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?
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.
Comments
cascaman
The reason why Apple adapters doesn't feed properly the Raspberry Pi is NOT that they are not "robust" as you say. They are CHARGERS (not only "adapters"), and they are clever and they try to provide the adequate voltage and current for charging the battery of Apple devices. And the voltage provided varies while the battery is getting charged. So, they are not designed for continous feeding of devices such as raspberry pi and the behaviour could be not as expected (as in your tests).
chester
Even though I did not mention "robust" anywhere on the text (unless you refer to the video - just adding a clarification in that case), you are correct: the iPad charger was likely not designed to do that - most likely it relies on some sort of voltage regulator inside the iPad itself. Still, one would reasonably expect such a charger to work as a power source (as the BlackBerry ones do).
cascaman
From eLinux.org "Verified Peripherals" for RaspBerry Pi:
"Note that apple designs its charger products to work optimally as chargers, in practice this means that apple chargers drop their output voltages somewhat with output current, so that the charging circuits do not need to dissipate more heat than is strictly necessary. Because of this, and although many people have reported apple products to power their basic PI setup reliably, its still not an optimal choice for a PI system that uses power hungry USB devices"
If I know that behaviour, I wouldn't expect such a charger to work as a power source. BlackBerry or other brands can do, but it is not a sign of Apple charger should do. BlackBerry has not thinked about reduce voltage when it is not needed, because it is cheaper to don't do it. It is good for using it with RPi, but worse for the charger itself and probably, the blackberry battery too. Anyway, not a "robustness" problem (yes... you say it in the video).
Look: http://www.righto.com/2012/...
Overpriced? Perhaps. Lacking of robustness? No!
chester
Guess I wasn't clear enough on both the answer and the "CLARIFICATION" added: it is *not* me on the video. NOT ME. CE N'EST PAS MOI. NÃO SOU EU. NON SOY YO. 私はだめです.
It is *another* gentleman on a site called YouTube that allows puny bloggers like me to embed their videos on our posts - which I did to illustrate the fact that voltage plays a role (and *that* I indeed wrote), and also because it shows how to measure it, which is quite helpful.
Here is (again) the URL of the original video: http://youtu.be/XX3kiRUf7mg, which has a discussion forum and links to the video author's website. It is the perfect place to discuss the matter with him, delivering the twenty lashes he surely deserves for such an unforgivable mistake.
Have a nice day, sir.
Pherja
Darn. I was figuring on using my iphone or ipad charger. Well I might stop at the plae I bought my Pi, they had a charger rated to work for about $10 (¥1060). So what problems does the slightly lower voltage cause?
Hey btw, 私はだめです kind of means "I'm not good" or "As for me, don't". Try Watashi no janai yo. 私のじゃないよ。"it's not mine!" :)
chesterbr
Hi! First of all, thanks for the correction - my Japanese is pretty crude, and I just wanted to grab the attention of a reader that was not paying much.
I have personally seen the occasional reboot while doing heavy (for Pi standards) stuff, like playing videos.. With the new power source, I have at least one Pi running for months (a server for a game, which might be featured on a future post). I'd recommend that you consider a pre-tested one - I'm not sure about Japan, but $10 would be a decent price here in Canada.
Good luck!