Do you sometimes develop with USB? I do. So I need to access data lines, bus voltage and I'd like to easily monitor the current too. I made myself a small companion (43.5 mm × 22.5 mm, 9.5 g) to do that. Just a simple breakout board with one connector in and one out. And a little extra to make things easier. I call it USBuddy.
|USBuddy between host and device.|
It has one pin header to break out all the lines plus one LED to indicate that the USB is on. And the current measurement system: It is a very simple system, just a current shunt resistor and a current shunt amplifier. I chose INA199B1 with 50 gain which consumes less than 100 µA. This allowed me to use a very small current shunt, just 20 mΩ. This means that at 0.5 A, I will be losing only 10 mV. And it also means that the output will be a convenient ratio of 1 mV = 1 mA. The current shunt has 4-wire (Kelvin) connection for more precise measurement.
But why stop here? Often, USB is used for charging at larger currents. The design allows for up to 4 A and 24 V. While putting more than 5 V on USB is typically not a good idea, the USBuddy is designed to handle that just well.