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I am new to use RPi2 and all I did is following the documents and procedures given in raspberrypi.org. I have installed the latest NOOBS_v1_9_2 on my Pi2 and did some basic execution using Scratch. Good its all done, now I need to start doing stuffs which are needed for me.

I have utilized all the 4 usb ports for Keyboard, Bluetooth dongle for mouse, USB barcode scanner, USB Wi-Fi(Wi-Pi) adapter. Note: I need not boot from USB stick.

After I do a power off/on it doesn't boot, so I removed USB WiFi Adapter and powered up again, it booted again. Somewhere while googling I read that when connecting WiFi adapter, we need to use Powered USB hub, since RPi cannot provide sufficient current to drive the WiFi adapter requirement.

I tried with that option and identified it boots fine. So I need to know, why this is happening? The same Wi-Fi adapter works when I connect it to RPi after it boots. I understood that RPi is looking for code inside the Wi-Fi adapter. Is it because of USB boot option?

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    insufficient power -either a weak power supply or cheap cable – Steve Robillard Jul 22 '16 at 15:43
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The default USB current for Pi2 is 600mA which can be doubled by setting max_usb_current=1 in /boot/config.txt.

Of course, this only helps if the power supply is adequate.

WiPi does not need excessive current - I use with my Pi2.

You need to provide more detail. What PSU? What lights are on etc.

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Most likely a cheap power cable or a bad adapter if you are using one might I suggest

Something along the lines of https://www.amazon.ca/gp/aw/d/B00ITYAASG/ref=mp_s_a_1_4?ie=UTF8&qid=1469218190&sr=8-4&pi=AC_SX236_SY340_QL65&keywords=5v+1a+power+supply+micro+usb&dpPl=1&dpID=41sU%2Ba4E23L&ref=plSrch

For a power supply, it's not worth it to get an adapter because most do not supply enough amperage from my experience

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