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Raspberry Pi 2 "5 V Power Supply" Notes

Former Member

Some time ago, I ordered and received the RPi 2,.  ordered and imaged a new SD card and found the board not booting.

 

Having been using a RPi 1 with power supply, monitor, mouse and keyboard and never seen any problems starting,. I expected

the new board also to boot with the new SD card and the rest of the hardware.

 

After spending quite some time with changing to another RPi 2 board ( exchange )  different SD cards, I had the conclude that

my problem most likely had to do with the the power supply.

 

All the used SD cards and hardware  did work with The RPi 1.

 

Up to this point both boards did not work while the RP1 1 never failed to boot.

 

The power supply I was using provided about 5.4 V measured on the miniature USB connector on the board. Changing start sequence

had no effect, 110 Volt first, USB switching with Power Supply already powered up. No variations in the board voltage were noticed.

during the start-up.

 

The power supply ( Chinese switching ) had a relative thin output power cord ( by closer inspection )

 

Finally, by using an home built power supply with adjustable output voltage and the heavier gauge output power cord, I got both computer

board to work with all the SD cards I had required in the previous process.

 

Not having an oscilloscope available, I was not able to look for any non wanted effects at the time of switching on the RPi 2.

My final conclusion is that the RP1 2 is much more sensitive to possible voltage changes than the RPi 1 during start-up and booting

With the adjustable 5 V power, I experienced no problems running between 4.5 and 5.5 V input to the board.

 

Wished I had known this a little earlier....!

 

Rein Smit

  • You need to be very careful providing in excess of 4.25V to the PI or Any 5V rated logic device, this is exceeding the recommended max volts and could damage something

     

    With the right adapter at 5V or no more than about 5.2V with 2A Capacity or more and a quality USB cable you should be fine. The 2A is to allow for any additions you may plug into the PI.

     

    I have performed a review and analysis of many cables here should you be interested in learning more about it

     

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

     

    Regards

     

    Peter

  • I ordered one of those "5.25 volt" power supplies from Adafruit and unfortunately it spits out 5.35V according to my multimeter.  I'm worried that at very low currents 5.35V could damage something.  So the new supply is sitting in a box until I can check the image Raspberry Pi 2  schematics image and see which components are hooked up to that 5V power input.  (Yes, jamesh, we really do need schematics.)

     

    I'll pick up a Schottky diode next time I order some parts and get a 0.2V drop.

     

    For now, I've been able to run a RasPi 2, keyboard, mouse, and Papilio DUO with a 700 mA supply.  With Ethernet plugged in, too! image

  • in reply to johnbeetem

    Hi John, there's obviously not a lot on that board and really nothing I can see that uses the 5V directly it all goes via the DCDC. The USB may be a different kettle of fish, so the juice coming out of there maybe a bit hot for some things, there again most USB devices are pretty robust and probably regulate down to their own 3.3V supplies any way.

  • I've not had any problems taking the RPI2 down to 4.5V and below maybe you are just unfortunate with your particular 'Pi or the PSU  supply is full of noise?!

     

    I'll take a look at it when I get some time ..no doubt producing a few videos image !

  • in reply to Robert Peter Oakes

    Peter, any chip that allows "5V nominal" powersupply will implment that as "5V +/- 10%". Now, if a powersupply were specified to provide 5V +/- 10%, it could in some cases provide one of the extreme voltages of that range. Then things would be "tricky". So for most powersupplies the range they provide is the nominal voltage +/- 5%.
    Another consideration would be that there is a slight voltage droop across the cable from the powersupply to the load. Expensive powersupplies will measure the resulting voltage at the load, and automatically compensate for that. However, cheaper powersupplies (e.g. the ones you might use to power your 'pi) cannot afford the extra electronics/wires. So it could be useful to pre-compensate a bit for the voltage drop. So now you see powersupplies that provide 5.25 nominal. A voltage like 5.35 coming out of such a powersupply will still be well within range for all chips that nominally take 5V.

  • in reply to rew

    I would largely agree except you do see many that are 5v +- %5 which would take the PSU out of spec.. All depends on how much you invest in the paranoia!

     

    Probably OK but John probably doesn't feel lucky image

  • in reply to Problemchild

    I disagree with the word "many". I can't find any.

     

    Some "special" chips might have stricter requirements than 5V +/- 10%. However, those are exceptions. For example, a sensor normally built for 3.3V might "stretch" to "5V + a little bit".

  • Certainly when I was learning Electronics with the good old TTL and CMOS chips along with the CPU/ ROM/ GPIO chips of the time, it was all 5V +/- 5%, not 10% but having looked at a few of the common PDF details for some chips I'm using now, they are indeed all 10% (1970s - 1990s) compared to 2015

     

    This does not imply ALL chips are now 10%, just that some are, so I would still be cautious of assuming 10% is good for all and as has already been stated, without schematics no-one can be sure

     

    On a separate note, this is a USB connector and the PSU can be (And at some point will be) used on other devices. the USB specification clearly states +5%, - 11% so between 4.45 and 5.25V MAX. but up to 5A in the case of a charging interface so a wall brick with USB interface outputting more than 5.25V is OUT OF SPECIFICATION.

     

    Hope this helps

     

    Peter

  • in reply to Robert Peter Oakes

    Oh do not use a cheap USB powered hub. I did the other day and it blew out the laptop I was using.image The battery is so dead, it will not power the laptop while inside.image Luckily no battery works.

    Clem

  • in reply to Problemchild

    The +5V is used to supply power for USB, HDMI, the GPIO connector (and anything connected to it of course) and is connected to the VDD_BAT1 - VDD_BAT4

    and VDDBAT2 inputs on the BCM2835 SoC.

     

    All of the old 74xx(x) series and their variations (LS, ALS, S, F) are +-10%. The 54xx(x) series (these are miltary grade versions of the 74) are +-5%. I

    also found some old PLDs (16L8 etc.) that are +-5%. There are probably more. I have noticed that more of the modern 5V ICs are +-5%. The absolute

    maximum voltage ratings usually allow for up to 6V but in the long run this will probably affect reliability.