I built this to measure the (mA) currents drawn by the Arduino devices I have been building.
I could have made a few break out cables - I have some already but that ties up my multimeter and as soon as I start testing I need to use the multimeters for something else.
There are very cheap but less accurate, or more more accurate but also more expensive, V/A units out there but I just wanted to see how I would go making a dedicated unit that I can leave plugged in while testing.

I already had quite a few of the parts mainly salvaged from other dead gizmos, and the rest of the parts the parts cost me about $25

Fuse (F) limits measurable current to 1500 mA.
"S "switches between internal and external ammeter (EA) e.g. during calibration.
"AP" is external power for the mA panel meter provided by a USB Li-ion battery pack - this PS has to be kept separate from the panel Voltmeter.

The (1Ω power resistor) that acts as a shunt resistor for the ammeter is that white block in between the two meters.
It has a 50Ω trimpot in parallel with it to assist calibration.


Fpanel.jpg


USB and 2.5mm DC I/O power sockets on either side of the unit.
Can monitor a, USB to USB device, or USB to 2.5mm DC, or 2.5mm DC to 2.5mm DC.
Side.jpg

Here is the testing setup and you can see he USB battery pack that powers the mA meter.
Testing.jpg
The gold looking cylinder on the lower left is a ~50 ohm 50W power resistor (at 6.467V its only developing ~750 mW) that acts as a load.
Flipping switch S allows the external ammeter to be read and compared to the mA reading shown on the unit.

A comparison between the internal and external meter is shown on the graph below.
Between 4 and about 12V the differences are around 0.5% .
I could tidy this difference up a bit by playing with the trimpot but i'm not going to bother as I'm pretty happy with that


Calibratin.jpg

When running an aArduino device you can see the current changing as the program loops in and out of various processes.