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Topic: Noob question about current (Read 827 times) previous topic - next topic


I suspect you have your test meter, when in mA/A mode connected between the +5v and ground.   This places a virtual short across the supply.

Basically current flows "along" a wire so when measuring current you need to place the meter into the current flow.  You achieve this by connecting your supply voltage + wire to the meter + terminal and you connect the meter - terminal to device + terminal.

Voltage is measured by setting the meter to volts and connecting meter + terminal to the point where yo wish to measure and the meter - to the device ground point.


       The reason your A/mA (amperage) reading was fluctuating is because when a multimeter is set in A/mA mode, its internal resistance is almost zero. So when you attach it to the Arduino's power supply, it shorts it, and the Arduino struggles to supply enough power. This causes it to fluctuate.

       As for learning about the basics of electronics, here are some good books to look at:

       Getting Started in Electronics by Forest Mims, III

       Make: Electronics by Makemagezine


Yes, you are misusing your meter and subjecting it and your arduino to damage. Current measurements are always made with the meter's current leads in series with an existing circuit, never in parallel. You meter leads when in the current mode are a portable short circuit waiting to cause much damage and drama if not properly in series with an exiting circuit. Voltage on the other hand is measured in parallel across components or power supplies + and - terminals and as the meter in voltage mode is a very high input impedance can cause no harm.

So you need to study and understand the differences in series and parallel circuits.



You may want to read this How to use a Multimeter

How to measure current is about half way down the page, and how to change the fuse in your meter is just below that.   

If you were measuring anything other than power from a USB port, your fuse would be blown at this time (unless you had it in the 10Amp range)


So... based on your responses, sounds like I was shorting the circuit, then. :smiley-roll: Thanks for the info, I will avoid doing this in the future, and I'll definitely check out those book suggestions.

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