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I have an RPI Model 2B running Raspbian that works (almost) perfectly.

I recently bought a TP-LINK WN725N V2.0 that I thought would work if I followed the instructions listed here. However, when I use the lsusb command, I don't see the WiFi dongle listed. I though it could be because I booted the RPI with the WiFi dongle attached, so I booted it without the dongle, and then attached it when it was booted. The rainbow thing in the top right corner appears, and the RPI freezes. I try typing, but the text lags about 10 minutes behind my key strokes. After what seems like an eternity of waiting, the RPI finally becomes responsive, but I still don't see the dongle when I type in the lsusb command.

Could this be an issue with the RPI or the dongle? Is the RPI not receiving enough power? If that's the case can you please recommend a cheap but reliable power supply I can purchase?

I should probably mention (not sure if this is important) that connecting to the internet via ethernet works perfectly.

  • Any "wall wart" type PSU that supplies 5V and a minimum of 1 Amp of current should suffice. If it is capable of supplying 1.5 to 2 Amps then that would be better, but not necessarily required - It depends upon the current draw of the WiFi stick. – Greenonline Jun 23 '15 at 14:56
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The rainbow square indicates that the Pi voltage has dropped beneath 4.65 volts.

You need to improve your power supply, either by buying a new power supply or using a better quality power supply cable or possibly both.

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I'm using this power supply for my rPi 2: http://www.mcmelectronics.com/product/28-17985 . Now, your choice of supply depends on where you live (US/EU/AP) and what is available. Sometimes you can use a power supply coming from another appliance (a tablet power supply typically has an output close to 2Amp). Just note that what is on the label is not always what you get - sometimes the Amp rating is just peak Amps, and the supply cannot provide this kind of current for any period of time. I've invested in a USB Amp/Volt meter like this one to review the power needs of all my adapters - it has helped filtering through the Amp claims and sorting out the good and the bad ones.

PS: Shopping questions are not in scope of this site, so you might get downvoted or closed ....

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Specific shopping recommendations are explicitly off topic. "Cheap but reliable" is probably not too much to ask, but note that "the absolute cheapest, but still reliable" is not worth bothering with -- you are going to spend hours searching around only to make a gamble that might save you $10.

You want a decent 2 amp, 5V supply. Find somewhere that has a clear return policy such that if it doesn't work, you can return it within a few days no questions asked. At least where I live, places that don't have such a policy usually rhyme with wanting the "absolute cheapest" and that comes with risks.

I have this exact same TP-Link dongle and it works with anything from a 1.2 A cell phone charger on up -- although I would not recommend that dongle to other pi users. It seems to get too hot and have minor issues that it does not have on, e.g., a linux laptop.

Although I don't think it will make any difference in this case, you could also try adding

max_usb_current=1

To /boot/config.txt.

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