Raspberry Pi Forums

Hello All,

I am very sorry to ask another power source question, I know a lot have already been asked, but I couldn't find an answer to my query.

I have tried powering my Pi using 2 different sources; 1 is an iPad Charger (5v, 1A) and the other is a Powered Belkin USB Hub that has 5v 2.5a on the adapter and I have nothing else plugged into the USB Hub.
I bought myself a multi metre to check the voltage etc... and get the following;
iPad charger with nothing plugged into the Pi (no SD) - 4.97V
USB Hub as above - 4.97-4.99v

As soon as I plug the SD card in (a Kingston Class 10 8GB), the voltage using either charger drops to 4.30-4.40v.

I am wondering if this is causing the issue I have been having where when using OpenELEC, it just crashes out sometimes. I have the log files from OpenELEC if they are of any use to anyone. It appears to be crashing when the Pi/OpenELEC is accessing the Networked Hard Drives.

I will go out and buy a better power source if this will resolve the issue, but wanted to check if anyone had any ideas.

Cheers

thekingcat

That sounds like a pretty dramatic power drop off for it just to be the power supply, but it could be the power supply.

I would suggest you read this thread:

http://www.raspberrypi.org/phpBB3/viewt ... 26&t=22318

all the way through once. You will see a post by "pluggy" that describes how to test the F3 fuse on the bottom of the board.

Be aware that any of the talk about measuring resistance, not voltage, but resistance, is done with the raspberry pi powered off, you should not test resistance with power going through the board.

For sure that low of power could be causing crashes.

Thanks for the info. I will have a look through that when I get home tonight. I thought it was something I was doing wrong. Must have reinstalled openelec and raspbmc about 20 times. Tried different SD cards. Now after gettin my multi meter today I realise I am not going mad, ....yet!

Right bgirardot.

I followed the instructions from 'pluggy' and measuring from the back of the TP2 to the outside edge of the F3 fuse I get 4.97-4.99v reading that is with the SD card in, without the SD card in i get 5.08v(ish). So does that mean it is a problem with the Power units or the USB lead I am using?

I have technically tried 2 power sources, the iPad adapter and the USB Powered Hub, but I have onyl tried the one USB cable, so I will try to get hold of one and try that. The cable I am using is the data cable for my Logitech Harmony remote so may be limiting as its only for data on the remote.

Cheers

Did you measure both sides of F3?

You should just hold one test lead on TP2, keep it there, and then put the other test lead on the board edge side of F3, note the voltage, make sure you have a good contact, then move the lead to the SD card slot side of F3 and see what it is there (first lead is still on TP2 this whole time).

We are trying to figure out the voltage drop across F3.

Because the problem is, you get good voltage coming into the board, that is the value of TP2 and board edge side of F3, but your voltage across TP1 & TP2 is only 4.3v.

So I think the question is: what is the voltage on the SD slot side of F3 and TP2?

Ok... I have now tested the voltage on the SD card side of the F3 fuse and there is a massive drop-off down to 4.39v.

So am I best contacting Newit.co.uk to ask for exchange?

Cheers

EDIT: Newit.co.uk have kindly offered an exchange on my Pi. Thanks for all the help on this issue.

thekingcat wrote:Ok... I have now tested the voltage on the SD card side of the F3 fuse and there is a massive drop-off down to 4.39v.

So am I best contacting Newit.co.uk to ask for exchange?

Cheers

EDIT: Newit.co.uk have kindly offered an exchange on my Pi. Thanks for all the help on this issue.

Great, glad to hear you tracked it down. And all I ever hear are good things about NewIT.

Glad to help

I have just received my replacement Pi from NewIt today, but I am still getting voltage drop across the F3 fuse, not as much this time, it is only dropping to around 4.5- 4.54 v.

Is there anything else I can try without having to contact NewIt and send this one back too?

What do you have plugged into each of the Ports/ Sockets of the RPi?

I ask because the F3 rating of ~750mA was designed to power the RPi Board and no more than 200mA off-board. This was originally limited by including 100mA Polyfuses on each of the USB Sockets. On late Rev1 Boards and all Rev2 Boards, the 100mA Polufuses were bridged with 0 Ohm Resistors and therefore, the F3 Polyfuse could be driving 500mA per USB Socket and more if used with an unpowered Hub. Also, if you are using an unpowered HDMI to VGA Adapter, this could be drawing ~ 200mA.

The answer to this issue is to power the RPI by backfeeding it from a POWERED USB Hub with a 5V 2A Power supply .... and ignore the MicroUSB Socket, (assuming that you have a late Rev1 or Rev2 board)

I have a Rev2 board 512mb RAM.

I have tried back powering from my 5v 2.5a USB hub but the TP1/2 readings are around 4.3v.
So I reverted back to the MicroUSB, have tried 3 different solutions for this (iPad charger, USB Hub, and a Nokia Charger 5v 1.2a). The USB hub power gives the best readings overall, but still around 4.5v after the F3 fuse.

When I have been testing the board I have nothing but the SD card and the power plugged in.
In normal use I would have the Ethernet and HDMI plugged in as well. HDMI goes direct to the TV.

I really dont know what to do.

If you are backfeeding from a Hub, here are some sources of Volt Drop:

1) A GOOD quality Hub will have Polyfuses or some other current limiting device on each Downstream port but should NOT have any current limit on the upstream cable to the RPi. Check the resistance of the upstream cable to the RPi.

2) Check the resistance of the Power cable to the Hub. I discarded the power supply that came with my Hub and now use a 2.1A iPad PSU with a USB to 3.5mm coaxial connector that I made up myself.

3) I have yet to find a USB to MicroUSB cable with acceptably low resistance other than the ones that came with my HP TouchPads and my Nokia N900 Phone.

Sorry to be a pain, but I am completely new to electronics....

How do I do all that?


Cheers

Could you post a list of the equipment you have; Hubs, cables, Power Supplies, Multimeter etc. with pictures if possible so I can suggest what you need to do/ get to make the backfeeding solution work for you.

When I get some time, I will take some pictutes of my setup(s) which give me a rock solid 4.95V to 5.1V measured on the RPi.

Here is a picture of my Backfed power setup.

The Hub is a 7 Port hub with two ports on the end. The Keyboard is plugged into one of the end ports and my WiFi Dongle with Antenna is plugged into the other. The upstream cable is plugged directly into the RPI. ( ignore the two MicroSD cards plugged into the top of the Hub.

The only other connections to the RPi are the HDMI Cable and audio cable to the Monitor.

The Rig is powered by an Apple Ipad (generic) PSU and a USB to 5.5/2.1mm Barrel connector with a short adapter cable to match the 5.5/2.1mm connector to the 3.5/1.3mm socket on the Hub.

Note that on my setup, I plug the Mouse into the USB socket on my Keyboard but it could equally well be plugged into the Hub or the spare RPi USB socket.

Well I recieved my 2nd replacement Pi this morning, a Made in UK one and ALL the problems seem to go. The voltage drop across the F3 is only down to 4.9v and thats even with my keyboard, mouse, ethernet and SD card in.

So I am very happy now with that, and a big thanks to NewIT.co.uk - absolutely fantastic customer service!!!!