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Powering a Raspberry Pi

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The Raspberry Pi is powered through an standard micro-USB conector. That is great, since it allows you to use pretty much any phone charger you got lying around. Or at least one that supplies 700mA of current (maybe a bit more if you plug extra USB stuff on the Pi).

I thought I had it covered with my iPad charger and its juicy 2.1A, but the video below shows that voltage also plays a role (and that the iPad charger doesn’t really deliver in that respect):

USB specs say you should have 5V ± 0.25V from a source, and the Pi also expects that, so I bought a $9 KDL-5100A at my electronics parts supplier. It is physically identical to the FY0501000 linked on the video, and indeed, performed better than the iPad charger… but still below 4.75.

After some head-scratching, I found the issue: the cable. Apparently, cheap cables have quite some resistance, which causes voltage drops as you need more current (Ohm’s Law, I suppose). Replaced it with a Samsung one, and voilà: iPad charger got almost good, and new charger worked great.

An LG cable (with no Part number) got me pretty much the same results. Also tested the cable on an Apple Cinema Display USB port (okay) and a BlackBerry Playbook charger with built-in cable (excellent). Heard good things about the Kindle Fire charger, but could not test it yet.

Guess who was causing trouble?

Guess who was causing trouble?

Below is a wrap-up of my measurements (Wi-Fi and keyboard dongles plugged); recommended options in bold. In short: get a proper charger, avoid $1 cables and always measure.

Charger Cable Power (V)
Apple A1357 Cheap unbranded 4.16 – 4.56
Apple A1357 Samsung APCBU10BBECSTD ~4.75
KDL-5100A Cheap unbranded 4.65 – 4.75
Cinema Display USB Port Samsung APCBU10BBECSTD 4.75 – 4.81
KDL-5100A Samsung APCBU10BBECSTD 4.90 – 4.95
BlackBerry HDW-34724-001 built-in 4.99 – 5.01

CLARIFICATION: The video above is not mine. It was just the inspiration for my own measurements, so I included it for illustration purposes.

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