what adapter to get to power my arduino duemilanove?
the "Getting Started with Arduino" book suggest to get an AC adapter 9 volts recommended, 2.1 mm barrel tip, center positive. page 20.
but doesn't say anything about the mA... so i don't know which one to get, since they have several different models with different mA or mW... so please advice, i was about to get this one:
thank you.
Anything bigger than 500mAmps should be okay. So the Radioshack adapter should be okay.
My logic is:
I bought one of these 9V 1A adapters (but with a UK plug) from eBay, which is under 1/5th the price of the Radioshack adapter, and it works okay:
GB-)
Agreed, anything over 500 mA is fine.
As a rule devices draw the current they need once adequate potential is provided. For safety's sake lower current is better, but anything between 500 mA and 1 A is fine.
Dan, I'm unsure what you meant by:
For safety's sake lower current is better
I don't understand how driving the Arduino with, say, a 4A supply is less safe than driving it with 550mA.
I would go further, and say a 4A supply is more safe than a 550mA, as a 4A supply is running well within spec. and so should be unstressed.
If the Arduino tries to draw more than 500mA, and it is a normal Arduino, it's onboard polyfuse would (temporarily) blow, protecting the whole system.
Are you thinking of something else?
GB-)
I don't understand how driving the Arduino with, say, a 4A supply is less safe than driving it with 550mA.
Try fiddling about and getting a short. Four amps is going to melt a lot more that half an amp.
If the Arduino tries to draw more than 500mA, and it is a normal Arduino, it's onboard polyfuse would (temporarily) blow, protecting the whole system.
Which poly fuse is this then? I haven't got one on my arduino.
Anyway a poly fuse takes a long time to blow so is no good at protecting semiconductors.
The Arduino Duemilanove board has a polyfuse on the 5v USB line.
Yes that poly fuse is in the USB 5V supply and is designed to protect the USB power supply in the PC. They are part of the USB spec.
It is not designed to, nor will it, protect the circuits beyond the fuse. It will also not even be in the circuit while powering the arduino from an external power supply, the purpose of this thread I think.
As I said a poly fuse takes a long time to blow, sometimes as long as a second, depending on the degree of overload and will only protect components that are damaged by prolonged exposure to excessive power dissipation. It will not protect components from peak excessive current.
Try fiddling about and getting a short. Four amps is going to melt a lot more that half an amp.
Understand. Good point. Agree in principle.
I guess the bigger power supplies I've used are better protected than wall-warts, but agree, that isn't the question.
GB-)
The arduino itself will only pull about 30 mA so if you're not doing too much with external power, you can get away with a lot less than 500mA.
So, given the question:
but doesn't say anything about the mA... so i don't know which one to get, since they have several different models with different mA or mW... so please advice
what is a reasonable answer?
Surely we can give some consistent advice to folks looking for an answer?
I will stick to lower-end 500mA based on my logic above. The rationale is that if you build something powered from USB, then you might like to run the same thing from the adapter, and you might get grumpy (no disrespect 
if it doesn't work.
Accepting Grump_Mike's helpful reminder that more power isn't automatically safer, there isn't a lot of point having oodles of power just waiting to melt something, so 500mA is enough for folks who use a USB-based development approach.
Pluggy observes that the Arduino itself only pulls 30mA, so if you feel that you are only going to run the Arduino with minimal external electronics, even lower current would be better.
EDIT: I've always been taught to have some headroom for power, up to 2x. The rationale includes: estimates might have small errors, start-up power can exceed the average, circumstances change, and it'd be annoying to have to change power supply because of a small change, heat damages parts, so running something at half capacity allows the thing to stay away from its maximum temperature, and hence should last longer.
So if you are calculating the power output of your power supply, give yourself upto 2x headroom, and multiply your estimate by 2.
Having said all that, 1A 9V supplies are dirt cheap ($3.50), and work fine.
The calculation of power is: Power = Voltage * Current, so
Power = 5V * 0.5A = 2.5W,
Power = 5V * 1A = 5W,
Power = 5V * 0.03 (30 mA) = 0.15W, or 150mW
The amount of power consumed is based on the amount of current that the Arduino and electronics uses, and not the rating of the adapter.
There will be some losses in the adapter, and these can vary. So you may find that an adapter rated at 150mA actually waste more power than one rated at 0.5A. Efficiency is based on the technology and quality of its implementation, not its rating.
(I would expect the adapter to be optimised for its rated power output, so don't go crazy and get a 88% efficient 100A power supply and feel disappointed that it seems to use more power than a notionally 70% efficient adapter rated at 200mA.)
I'm sure folks can improve this suggestion. I am not saying this is definitive. But may I ask that we try to summaries for the poor souls trying to understand, so they get a clear-ish view, at least of this thread, in one spot? (IMHO, the time to find something as concrete and direct as this should be small).
HTH
GB-)
Surely we can give some consistent advice to folks looking for an answer?
I have found over the years that given N engineers there are AT LEAST N+1 firmly held opinions.
thank you so much for all of your answers! i am going with the one i selected
I have found over the years that given N engineers there are AT LEAST N+1 firmly held opinions.
Only N+1
That's probably because you are a management genius, and only ask each one once 
GB
Only N+1
I did say "AT LEAST"
But it is true about my management skills.