wtf?  why can't I measure current? (mA)

I'm trying to determine the total current draw of my device. I tried setting my meter up to read mA (switched the lead) set if for DC, broke the 5V power lead from the Arduino to my device, and put the meter in-line. Nothing. So then I tried the ground wire. Nothing, no matter which order I put the leads. It seems like this would work - what am I doing wrong? Everything I have read says you need to break the circuit and put the meter in line and the meter will complete the circuit and give you current readings. But it just acts like a dead circuit. If I switch the leads I can measure 5V between the wires so...? It's a nice Fluke meter and it's great at everything else so I doubt it's the meter. I checked an old AA battery and that gave me .25 mA. Maybe I am just being stupid but I'm usually not quite this stupid.

Sounds like you are hooking it up corrctly. Switching the leads will result in a negative sign error.

I wonder if some time you've mistakenly measured voltage when set up for current measurement? That will easily blow the fuse in your meter. Don'y ask me how I know this, or how I know that Fluke likes to use a 440mA fuse in their meters for overcurrent protection. |:

-j

Your comment raises a question for me - how could I NOT accidentally measure voltage if I'm putting the meter inline with +5V, but setup to read mA?

I opened it up and there are two large ceramic-looking fuses - any way to tell visibly if these are blown? Don't appear damaged.

Update: Fortunately the meter still works to test continuity with no fuses in it - one of them tested open so that must be it.

Thanks.

The meter's internal resistance on the mA scale might be too big and the voltage drop too high for your device to even start. Try measuring on the Amps scale first.

I'm trying to determine the total current draw of my device. I tried setting my meter up to read mA (switched the lead) set if for DC, broke the 5V power lead from the Arduino to my device, and put the meter in-line. Nothing.

Make sure the Arudino is not plugged into the USB socket because if it is, then it will draw current from there instead of the 5V input.

when you say "switched the lead", do you mean you moved the red lead to the "Amps" jack? should work fine in that case.

What model of meter is it?

D

Yes, that’s what I meant by moving the lead. A previous poster had it right - I had blown the fuse at some point probably by forgetting to switch the leads back before measuring some high voltage. I have a new fuse in there but haven’t had a chance to try the measurement again yet.

Even without my amp-meter I was able to get the solution with some empirical (e.g. “screw it, let’s see what happens”) testing - and the 1024 leds all light up at once - w/ only USB power! I had several very credible people telling me there was no way this would work without external power. I think the trick was reducing the overall brightness of each matrix with a 47K resistor at ISET on the MAX7221. Still plenty bright, though I might try some different values to see the change in current draw.

The MAX7221 datasheet recommends the 47K resistor if you are using high-power requirement leds powered externally, I guess to reduce the power requirement of the 7221. Not sure I understand it, but hey, it works!

Greetings,

USB is only [u]rated[/u] for 500mA per port, and most LEDs need about 20mA for full brightness, so that's a maximum of 20-something LEDs with some current available for the Arudino. But the keyword in that sentence is [u]rated[/u], so you may be able to get a lot more than that before you have problems. This might be fine for a hobby application, but nobody would want their life to depend on it.

Regards, David

Greetings,

USB is only [u]rated[/u] for 500mA per port, and most LEDs need about 20mA for full brightness, so that's a maximum of 20-something LEDs with some current available for the Arudino. But the keyword in that sentence is [u]rated[/u], so you may be able to get a lot more than that before you have problems. This might be fine for a hobby application, but nobody would want their life to depend on it.

Regards, David

Ive had 80 high intensity LEDs running off Arduino, using this method:-

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1185507207/45#45

With 595's coded per the above link, only 8 LEDs are ever on at one time. Due to the speed at which Arduino processes the code, the eye is tricked into thinking all 80 are on.

I stopped at 80 because I ran out of 595's (and time). The code can be further optimized and theoretically, you could power as many 595's and LEDs as you care to add.

Some meters (All ??) have a built in fuse in the that protects it when measuring Amps.

I recently spend quite some hair pulling time figuring out why i could not meassure mApms with my meter.

The fuse was blown

i have a stockpile of spare 10 amp and 250 milliamp fuses cos its far too easy to blow them accidentally in your meter ;)

actually the 1st thing i did wen i got my meter was to blow the fuses :-[

so if u cant get a reading on milliamps then u blown ure fuses most likely :D