POWER in, LOAD out
The block goes in the positive wire: supply into POWER, load out of LOAD, both negatives into the minus screws. Put it anywhere else and it either short-circuits the supply, sends the load's current home through your USB cable, or reads a voltage with nothing to measure it against.
In the positive wire
Cut the positive wire between your supply and your load, and put the block in the gap. Supply + into POWER +, supply − into POWER −; LOAD + to the load's +, LOAD − to the load's −.
Now everything the load draws goes in at POWER +, through the shunt and out at LOAD +, which is the way the printed arrows point. This is called high-side sensing: the shunt is on the positive side of the load.
Three places that look right
Across the load, the way a voltmeter goes, is the one that does damage. A tenth of an ohm across a supply is very nearly a short circuit.
In the negative wire looks as if it should work, and on a board built for it it would. On this one the minus screws are the header's GND, so the load's return current finds a path back through your microcontroller's ground and its USB cable instead of the shunt.
Without a shared ground there is nothing to measure the bus voltage against. If your supply's negative goes to POWER −, this is already taken care of.
One ground for everything
Both minus screws and the header's GND are one piece of copper. Wiring the block in joins the negative of the supply you are measuring to the ground of the board reading it.
That is what makes the voltage reading possible, and it is almost always what you want. It also means the block cannot measure a rail whose negative must stay apart from your microcontroller's ground. The block has no isolation.
The limits
The rail may be anything from 0 to 26 V. 26 V is the chip's absolute maximum on both inputs, not a working voltage, so a 24 V supply is fine and an unloaded supply that floats up to 28 V is not.
Current is limited by what the chip can read, 3.2 A on its widest range, before anything gets hot. Above that the reading stops and the shunt keeps warming.
When it does not work
The supply and the load are in each other's terminals, so current flows LOAD to POWER, against the printed arrows. Nothing is harmed. Swap the two pairs of wires. With them swapped the bus voltage was the supply's rather than the load's, so that number changes too.
Only if you are happy for their grounds to be joined. The block has no isolation: its minus screws are its GND pin, so wiring it in ties the measured supply's negative to your microcontroller's ground. For a bench supply and a USB-powered board that is normally fine. For anything whose negative is not meant to be ground, such as the middle of a battery stack, it is not.
The chip survives it: it takes up to 26 V between its two inputs. The shunt then carries whatever the supply will give. At 3.2 A it makes about 1 W and past about 4.4 A it is beyond its 2 W rating, so set a current limit on the supply, or add a fuse, when testing something that might short.
Yes. The rail is measured, not used: the chip runs from the 3V3 hole on the header, 3 to 5.5 V, whatever rail is in the terminals. With the header unpowered the chip does nothing, even with 12 V on POWER.
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