitHub or you can download the project bundle (ZIP). Adafruit has a guide on how to install CircuitPython on the RPi Pico and how to install libraries.

The Adafruit library offers an easy to use interface with pre-calculated calibration options, function calls to read measurement registers, and a power down function. A list of functions available in the Adafruit library are provided below.

INA219(i2c_bus):

Class constructor to set the I2C address to i2c_bus and initialize calibration. If i2c_bus is not provided, then the default is 0x40. This constructor initializes the calibration with a maximum bus voltage of 32V and maximum current of 2A.

None set_calibration_32V_2A():

Highest range setting with a maximum bus voltage of 32V and maximum current of 2A. Uses a PGA gain of 1/8 that has an FSR of 320mV across the shunt resistor. The current LSBThe Least Significant Bit (LSB) in a binary number is the bit that represents the smallest place value carrying the lowest weight. It is also sometimes referred to as the low-order bit or right-most bit due to the convention in positional notation of writing less significant digits further to the right. is 100μA/bit and the power LSB is 2mW/bit. Counter overflow occurs at 3.2767A.

None set_calibration_32V_1A():

Uses a maximum bus voltage of 32V and a slightly lower maximum current of 1A range with higher precision on amps. Uses a PGA gain of 1/8 that has an FSR of 320mV across the shunt resistor. The current LSB is 40μA/bit and the power LSB is 800μW/bit. Counter overflow occurs at 1.31068A.

None set_calibration_16V_400mA():

Uses a lower 16V maximum bus voltage and 400mA maximum current range with higher precision on volts and amps. Uses a PGA gain of 1 that has an FSR of 40mV across the shunt resistor. The current LSB is 50μA/bit and the power LSB is 1mW/bit. Counter overflow occurs at 1.63835A.

float bus_voltage():

Reads the bus voltage in Volts. The bus voltage is across the load: V_IN- - GND.

float shunt_voltage():

Reads the shunt voltage in millivolts.

float current():

Reads the current through the load in milliamps.

float power():

Reads the load power through in Watts.

boolean overflow():

Overflow bit in the Bus Voltage Register that can be used to check for overflow. Returns True if there is overflow and False if there is no overflow.

The Pico CircuitPython code provided below reads voltage, current, and power from the INA219 over I2C at a sampling rate of 1 second. The load current is expected to be around 50mA, so the calibration is configured to the lowest setting to maximize the resolution: 16V bus voltage and 400mA current range. The output is printed to the computer console over the USB port.

The output of the measurements from a 5V source through a 0.1Ω shunt and 100Ω load resistor is provided below.

pyb_ina219 MicroPython Library

The pyb_ina219 MicroPython library available on GitHub was created by Chris Borrill to provide more functionality and control over the INA219 and can be used with other microcontroller boards such as the pyboard, ESP8266, and ESP32. A list of functions available in the pyb_ina219 library are provided below. The GitHub README page has more details on the library usage and functions.

INA219():

Class constructor to set the shunt resistance, maximum expected current, I2C address, and logging level. The input arguments are defined below.

• shunt_ohms:
  (required) Value of shunt resistor in Ohms.
• max_expected_amps:
  (optional) The maximum expected current in Amps that can be used to calculate the best gain to achieve the highest resolution when GAIN_AUTO mode is set in configure().
• address:
  (optional) An instance of the I2C class from the MicroPython Machine module, either I2C(1) or I2C(2). Defaults to 0x40.
• log_level:
  (optional) Sets the logging level for the MicroPython logging module. Default is logging.ERROR. To understand the calibration calculation results and automatic gain increases, informational output can be enabled with logging.INFO. Detailed logging of device register operations can be enabled with logging.DEBUG.

None configure():

Configures and calibrates how the INA219 will take measurements. The input arguments are defined below.

• voltage_range:
  The bus full scale voltage range, this is either 16V or 32V set by the constants RANGE_16V or RANGE_32V (default).
• gain:
  The gain which controls the maximum range of the shunt voltage set by the constants GAIN_1_40MV, GAIN_2_80MV, GAIN_4_160MV, GAIN_8_320MV, or GAIN_AUTO (default). If GAIN_AUTO is configured without setting the max_expected_amps, then GAIN_1_40MV is used.
• bus_adc:
  The bus ADC resolution (9, 10, 11, or 12-bit) or the number of samples used when averaging results. Set by the constants ADC_9BIT, ADC_10BIT, ADC_11BIT, ADC_12BIT (default), ADC_2SAMP, ADC_4SAMP, ADC_8SAMP, ADC_16SAMP, ADC_32SAMP, ADC_64SAMP, or ADC_128SAMP.
• shunt_adc:
  The shunt ADC resolution (9, 10, 11, or 12-bit) or the number of samples used when averaging results. Set by the constants ADC_9BIT, ADC_10BIT, ADC_11BIT, ADC_12BIT (default), ADC_2SAMP, ADC_4SAMP, ADC_8SAMP, ADC_16SAMP, ADC_32SAMP, ADC_64SAMP, or ADC_128SAMP.

float voltage():

Reads the bus voltage in Volts. The bus voltage is across the load: V_IN- - GND.

float supply_voltage():

Returns the computed bus supply voltage in volts. This is the sum of the bus voltage and shunt voltage. A DeviceRangeError exception is thrown if current overflow occurs.

float shunt_voltage():

Reads the shunt voltage in millivolts. A DeviceRangeError exception is thrown if current overflow occurs.

float current():

Reads the current in milliamps. A DeviceRangeError exception is thrown if current overflow occurs.

float power():

Reads the load power in milliWatts. A DeviceRangeError exception is thrown if current overflow occurs.

bool current_overflow():

Checks whether one of the data registers is overflowed. Returns True if there is overflow and False if there is no overflow. If overflow occurs, select a higher PGAIN.

void sleep():

Puts the INA219 into power down mode.

void wake():

Wakes the INA219 from power down mode.

void reset():

Resets the INA219 to its default configuration.

The Pico MicroPython code provided below reads voltage, current, and power from the INA219 over I2C at a sampling rate of 1 second. The load current is expected to be around 50mA, so the calibration is configured to the lowest setting to maximize the resolution: 16V bus voltage and 400mA current range. The output is printed to the computer console over the USB port.

The output of the measurements from a 5V source through a 0.1Ω shunt and 100Ω load resistor is provided below.