Logging Power Consumption of Pi with ACS712 Current Sensor

Question

Here's the experiment;

Measured consumed Power of Pi using an Arduino and a voltage divider and an ACS712 Current sensor. Both sensors have been calibrated on a power supply with about 5% accuracy.

Cut a USB cable to expose +5V and 0V. Brought the 5V out onto a breadboard, plugged in micro USB end again with broken out 5V and 0V pins into the breadboard. So all I've done is access the individual cores.

Connected Raspberry Pi 3 and was able to remote desktop in and use.

Next I added Current sensor in series on the 5V line to the Pi. The Pi becomes unstable and unreliable.

Any thoughts as to why this should happen? Used this sensor in another application and worked perfectly fine, what's more I've proven and calibrated this sensor prior. I know it's working.

Background, I've also read various posts on here about power consumption and methods to reduce. I wanted to apply some these and log the data. I also have a Power doctor to connect inline with USB cable but not sure of its accuracy.

Thoughts or suggestions anyone?

Edit: Here's an earlier photo, it's been powered by a USB battery pack, later tried mains supply via mobile charger. In this photo just twist connected the black ground cables. In later experiment I soldered breadboard cable to all +5V and Ground connections.

Answer 1

First guess is that the current monitor and/or the wires and joints you added to connect have together enough resistance to drop too much voltage.

In software: the kernel log will on some (newer?) hardware versions show when the supply voltage drops a little. Try watching it with dmesg -w -e, if you can get the machine booted that far... but the most this will show is that you have a problem!

Current demand will vary depending on what the machine is doing. You can measure the voltage drop in the steady state by replacing the Pi with resistors. Each 33 ohm resistor will use 750 milliwatts, so they need to be rated at one watt. If you get a bag of ten and add them in parallel, one at a time, you can both

1. measure the voltage across them, and know that anything short of the supply voltage has been dropped in the other connections
2. check the calibration of your current meter, bearing in mind the tolerance of the resistors or measuring current separately with another meter.

Caution: do your calculations and beware that things will get hot, possibly hot enough to burn fingers and melt plastic.