cannot measure current for some reason?

Using Arduino General Electronics
1

Hey guys I've built a lap timer using an atmega328pu and im trying to see how much current it draws when it's on. The atmega328pu is powered by a 7805 5v regulator and that is powered by a 9v battery.

So my problem is this: I disconnect one battery terminal and put my tester in series to close the loop and I put the setting on ma. Then I switch the timer on and it shows 0.75mA which is kewl, but when I trip the timer mechanism, (laser shining on a photodiode) then the program is supposed to go through a few if statements and then display a lap time. But it doesn't do this with my multi meter in series. It works when I connect the battery directly?

2

Check your multimeter fuse.

3

If the fuse is OK then turn the meter onto a higher current range. I would expect somewhere in the 50mA region so setting it to 100mA range or larger would seem good.

4

Test your multimeter with a battery, a resistor and a LED, if it didn't work:

  • Change your fuse (if you have some) and make sure the new one is the same value of the old one.
  • Solder a wire to the both fuse ends (this should be temporary until you have a new fuse) - Be careful with Amp measuring after you do this!!!!
5

I have checked my fuse and I realised it's non existent!!!! Hahahahaha!!! But what's confusing now is why does it show 0.75mA without a fuse at all?

6

calvingloster:
I have checked my fuse and I realised it's non existent!!!! Hahahahaha!!! But what's confusing now is why does it show 0.75mA without a fuse at all?

Have you tested it out with a led?
Fuse - Are you sure about that? Can you send us a picture?

7

calvingloster:
I have checked my fuse and I realised it's non existent!!!! Hahahahaha!!! But what's confusing now is why does it show 0.75mA without a fuse at all?

Some multimeters do that

(no, I don't know why...)

8

tsunamy_boy:

calvingloster:
I have checked my fuse and I realised it's non existent!!!! Hahahahaha!!! But what's confusing now is why does it show 0.75mA without a fuse at all?

Have you tested it out with a led?
Fuse - Are you sure about that? Can you send us a picture?

Yes I'm sure about the fuse cause I remember now that I took both of them out about 2months ago and forgot about it. I have not tested it with an LED yet but I will tomorow.

9

Without a fuse that area of measure (Amp) will not work.
The value you are getting it might be a false positive because something is missing there

10

It sounds too low to me. Have you swapped the leads over to the current position?

11

A multimeter on a low-current range has very significant series resistance - most multimeters
require 100mV across the current shunt for full scale, so on a 0..2mA range the shunt is 50
ohms, on a 0..20mA range the shunt is 5 ohms. On a 10A FSD range the shunt would be
0.005 ohms (its really a 20A range but the shunt/leads aren't rated to that power)

The fuse protects the probe leads and PCB from catching fire on overcurrent, only a
possibility with a low-resistance shunt, so low current ranges aren't fused (the shunt
resistor acts as the fuse in fact).