Farnell power adaptor for the Pi

There was a recent post on the raspberrypi home page about fault power supplies, take a read: http://www.raspberrypi.org/archives/2151

Thanks for pointing this out, but the raspberry pi blog post was about fake apple power supplies. What I am discussing is entirely different: The power supply proposed by farnell in its "raspberry pi accessories" page is severely out of spec - in fact it is totally inadequate for the pi. I am slightly amazed that nobody has reported that yet - there is an ever so slight possibility that the specific PSU I tried was defective. I could check this by testing the second one I bought, but as I intend to return them in case they are both defective, I am hesitating to do it. I 'll wait a few days in case someone from Farnell / Element14 sees this and comments, then I 'll try to contact them directly.

alfon1917 wrote:

 

I got two such adaptors and tried one of them. Tha voltage between tp1 and tp2 on my Pi, after it boots, is 3,88V - way below the nominal 5V. As is to be expected, the Pi is not very stable with this supply. In contrast, using the PSU recommended by another distributor, I get 4.90V and the Pi works like a charm.

Are both power supplies you tried using the same Micro USB cable?  People have reported a lot of variation in Micro USB cable resistance due to varying size of conductors.  Generally, you want a short Micro USB cable with large conductors.  I have a no-name 1m Micro USB cable with 28 AWG wires and it works pretty well: 0.1 to 0.2 V drop IIRC.

It appears that CPC (part of Farnell) are recalling one of their recommended ones:

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=22&t=19020

Can't tell if that's the same as yours as the links you posted re-direct to the export 'choose your country' page, when I extract the 2210244 part number from your link and do a search I get a molex crimp terminal instead, and the CPC part number mentioned in that thread no longer exists..

Interesting to see issues with crap usb chargers finally getting some exposure though.

John is absolutely right on the cable though. I bought a supposedly decent one from farnell earlier this year and there's 0.5v or more drop at 400mA, and none of the other cables I could find were much better.

Dear John,

Are both power supplies you tried using the same Micro USB cable?

Nope. The PSU that worked fine from the beginning has its own cable with micro-USB output (the cable is fixed on the PSU, so I could not remove it and test it with the other PSUs). The Farnell PSU has a regular USB output, to which I plug a USB type A <-> micro-USB cable.

People have reported a lot of variation in Micro USB cable resistance due to varying size of conductors.  Generally, you want a short Micro USB cable with large conductors.  I have a no-name 1m Micro USB cable with 28 AWG wires and it works pretty well: 0.1 to 0.2 V drop IIRC.

You seem to be on to something - and I seem to owe an apology to Farnell. More testing needed, but It is already clear that with different USB cables I get different results. The best so far is 4,78V vs 3,88 which I was getting with my old setup. The real lesson for me is that there are far more crappy USB cables than PSUs (I thought it was the other way round).

Thanks for pointing me to the right direction.

As a newbie and a broadcast video engineer (who knows HDMI and video interfacing) I just received two boards, bundled with power supplies and pre-programmed SD cards

from element14. No combination boots. Only a single steady Red LED.

Will try a new power source and report findings.

SD cards both have correct image files.

Steve

Did you try a different cable?

I had trouble with three power supplies.

One was a Toshiba 2.1A version for my tablet, that wouldn't run the Pi, but happily charged the tablet.

The other supplies seemed to work one day, then wouldn't work at all and I suspect they had too much ripple which was causing the supply to reduce the output voltage.

I now seem to have about 5 mains to usb outlet devices, but I suspect they are designed to charge a 3.6v LiPo battery rather than be a 5v supply, like everyone thinks.

I also had cable suspicions, and was able to measure it at 0.1v. Follow the guide to measure the voltage and check its above 4.8v (which seemed to be the magic screen or no screen).

Funnily the best one was a cheap one that coiled up.

Mark

Voltages are good on board. Measured with Fluke DVM. I suspect an SD card issue. Going to write image to a new card. Seems odd that a purchased bundle with pre-loaded SD image wont boot two out of the box Pi's.

No green activity light and it appears that the green light is flashing three or four times reporting a SD image not found.

Steve