Safely measuring high-side current using Adafruit INA219

Question

I've been thinking about monitoring the power draw within my solar-based chicken coop setup to get a better understanding of the true power consumption for estimating battery life.

I have a 12 V deep cycle RV battery taking in solar power (through an appropriate charge controller) and supplying power to some USB cameras and a 12 V RV fan, controlled by a thermostat circuit.

I want to use an Arduino with an INA219 Adafruit breakout to monitor and log the current draw of the various components I have hooked up.

My understanding is that I would split the +12 V wire and use those to feed the breakout's Vin+/Vin- terminal block.

I have a couple of questions to make sure I don't fry the board:

1. The Adafruit page has a big disclaimer:

Be careful inserting noisy loads that can cause a sharp current draw, such as DC motors, since they can cause problems on the power lines and may cause the INA219 to reset, etc. When using a DC motor or a similar device, be sure to include a large capacitor to decouple the motor from the power supply and use a snubber diode to protect against inductive spikes.

What do I need to do to protect the circuit, I assume mostly from the fan? I'm not sure how to decouple the other parts of the circuit with a capacitor and snubber diode, so any advice here is highly appreciated. This is the part I'm most worried about since I understand inductive loads can supply a pretty strong back-EMF if not accounted for.

2. The Adafruit wiring guide shows connecting the breakout ground, the Arduino ground, and the "high-side" ground all together. Is this normal, and are there any concerns about connecting the 12 V- (ground) and the Arduino ground (powered by a car 12 V to USB hub) together? I guess in theory it's all the same ground reference, but I'm unsure here. Maybe in the picture below they are powering the breakout using the shield's power, so this may be misleading, but I'm still uncertain.

3. I also believe I need to swap out the sense resistor for a lower ohm rating. Is there a specific Digi-Key part number to use to get more than 3 A of measurement? Do I need to do something special in the code to adjust the breakout to be able to measure ~5 A?

Answer 1

I'll keep it brief -- feel free to add comments if you need more clarity.

1. A snubber diode is nothing more than a diode in parallel with your load in reverse polarity. It essentially gives the back-EMF a place to go when the load is switched off. Also called a flyback diode at times - https://en.wikipedia.org/wiki/Flyback_diode. I'll address the decoupling capacitor next.

2. You actually NEED the grounds connected together for the INA219 to work reliably. It needs a reference to the main voltage. If grounds aren't connected, it's not guaranteed that the current sensing inputs will be within their absolute maximums. So connect the ground of the Arduino to the ground of your main power. The fact that you're using a 12V to USB hub to power it actually helps with the decoupling you're asking about in your first question. My best guess is that the "resetting" it's talking about would likely be due to the large motor load pulling down the voltage supplied to the small INA219, causing a brown-out condition. The USB hub should automatically provide some level of decoupling. Try without any extra capacitors. If operation is spotty, add a 10 uF electrolytic cap across your +5V/gnd power lines for the Arduino, and then add a 0.1 uF ceramic cap across your power pins for each of your boards as close to the device as possible. The idea is for the capacitors to provide little "boosts" of energy as needed whenever the main line voltage sags down, and by doing so, keep the microcontrollers from experiencing low voltage conditions.

3. I can't be sure (haven't found a BOM for the board), but it looks like it's just a precision 2512 surface mount resistor.

"A precision amplifier measures the voltage across the 0.1 ohm, 1% sense resistor. Since the amplifier maximum input difference is ±320mV this means it can measure up to ±3.2 Amps."

So if you want more current range, use a smaller resistor. For example, a 0.05 ohm resistor would give you up to ±6.4 Amps of measurement range. Resistor TLRP3A30WR050FTE could be one option. I chose it based on resistance, being the correct size, having a low tolerance, and high power rating just in case there are current surges so it doesn't burn up.