The RMS fluctuation in many wall-wart power supplies is often large enough,
especially in the cheap ones, to create all sorts of problems, none of
which will produce any error resembling "Hey, I need some more voltage". I
have a bunch of cheap chinese 5V power supplies that have something like
0.3-0.4V RMS atop 5.05-5.1V and this alone can and will corrupt images on
SD Cards. More typically, what you will observe will be USB devices not
functioning, such as WiFi, and memory read errors from the SDCard. My
general rule is that if I see errors in a standard image, it's the power
supply. I'm sorry I did not think of this and mention it sooner.
The recommended power supplies are at the high end of the USB power
standard at 5.2-5.25V. I haven't had enough time to futz with my cheap-o
power supplies to see if some simple caps will do enough smoothing to help,
but my scope suggests not. I did not, however, scope them under load, so
who knows. I've got the cap slots on the board so I'll try anyway.
Colin
On Mon, Feb 3, 2014 at 1:49 PM, Vajk Fekete <[email protected]> wrote:
> One of my recent lessons learned: a lot of small devices have a mini usb
> connector for power. And they consume around say 500mA. And the cheap usb
> cables, even the short ones have so little copper in them that 500mA may
> cause 2-300mV drop on a 6in cable.
>
> Vajk
>
>
> On Mon, Feb 3, 2014 at 10:43 PM, Ziggy <[email protected]> wrote:
>
>> Well. It would certainly be interesting to understand THAT failure mode
>> :)
>> Thanks for getting back to us and letting us know your solution. So many
>> times you never hear or someone just says :N/M I fixed it.
>>
>> Paul
>>
>> On 02/03/2014 11:12 AM, Colin Tinker wrote:
>>
>> Just to let you all know I sorted this one. New power supply for the
>> Raspberry Pi was not good enough for the job and could no cope for some
>> reason when I added a long cable to the 1-wire network.
>>
>> Colin
>>
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