Measuring current draw?

Projects General Guidance
1

I've been trolling google for the past 10 minutes, I've barely found anything, I want to go about measuring current...

Max current draw will be around 100ma... I presume a hall sensor is over kill? what's the easiest way to do this type of thing?

Thanks!

2

http://www.vwlowen.co.uk/arduino/current/current.htm

typical! - last search i did....

So, basically I shunt it then read the voltage, makes sense... but would that not entirely depend on the voltage spit out being always identical to get an accurate figure?

anyway if you have anything to add, please do.. thanks :slight_smile:

3

Would this work?

Edit:

Looks like we posted at about the same time. Anyway your link is broken.

4

Power the Arduino from a 20 farad super capacitor and obserce how fast it discharges.

i = C dv/dt

example :
i = 50mA
dv = 0.1 volt drop in capacitor voltage

time = dt = c dv/i = 20 * .1 / .05 = 2*20 = 40 seconds

5

but would that not entirely depend on the voltage spit out being always identical to get an accurate figure?

No that is not what ohms law says.

6

What resolution do you need?

Those ACS boards are really nice but 0 to 5A in 1024 steps is about 5mA resolution, which I consider to be coarse. Since you only go to 100mA, the vast majority of the ACS range is wasted.

If you want to read at 1mA or 0.1mA resolution, a different approach is needed.

7

Grumpy_Mike:

but would that not entirely depend on the voltage spit out being always identical to get an accurate figure?

No that is not what ohms law says.

I was refering to the usb voltage..

8

tylernt:
What resolution do you need?

Those ACS boards are really nice but 0 to 5A in 1024 steps is about 5mA resolution, which I consider to be coarse. Since you only go to 100mA, the vast majority of the ACS range is wasted.

If you want to read at 1mA or 0.1mA resolution, a different approach is needed.

What i had in mind to do has changed some what since my post.... but yeah it's food for thought, thanks...

9

You need two things to accomplish this: A shunt and an amplifier.
But you are much better if you use a current shunt monitor, to measure the load, and amplify it with a known value. I wrote about this some time back:
http://nandblog.com/current-shunt-monitors/