"Best Power Supply for the Raspberry Pi"?

Hi, I came across this video comparing USB chargers for the Pi today:

I've been using the Adafruit power supply he shows. Interestingly, it is designed to output 5.25V:

http://www.adafruit.com/products/501

This adapter was specifically designed to provide 5.25V not 5V, but we still call it a 5V USB adapter. We did this on purpose to solve a problem that occurs often with USB-powered gadgets: they draw so much current than the resistance of the cable causes a voltage drop, so instead of 5V, the device sees 4.75V or so. To avoid this problem, we made the adapter 5.25V. This is because the USB power specification allows up to 5.25V, and its safe for all 5V electronics, and has the benefit of making up for any loss due to thin USB cables. Basically, you can use it where ever USB 5V power is needed, with no risk of damage, and it will happily work at the full current range, no matter what cable you use!

Is there any downside to having a USB power supply output 5.25V? If it's within USB spec, then I wonder why all supplies without a built-in cable don't do this.

Cheers,

Drew

UPDATE: that YouTube user also has an interesting Pi temperature video, too:

morgaine over 13 years ago

There's nothing particularly wrong with the concept of purposely supplying a voltage at the top end of the USB spec, but Adafruit had better be careful with their QA so that they don't accidentally send out chargers that output more than that. And I hope that they've done some long-term testing too to see how their hardware varies with age.

Clearly the strategy is a good match for the Pi's loopy power management which deregulates the voltage by design.

PS. The video commentary was excruciatingly aweful though, LOL. It drove me up the wall by its utter imprecision.
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johnbeetem over 13 years ago

Most chargers I've seen say "5.1V". This permits a 100mV drop across the Micro USB cable, e.g., 1A across a 0.1 Ohm cable. To amplify Morgaine's comment, you really don't want to try to manufacture a 5.25V supply, as there needs to be some tolerance. An excellent regulator will give you 2%, which is 100mV. 5.1V is 3% away from max USB voltage, which gives you a comfy margin.

A potential problem with RasPi is that its 3.3V linear regulator RG2 gets hotter with higher 5V, which could expose a heat-related problem such as an iffy polyfuse F3. YMMV, but it could affect some RasPis.
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fustini over 13 years ago in reply to johnbeetem

Thanks for both of your comments. I have the Pi in the Adafruit Pi Box powered with that adapter. I've not had any instability yet, but I'll keep that in mind if I run into troubles. I'm interested now to look at the printed output voltage on my other USB adapters.
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Former Member over 13 years ago in reply to johnbeetem

This permits a 100mV drop across the Micro USB cable, e.g., 1A across a 0.1 Ohm cable.
All of the micro usb cables I was able to get my hands on were about 0.5 ohm in both ground and 5v lines, so lots of voltage drop @ 450-ish mA
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SFCAirborne over 13 years ago

I just went with a Apple IPAD Power Supply, Output 5.1V at 2.1A. Haven't had a single problem sofar.
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jamodio over 13 years ago

After testing many of the cheap-o USB got tired and got one of this to power the Pi
Still trying to figure how to solder the 10mm thick cables onto the micro USB connector .... No USB power limitations for me !!!

-J
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SFCAirborne over 13 years ago

LOL what is that 10 or 20KW, That's enoungh to power everyones Pi.
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