Noob question about current

[Desade]

Hello all, this is my first post, although I've searched and lurked the forum for a while. My electronics experience is very rusty and rudimentary, so I'm going to ask some very dumb questions. Please feel free to mock. 

I'm trying to understand current by experimenting with my Arduino Uno and a digital multimeter, and there are a couple of things I don't understand:

1) If I switch my meter to A/mA and connect it to the 5V or 3.3V power pins on the Uno, I get constantly fluctuating numbers - so fast that I couldn't even tell you where the decimal point is. Is this because the multimeter is varying the drawn current to remove its own internal resistance from the equation? If I include a resistor in series, it behaves more like I expect: a 270-ohm resistor causes my meter to show 18.5mA on the 5V pin and 12.2mA on the 3.3V pin. From a straight equation standpoint, I=V/R with no resistor in series would yield I=5/0=undefined. But that can't possibly be true in the real world - can it?

(edit: the spec sheet says the 3.3V pin carries a current of 50mA, which doesn't clear up my mysterious current readings at all)

2) The spec sheet on the Uno says that the digital output pins generate 5V at 40mA. But if I write a simple sketch that sets the pin (I'm using #7, but it shouldn't matter) HIGH, my multimeter reads 5V at like 87mA. What gives? Technically, I'm setting it high for 10 seconds, then setting it low for 10 seconds in a loop. The multimeter readings vary between 87mA and approx 0.15mA.

As you may have guessed, I'm still having trouble wrapping my head around current vs voltage, and how it all works together in a circuit.

Thanks...

[AWOL]

If you connect a current meter between ground and a supply, you're effectively presenting a short across the supply.
Don't do it.

Measure current in series with the circuit.

[zoomkat]

As you may have guessed, I'm still having trouble wrapping my head around current vs voltage, and how it all works together in a circuit.

A good place to start.

https://www.google.com/search?num=100&hl=en&lr=&tbo=d&as_qdr=all&q=current+voltage+tutorial+multimeter&oq=current+voltage+tutorial+multimeter&gs_l=serp.3...2846.8848.0.9307.11.11.0.0.0.0.251.1052.8j2j1.11.0.les%3B..0.0...1c.1.AQFx8-0Lsgg

[fungus]

I get constantly fluctuating numbers

You're short-circuiting the Arduino.

I get constantly fluctuating numbers

I assume the power supply (USB?) is trying to save itself from death-by-short-circuit.

[oric_dan]

@Desade, when you edit a msg, you're supposed to go to your original msg, and click
on "modify" rather than start a new thread, or whatever you did to create 2 of the
same threads.

[jackrae]

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.

[Mr_Blah]

       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
http://www.amazon.com/Getting-Started-Electronics-Forrest-Mims/dp/0945053282/ref=sr_1_1?s=books&ie=UTF8&qid=1356817141&sr=1-1&keywords=getting+started+in+electronics+by+forrest+m.+mims+iii

       Make: Electronics by Makemagezine
http://www.amazon.com/Make-Electronics-Discovery-Charles-Platt/dp/0596153740/ref=sr_1_1?s=books&ie=UTF8&qid=1356817190&sr=1-1&keywords=make%3A+electronics

[retrolefty]

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.

Lefty

[brass]

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)

[Desade]

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