I am wondering if I need to be aware of any voltage drop with wire length or other issues measure current through a shunt.
I'm using a 3 channel ina3221 breakout board but with external shunts (datasheet). The device measures current and voltage.
I believe the datasheet says 5ma input into any digital pin. If that's correct then the current is so low then wire voltage drop isn't going to be a large issue.
Datasheet also mentions shunt should be ideally placed as close as possible to the input pins.
"These errors result from additional trace inductance and other parasitic impedances between the shunt resistor and input pins. Longer traces also create an additional potential for coupling noise into the signal if they are routed near noise-generating sections of the board."
Well my question is what is practical? What wire size should be used to shunt? What length is acceptable? 100mm? 3m?
That spec is irrelevant to the ADC input, which is high impedance. Voltage drop is not an issue. The noise picked up by long wires will probably be the most serious problem, and using twisted pair should help to reduce magnetically coupled noise. Using shielded twisted pair, with the shield connected to ground only at the Arduino end, may also help.
You should be able to use a few meters of twisted pair, (e.g. land line telephone cable) but if you run into problems, suspect that long cable.
There can't be any long/thin wire between external shunt and the 0.1 ohm shunt on that module.
If it's more than a few inches, then you also have to add/calculate that wire resistance.
The module uses I2C communication, which could have issues over some distance.
So what distance are we talking about.
Leo..
No, I removed the resistors to use larger external shunts. It's on a yacht. I connected it to the alternator yesterday and it's great to see 80 amps pumping through.
Wire resistance should be very minimal if the current is low.
And I2C is from the board to the MCU which I understand is 1m maximum.
So do you have that INA board near the shunt or near the Arduino, and what distance.
Extending (hot) shunt wires sounds scary, especially on a yacht.
Max I2C length depends on wire capacitance, not wire resistance,
and if you pull up right, both ends, it can be much more than 1metre.
Ethernet cable is good, but don't put SCL and SDA on the same pair.
Leo..
I tried the ethernet cable. Interestingly it only works with 1 cable connected and the other connection with a short jumper to board. 2 wires of the ethernet cable doesn't work.
I'll give your pull up resistor advice a shot soon. Thanks.
I have a setup with two I2C slaves and a Mega master without extra pull up
(a Mega has 10k physical pull up).
With 5-6 metre of Cat-6 in between.
SCL/ground on one pair and SDA/power on another pair.
Running problem-free 24/7.
Leo..
