RPI 3 nousb

It's usually the USB cable that is to blame. My Raspi 1 with USB wireless mouse, keyboard and WiFi dongle worked fine with a 1m cable. The Raspi 2 takes more power and will only work with a 1/2m cable. The Raspi 3 takes even more because of the on-board BlueTooth. It's all laid out in the specifications.

A Comprehensive Raspberry Pi 3 Benchmark

Even if the PSU produces bang on 5v, the Raspi 3 will still not work with a 2m cable because of the resistance.

I tried to find my measurements, but cannot. I measured the voltage at a USB port by connecting it via a cable to a PCB USB through-hole socket and using a meter on the unsoldered pins. However, here is an excellent post on element14 that covers the issue and gives measurements (you may need to convert from feet to metres).

Raspberry PI:- USB power cables, crashing and other problems

The cables measured in the above link were around 0.3 Ohms to 1.2 Ohms per metre.

I seem to remember that my cables were all around 0.6 Ohms per metre including those sold as 'charging' cables. If a PSU is delivering 5V at 1 Amp via a 1m USB cable of 0.6 Ohms resistance, then the voltage at the Pi is 5V - (0.6 x 1A) = 4.3V.

The maximum power draw from the Pi 1 and the Pi 2 is 1.8A. The maximum power draw from the Pi3 is 2.5A.

For a 5Volt PSU that can actually deliver 2.5A at 5V, the voltage drop across a typical (28AWG thickness) cable - often with 28AWG printed on the side - of one metre in length is 2.5 x 0.6 = 1.5V. The voltage at the Raspberry Pi 3 is 5V -1.5V = 3.5V. This is the worst case at the maximum power draw, but it is a problem even at the minimum power draw for a RPi 3 because of the extra on-board components.

The official power supply is rated at 5.1V which helps to reduce the problem. Don't use a PSU with a higher rated voltage because the Raspi might pop if the connected devices stop taking their normal power draw. The obvious way to solve the problem is to use 25cm cables rather than the typical 180cm cables or the shorter 100cm cables. Always buy cables rated as 'charging' cables, but don't try to use anything longer than 50cm because they are typically still only 28AWG in thickness and have relatively high resistance. 

I got some 20 AWG charging usb cables from amazon and tried again(without the lcd) with the 2.1a psu, usb works until I try to use Bluetooth which drops the voltage enough to kill the usb ports. Which means I cannot finish setting up Bluetooth.

So, I tried a 2.4a and a 3.4a usb psu and those turned out to be worse than the 2.1a psu. The 2.4a psu was 4.88 volts while the 3.4a psu was 4.91 volts. It looks like the oddball voltage requirements is the problem and not the amperage requirements.

Trying to find a quality psu for the pi 3 is getting to be a real PITA. Element14 has 872 while MCM has 0.

I have a 8a 5 volt psu that's running my RGB leds. 5.09 volts with a 3 amp load, 5.14 volts no load. I'll be trying that next when I have the time. If that doesn't work I'll send my pi 3 back to MCM.

BTW my other pi's say made in the UK while my pi 3 says made in PRC. WTH is that?

Dan,

     No way? That may be part of the problem and I would get an RMA based on the information you put forth here. Mine was UK made from Microcenter since on line was hard to get.

Clem

Dan,

     No way? That may be part of the problem and I would get an RMA based on the information you put forth here. Mine was UK made from Microcenter since on line was hard to get.

Clem