How to measure current drawn from 5V pin

Other Hardware Motors, Mechanics, Power and CNC
1

I need to measure the current drawn from the 5V pin.

I'll be powering up to 3 boards through this pin, and I only have the data for one of them. So I need to come up with a method for empirically measuring the current drawn when connecting each one of these boards individually. In my project, the Arduino will be powered with batteries.

I've connected a multimeter in series with a wire coming from Arduino's 5V to one of the other board's 5V pin. The Arduino itself was powered by USB for the test. I know the connection worked because the attached board was powered and the program flashed on the Arduino was running.

However the multimeter displayed 0 Amps :confused: I switched the multimeter knob all the way from 200uA to 200mA, yet I got no current in the display.

The multimeter works, as I've measured a battery's current through a 100k resistor.

The only knob position I didn't test was the 10A unfused, but this is meant to measure high currents (wall sockets?) and I was not sure if this experiment would damage the Arduino.

What am I doing wrong? Arduino's 5V should be DC, exactly as a battery, so the multimeter method should just work with both!

2

Multimeters have usually two different sockets for the red lead for current, high and low current ranges
use separate sockets.

I'd double check you connections and try again, this is how I'd measure the current consumption.

Note that some boards might well take extremely low currents when idle - which boards are you
talking about? A rough guess at the current consumption can be made if the function is known.

3

MarkT:
Multimeters have usually two different sockets for the red lead for current, high and low current ranges
use separate sockets.

I'd double check you connections and try again, this is how I'd measure the current consumption.

Note that some boards might well take extremely low currents when idle - which boards are you
talking about? A rough guess at the current consumption can be made if the function is known.

The connection was correct, otherwise the second board would not have been powered up. Its idle consumption is about 25mA according to the manufacturer.but can go up to 300mA. But even if it were on the 20mA range, this is about the same current I measured through a 100K resistor wired to a battery with the same multimeter. So most likely the multimeter is ok. This board has not a power regulator of its own, so it uses Arduino's 5V.

I made this test with a different board (a motor driver) and also got 0 amps. I don't know what is going on.

4

I would look for a different DMM to try, or just a VOM.

While the second board is running, if you pull the test lead out of the DMM, the second board stops working?

Strange things usually happen on Fridays...

5

A "beginners DMM" has usually a blown fuse in the low-amp circuit.

The 10A setting is perfectly ok to use. You just won't see small currents because of the bad resolution.

Remove the small current fuse from your DMM, and use your DMM to measure it.
Leo..

6

A "beginners DMM" has usually a blown fuse in the low-amp circuit.

If the connections are correct and the fuse is blown, I doubt the second board would power up.

7

Yep, missed that.
Maybe the DMM is on AC current.
Leo..

8

The blink code could be put on the board to be powered and then look for fluctuating current as the board LED cycles on and off.

9

But even if it were on the 20mA range, this is about the same current I measured through a 100K resistor wired to a battery with the same multimeter

20mA through 100k would mean the battery is 2000V

Something is wrong with your meter perhaps?

10

"While the second board is running, if you pull the test lead out of the DMM, the second board stops working?" Right?

11

MarkT:
20mA through 100k would mean the battery is 2000V

Something is wrong with your meter perhaps?

You are right. The DMM was in the uA scale for the battery. It measures 75uA now, which is 7.5V through a 100K resistor.

12

Correct.

I think the DMM is faulty. It sometimes measures current, I note it, and then suddenly it reads 0.0. Then I change the dial scale and again reads. Then back to zero.

If the current were off scale it should display a 1. A 0.0 reading usually means the fuse is kaput, but I've replaced it this evening and the malfunctioning is still observable.

But hey, it's a 20 bucks multimeter after all.

13

I believe if the fuse was blown, then the second board would not get any power to run. So I suspect the fuse is fine.

Find another meter.