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

How long were the cables you used? Try using one that is only a quarter of a meter long.

A 1.8 meter cable can easily lose 1 volt.

If your 20 AWG cable was 0.3 Ohms per meter and (a typical) 1.8 meters long, then, at the full current rating of your 2.1A PSU and an output of 5Volts, the voltage loss along the cable will be:

2.1 x 1.8 x 0.3 Volts

= 1.134 Volts

Even though the PSU is producing 5 Volts @ 2.1 Amps, the Pi will see 5 Volts - 1.134 Volts =  3.866 Volts.

With a quarter of a meter cable the voltage at the Pi would be 4.842 Volts. Even with a good 28 AWG cable it only drops to 4.7 Volts.

The approved PSU produces 5.1 Volts which makes the loss at high currents even more acceptable.

Bear in mind that 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.

Not that it will make much difference, but the official PSU for the PI is 5.1V, not 5V and if you use the official PSU it has a good sized cable and works well

If you are using a Standard USB power adapter and a not so good USB cable, you can be experiencing a whole bunch of power related issues

I have found the cables for a Samsung Phone to be very good (USB2, not the USB3 ones as USB3 wont fit of course )

Back in 2014 I did a complete post on the subject, you  can find it here Raspberry PI:- USB power cables, crashing and other problems , it seems people are still having the same issues

I know because we (top members at least) see the same posts over and over. It kind of makes me feel like a parrot.

Clem

I got my "official" raspberry pi power supply today and the rainbow colored square disappeared but the usb ports still do not work.

Now what do I do with it?

I got my "official" raspberry pi power supply today and the rainbow colored square disappeared but the usb ports still do not work.

Now what do I do with it?

Hi Dan,

I looked at the specification of the PSU,

http://www.farnell.com/datasheets/2020825.pdf

and found that the connecting cable was of 18 AWG gauge and 1500mm long.

The following site,

http://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table?

gave the resistance of 4.5 feet (about 1500 mm) of 18 AWG cable has a resistance of 0.029 Ohms.

At the maximum current for the Pi 3 of 2.5 Amps, the voltage loss would be 0.0725 x 2 Volts because there are two wires in the connection. The 5.1 Volts of the PSU would drop to 4.955 Volts at the Pi. I'm definitely ordering one of these!

Just to be certain, measure the voltage at the GPIO 5V pin after the PI has booted and make sure it is still around 5V (the actual voltage at the PI is higher, but I think there is an on-board regulator feeding the pin and the USB port).

It sounds as though the software is at fault. I suggest you download the latest Rasbian OS and flash it to the microSD card. Perhaps the voltage loss of your original power system has caused a failure to load the USB drivers.

https://www.raspberrypi.org/downloads/

lsusb -v

https://dragonboardcontest.com/vote/page/5

lsusb man page

lsusb — list USB devices

Synopsis

https://www.mankier.com/8/lsusb#Synopsis

lsusb [options]

Description

https://www.mankier.com/8/lsusb#Description

lsusb is a utility for displaying information about USB buses in the system and the devices connected to them.

https://www.mankier.com/8/lsusb#Options

Options

https://www.mankier.com/8/lsusb#Options

-v, --verbose

Tells lsusb to be verbose and display detailed information about the devices shown. This includes configuration descriptors for the device's current speed. Class descriptors will be shown, when available, for USB device classes including hub, audio, HID, communications, and chipcard.

Roy Leith wrote:

 

Just to be certain, measure the voltage at the GPIO 5V pin after the PI has booted and make sure it is still around 5V (the actual voltage at the PI is higher, but I think there is an on-board regulator feeding the pin and the USB port).

 

No. There is no regulator for the 5V. The incoming 5V is passed through a fuse (should be "automatic resetting"), and then passed on to USB (through some more components on the newer raspberries) and directly to the GPIO.

There IS a measurable voltage drop over the fuse, I expect the "new" components in the USB area to have less drop, but it will still be measurable.

I downloaded the image and used windisk32 imager and still no working usb ports. I got another pi 3 and psu to go with it, the second pi 3 say's made in the u.k. and not p.r.c. like the first one Before I ask MCM for a replacement or refund should I bother to compare voltages' between the two pi's?