hippy
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Re: Potential power source

Thu Apr 05, 2012 10:08 am

Jim Manley said:


according to the USB spec, the minimum current load a USB port has to support is 100 ma (if power is provided at all), and the most is 500 ma


The other side of the coin is that something plugged in to a USB host socket should not draw more than 100mA without negotiating that and no more than 500mA so a manufacturer may take the decision to save on active current limiting and simply install a cheap non-resettable 500mA fuse or similar, or may even take 5V un-fused straight from the power supply. That may not be considered best design but the problem is it's hard to know what they have actually done.

Plug an R-Pi computer into something with a 500mA non-resettable fuse and you will likely blow that, probably voiding warranty then or if you open it to repair it yourself. If there's no fuse and wired straight to a power rail it may work but you could be overheating or overloading the internal wiring or power supply leading to damage or failure in the future.

If there is no fuse and the power supply is accidentally shorted it could crowbar the supplying device's power supply with all the consequential problems that may cause.

Unless a USB socket is explicitly rated for supplying the current an R-Pi computer needs there is a risk in using it and I'd recommend against doing so. Better to use a power supply rated for the job than risk a TV, STB or other device IMO.

Blunderbuss
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Re: Potential power source

Thu Apr 19, 2012 8:14 pm

I got a twitter reply from @Raspberry-Pi stating that the model B boards require a minimum of 700 mA and will draw a max of 800 mA.

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jbeale
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Re: Potential power source

Thu Apr 19, 2012 8:25 pm

note, the official current draw specs are what a supply should be rated for to ensure reliable operation, although not necessarily the "typical" currents people see right now. From the few posts I have seen where people actually measured the current draw, their Model B RasPi units were using between 350 and 550 mA depending on what"s plugged in (SD Card/USB/Ethernet/HDMI), and what software processes are active.  The maximum current presumably happens when the GPU is also running flat-out, and that"s hard to do right now, with most of the current software not having hardware-accelerated graphics enabled.

In other words, you could find your Pi works just fine on a 500 mA port now, but causes trouble later if you use a different USB stick, plug in different things, or even run different software.

pygmy_giant
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Re: Potential power source

Thu Apr 19, 2012 9:52 pm

Hi - having read the other posts I have erred towards caution as recomended by the sensible part of my brain and upgraded my battery pack from 600mAh to 800mAh. Despite this the reckless part of my brain has asked whether there is however some way to wire up a capacitor in parallel to the power supply to cope with sudden power demand while still using power supply rated at less than the recommended 700mAh?
Ostendo ignarus addo scientia.

john_wage
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Re: Potential power source

Thu Apr 19, 2012 10:16 pm

@pygmy_giant

mAh is not an indicative for current draw capacity.

batteries don't usually have any fixed upper limit for current draw, if you draw too much they will heat up and sometimes destruct, this is in the areas where you are talking about partial shorting.

pygmy_giant
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Re: Potential power source

Thu Apr 19, 2012 11:13 pm

Yes – you"re quite right and I"m quite embarrassed. Thakns for pointing that out.
Ahem, still, I wonder whether placing a capaicitor in paralell with the powersupply would work as way to cope with unexpected energy demad?
Ostendo ignarus addo scientia.

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jbeale
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Re: Potential power source

Thu Apr 19, 2012 11:19 pm

A capacitor does indeed act like a small rechargable battery, but for practical capacitance values, it will only supply useful current for a very short amount of time- think microseconds, or milliseconds for the largest capacitor on the board. So they can reduce voltage ripple from high-frequency switching, but not anything sustained for seconds or longer.

john_wage
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Re: Potential power source

Thu Apr 19, 2012 11:25 pm

@pygmy_giant

You're going in the general direction of an "UPS" or Uninterruptible Power Supply.

These are commercially available for 5V setups, but building one with your own PSU + Battery will take a bit more than a capacitor, and I wouldn't say it's a very feasible beginner project.

pygmy_giant
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Re: Potential power source

Thu Apr 19, 2012 11:39 pm

ok
Ostendo ignarus addo scientia.

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