Is it okay to power arduino uno with 9v @ 200mA external power supply on it's Vin?
rebellion:
Is it okay to power arduino uno with 9v @ 200mA external power supply on it's Vin?
Yes, as long as you are talking DC voltage and not AC. 
mmmm.. ok .. thanks
And note that common "plug packs" (AKA "wall warts") consisting of only transformer, rectifier and capacitor can deliver wildly different voltage when they are minimally-loaded, to their nameplate.
so how do I address that problem? I'm planning to run arduino maybe for a 2-3 days and it seems that using a 9v battery won't last.
Batteries should be used when you absolutely cannot access a mains supply.
Switchmode (regulated) plug-packs are readily and quite cheaply available (eBay, Deal Extreme, etc.) by mail order, either 9V to feed the Vin on the Arduino, or 5V (such as - phone chargers - I scavenge these at garage sales for $1 each or less, can cut "odd" connectors off) to feed Vcc directly.
You absolutely must have a DMM (digital multimeter - $5 to $10 is generally adequate nowadays for simple stuff) - or two - to work with these things; you can check voltages and immediately figure out whether your supply is regulated or not.
I see.. but I do already have a 12v 200mA transformer, some diodes and caps, and a voltage regulator (7809) and build my own power supply. But I would consider owning a DMM to see if the voltage is regulated. By the way, thanks for the answer.., ^^