Measuring total current used by circuit with multimeter problem

[sheng]

Hi,

I am trying to calculate how long my 4 AA battery would last in my circuit running at 3.3v with my multimeter. So I need to find out the total current drawn. I cut the line after regulator 6v to 3.3v and place my probes of the multimeter in series touching my cut line. I got a read out on the multimeter in mA but the strange thing is my circuit does not come on. If I put the two probes together my circuit come back on. Am I doing something work? Have anyone encounter this before? Maybe the multimeter draining to much current?

Thanks,
Sheng

 

[chuckey]

The multimeter is not drawing too much current, all the current that is flowing into it down the red wire is coming out of the black wire. It is likely the the resistance of the multimeter is too large, if you change the range to a higher current one, the resistance of the meter will fall and your circuit may come back into life. Or it could be that you need somme added decoupling on the circuit side of your meter.
Frank

 

[D.A.(Tony)Stewart]

Chukey is correct


A DMM requires a low ESR supply for measuring current like a battery or big Cap and cannot tolerate the large external shunt resistor used in the mA scale.

 

[BradtheRad]

It is the same when I measure current with my analog meter.

I find I should start at the highest range (300 mA) because its resistance is smallest (1 ohm). It hinders current flow the least.

If the needle does not move up the scale enough to take an accurate reading, then I switch to a lower range.

 

[sheng]

Hi,

Thank you for the replies.

I have an Extech 22-816 multimeter, at the bottom it have three holes for the probes. The middle one is COM. The right is for 400mA Fused and left one is 10A label. At first I try the right (400mA), I got a current reading but the circuit does not turn on even I turn the knob to uA, mA, A. But when I use left (10A) side, the circuit comes on but 0 reading at first then when I start turn on a relay it have a reading. So I am confuse on what to use for a correct current measurement. Anyone know what is the difference between the left 10A vs the right 400mA? Which should I be using to get the current measurement? Sorry for the newbie question.

Thanks,
Sheng

 

[chuckey]

I would think that the 400mA and under, ranges are faulty. There could be an internal fuse that has blown. The 10A range does not use a fuse.
Frank

 

[FvM]

You won't get a reading with blown fuse. So more likely it's just multimeter voltage drop. Measuring current directly at the battery terminals should work better.

 

[sheng]

Hi Guys,

I just open two of my multimeter up and blown fuses. Thank you guys for the reply. Going to run to the store to get some fuse and see if I can make some sense of things.

Now the multimeter is out of the way.

Any suggestion on how to estimate how long 4 AA alkaline battery (series) would last on a system that take a Max 6.5v regulator it to 3.3V? I figure the system bottle neck at 3V for the relay coil. I believe the AA Alkaline battery is spec at 1.5Ah -2.8Ah for my protection I will used 2Ah. After replacing the fuse, I will measure with the multimeter in series with the 4 AA to see how much current is drawn.



Thanks,
Sheng