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Farnell power adaptor for the Pi

e14 Contributor

*** EDIT ***

 

In this message, I implied that the PSU supplied by Farnell is inadequate, out of spec etc. it turns out that THIS IS NOT THE CASE - the Farnell PSU apparently works just fine. If you are interested in the details (or if you have similar problems, either with the Farnell PSU or with others) please read on. The short story is, it seems that USB type A -> Micro USB cables are usually the culprits, so check this before discarding/blaming your PSU.

 

*** EDIT ***

 

I was wondering if anyone has used the Farnell "ADAPTOR, USB TO EURO, WHITE " (the last item in page http://export.farnell.com/jsp/bespoke/bespoke7.jsp?bespokepage=farnell/en_UK/promotions/rasp-pi-accessories.jsp - interestingly enough, clicking on the link leads to a page about an unrelated product, but clicking on "buy" from the first page mentioned adds the adaptor to the cart).

 

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.

 

- Is this a known problem?

- What is the procedure for returning the two PSUs?

Parents

  • 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.

Reply

  • 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.

Children
  • in reply to johnbeetem

    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.