Hello!
One time, before I knew how to correctly measure a current using a multimeter (or understand how current works), I tried to measure how much current the 5V pin of the Arduino can give off. I attached the red lead of the multimeter to the 5V pin, and the black lead to the GND pin. The Arduino then turned off its power LED, and the computer made a device removal sound. But, the multimeter still gave a reading of 0.8A! If I hadn't removed the multimeter leads, would the Arduino had been fried? The Arduino is supposed to have a 500mA fuse (overcurrent defence mechanism, etc.) at the USB port, right? Why does the Arduino still have power to it when I shorted it with a multimeter? It wasn't the only time I did that, either. So what happened when I did that? Did some defense technique kick in?
those fuses are not instant, and they don't really cut off current, their resistance simply increases with temperature, and the temperature increases with current.
if you left a screw driver between 5V and GND and went for a coffee, I think the fuse might trip completely
I wouldn't try that too often, shorting out power supplies is never a good thing, especially when the 30$ thing is connected to a probably more expensive computer
dkl65:
But, the multimeter still gave a reading of 0.8A! If I hadn't removed the multimeter leads, would the Arduino had been fried?
If you weren't measuring current correctly, then it really doesn't matter what the multimeter read...
dkl65:
Hello!
One time, before I knew how to correctly measure a current using a multimeter (or understand how current works), I tried to measure how much current the 5V pin of the Arduino can give off. I attached the red lead of the multimeter to the 5V pin, and the black lead to the GND pin.
Whoops. First, let's make sure we are actually measuring what we think we are measuring. Also, please never try to measure current this way on any device capable of more hurt than USB.
When I connected the leads, the computer made the device removal sound, and the Arduino's power LED turned off. When I removed the short, the LED came back on, the the computer made the device insertion sound. I thought that it was the defense mechanism breaking the power connection, because I think that I heard a click. But the Arduino still had power to it. Does anyone know what haapened?
dkl65:
I thought that it was the defense mechanism breaking the power connection, because I think that I heard a click. But the Arduino still had power to it. Does anyone know what haapened?
You did not active the poly fuse on the Arduino. Instead you activated your PC's supply protection. You saw readings on your Multimeter because the PC kept cycling power to the USB port to see if the short had been removed yet.
The Arduino's PolyFuse would not "click." That isn't how it works.
... and you measure CURRENT inline with the circuit path... not ACROSS the power leads. You placed very LOW low value resistance shunt across the power leads creating s short circuit on the 5V power feed from your PC. The PC is designed to sense these shorts and will disable the USB power.
This is a good way to fry the fuse in your meter as well, depending on settings (if you have a low current setting).
Note: the value you got from your meter, as stated already... is bogus.
Before, I tried to measure the current without anything in the circuit. Yeah, I know how to measure currents now, because I read a book, and looked at the instruction sheet of the multimeter. That was a few months before. The person in the video stated that the electrons go from the + to -; NOT TRUE! I know that it is a conventional way (due to past misunderstandings), but electrons move from - to +, because they are pulled from the ground (negative in DC) by the positive charge at the positive terminal.
The person in the video stated that the electrons go from the + to -; NOT TRUE! I know that it is a conventional way
True but it matters not a jot which way round the actual flow of electrons go does it.
What shut down was the PC providing power to the USB.
dkl65:
Before, I tried to measure the current without anything in the circuit. Yeah, I know how to measure currents now, because I read a book, and looked at the instruction sheet of the multimeter. That was a few months before. The person in the video stated that the electrons go from the + to -; NOT TRUE! I know that it is a conventional way (due to past misunderstandings), but electrons move from - to +, because they are pulled from the ground (negative in DC) by the positive charge at the positive terminal.
Note even that. Potential may appear to "flow" but mostly we have free electrons being exchanged along a path. You can take any model you want and it can be reduce to quantum levels where models like "flow" make little sense.
Not really related to your problem, but the practical advice on how to properly measure current from a technician's (as opposed to a physicist's) POV is still germane.
The primary advice here is "don't short out your computer USB power connects." Which is still pretty good advice, right?
Yup. I never intended that the direction of current was part of the problem; I only said that it goes from - to +. Short circuits draw a lot of current; to much for the power supply to handle. So there isn't a way to measure the full current capability of a power source. If the desktop cycles power to check if the short is removed, why doesn't the power LED on the Arduino turn on?
dkl65:
Yup. I never intended that the direction of current was part of the problem; I only said that it goes from - to +. Short circuits draw a lot of current; to much for the power supply to handle. So there isn't a way to measure the full current capability of a power source. If the desktop cycles power to check if the short is removed, why doesn't the power LED on the Arduino turn on?
Because you shorted the connection.
dkl65:
So there isn't a way to measure the full current capability of a power source.
Not /strictly/ true. You can dummy load any power source you want and watch the power draw in-circuit
Ignoring the impedance of the meter (which may or may not be a good idea, depending on your meter!) and things like temperature, this implies something like a 5.5-10 Ohm resistive load would not trip the computer's 500-900mA (depending on hardware) over-current protection and allow you to measure it.
(This is just back-of-the-envelope on my part -- I'm actually hoping someone will correct me if I'm wrong.)
Again, though, probably not really helpful or useful in this context.
If you are trying to measure how much current the whole Arduino is drawing, you can do that with a USB breakout circuit. Otherwise, you can measure various points in your circuit to see what the draw is. Using Kirchhoff's law you should be able to work out where this is the same and where it might differ across a circuit.
This is something I've been trying to take into consideration more when I think about stuff where current is important -- designing in test points to make measurements easier.
So there isn't a way to measure the full current capability of a power source.
Yes, the simplest way is to read what the rating is and don't exceed it.
As you do exceed it the voltage drops, so you can keep drawing out more current but with less voltage. As the load increases the current increases and the voltage drops again. So your simple concept of "full current" is some what meaningless.