I have a Uno r3, and would like to provide it with at least 9v. In reviewing the 3rd image down here, I notice we want "center positive". I found an adapter with output of 8-10v, but it seems like it's the other way around (center negative?). What does "Cetner [x]" mean? What would happen if I were to use this to power it? I would like to power a 5V dc step motor.
droidus:
What does "Cetner [ x ]" mean?
The barrel jack connector on the Uno has a pin in the center, with a tube on the outside. The center pin is the positive contact, the tube is the negative contact.
On power supplies and devices with barrel jacks, you often find this helpful design that indicates center positive:
droidus:
What would happen if I were to use this to power it?
It won't work. You can damage electronics by connecting the power supply with reverse polarity but there should be a diode to prevent damage.
When in doubt, I always check power supply polarity with a multimeter.
droidus:
I have a Uno r3, and would like to provide it with at least 9v.
The Uno runs on 5volt.
The regulator and reverse protection circuitry need a few volts more to function properly.
7.5volt on the DC socket is sufficient. 9volt is still ok.
Higher will just be burned off as unwanted heat in the regulator.
If you're going to use a 9volt supply, then make sure it's regulated.
A better (less wasted heat) way is to power the Uno with a 5volt cellphone charger, connected to the USB socket.
The Uno's DC socket is protected for reverse polarity.
The Uno just won't work with a supply with the wrong polarity.
Leo..
I decided to say "should" about the protection diode because I figure it's possible some cheap "Uno" derivatives leave that off.
It's probably difficult to find a charger that will plug directly into the USB socket of an Uno. I don't even see any micro to type B adapters. You could get a micro to type A female adapter and then plug a type B USB cable into that. They're pretty much giving the adapters away (I have a bag of them I won with $0.01 bids + free shipping on eBay because there are more listed than people even want to bid on) and everyone already has USB cables and phone chargers on hand so that could be the cheapest solution.
There is a problem with the antiquated documentation for the Arduino UNO and its predecessors.
It implies that the "Barrel jack" is the "correct" way to power it. In fact, it is essentially an ornament provided in the very beginning of the Arduino project when "9V" power packs were common and this was a practical way to power a lone Arduino board for initial demonstration purposes. And even then it was limited because an unloaded 9 V transformer-rectifier-capacitor supply would generally provide over 12 V which the regulator could barely handle. You say:
droidus:
I have a Uno r3, and would like to provide it with at least 9v.
implying you wish to feed it "plenty" of voltage. In fact, it is the exact opposite. The higher the voltage you feed in, the less current you can use before the regulator shuts down from overheating, hopefully reversibly. You could simply never power a motor from the 5 V terminal if powered via the "Barrel jack" or "Vin" pin.
Nowadays, 5 V regulated switchmode packs are arguably the most readily available in the form of "Phone chargers" and switchmode "buck" regulators are cheap on eBay so these can be fed into the USB connector or 5 V pin to provide adequate power for all applications. Unfortunately, many tutorials or "instructables" are outdated or misleading and have not been updated to reflect the contemporary situation.
If powering from batteries, as long as the battery pack cannot exceed 5.5 V, this must be connected to the 5 V pin.
pert:
It's probably difficult to find a charger that will plug directly into the USB socket of an Uno.
Many USB phone chargers have a USB socket on the supply, so you can use your regular USB cable that you use for uploading a sketch.
Leo..
Yeah, that would be perfect. I've never gotten one of that style with a phone (I have purchased a few on their own). On the other hand, I've only owned four cell phones and the last one was a used eBay bargain that didn't even come with a charger. So I shouldn't consider myself a phone charger expert.
pert:
Yeah, that would be perfect. I've never gotten one of that style with a phone (I have purchased a few on their own). On the other hand, I've only owned four cell phones and the last one was a used eBay bargain that didn't even come with a charger. So I shouldn't consider myself a phone charger expert.
In short, you haven't purchased a new phone in its package for quite a while. 
They now virtually all come with a charger with a USB "A" socket and a lead which can - of course - be used either with the charger or with your PC when you "sync" the phone.
Unfortunately I know that because I have a nasty habit of dropping phones. 
But my overwhelming source of "phone chargers" is garage sales. People do - as I do - use the cable/ charger from the previous phone and the "new" one languishes in the box till the next phone/ next clear-out.
I'm still using an old school bomb-proof phone with the non-touch LCD under thick plastic and real buttons. Goes about a month between charges because I almost never make or receive calls. I bought a used smart phone on eBay a few years ago just to have an Android device to mess with, but I have no desire to sign it up for a phone plan.
The OP here seems to be a bit of a "one-hit wonder" who does not actually engage in the discussions he initiates! 