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My question: it's my understanding, based on the official Raspberry Pi FAQ, the wiki, and numerous forum threads on Stackexchange and elsewhere, that the Raspberry Pi has a 1.1A polyfuse on the micro-USB input, which limits the total current that the Pi+any peripherals can draw to 1.1A.

I understand that it's worth using a power supply slightly bigger than 1.1A because:

  • you can get a more stable voltage by not stressing the power supply to its limits
  • sizes like 1.5A may be more common than 1.1A, 1.2A etc; and 1.0A is too low

However, given the existence of the 1.1A polyfuse, I can't figure out the purpose of a 2A supply. I think I've even seen 3A supplies marketed. Unless I'm really missing something, there's no way to draw that much current directly through the Pi (not counting splitting the cable for some custom setup), and I have a hard time believing that you get any added "more stable voltage" benefits by moving up to 2.0A from 1.5A. So, are the companies selling these bigger supplies just wrong? Or is my understanding wrong?

You can see my email exchange with ModMyPi support about this in this thread on Reddit (scroll to the bottom; note that it doesn't look like they were really aware of the 1.1A fuse). I also started a new thread there and on the official raspberrypi.org forums, but this is my first Stackexchange post so it won't let me post more than two links.

  • Rated for more amps means it's more likely to produce a stable voltage regardless of how much the Pi draws. A PSU rated at 1.5 amps might give out 5V when the Pi draws 700mA, but only a marginal 4.7 when the Pi draws 1050mA. All other things being equal a higher amperage is likely to produce a more stable voltage. – joan Apr 23 '14 at 13:55
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    @joan that is rubbish. Most of the "higher" rated PSU are in fact "chargers" - designed to rapidly charge the newer generation of phones, and many comply with the latest USB specification, which lets voltage drop to 3.6v on load. – Milliways Apr 23 '14 at 23:43
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**WARNING: Boring electronics theory, hardly any pictures or diagrams, just black text...

Amps do not make voltages more stable, Amps have nothing to do with that! What makes voltages stable is the quality of components and design of the power adapter.

Cheap 1A power supplies usually drop to 800ma when they are loaded because, they are cheap! Cheap (or non existent) filter capacitors, simple voltage regulator and rectifier bridge used, usually burn out after 6~12 months. But that still doesn't answer why they supply 2 or 3 amp power supplies!?

Even a 0.5 Amp will work (like plugged into TV USB for XBMC) but as long as you do not plug in any other USB devices, like WiFi it will generally not use above 500ma, but its a bad idea of course.

A good power supply is a switched (switching) power supply, like the ones used in computers, but obviously we do not need 450watts! But you can get 25 watt (5v * 5A) and they cost 15USD (cheapbay) but they supply 5V at 0% load or 5V at 99% load, and they can provide up to 25 watts of continuous power, on cables up to 2 metres! Just because they are built better and have better electronics to filter (stabilise) voltages!

As demonstrated in the graph below, the only reason that increases the load on the power supply (Amps) reduces the voltage is because unregulated power supplies (cheap) ones inherently have this flaw to them. So in the graph you see unregulated the voltage drops because the design is cheap, but really, in proper power supplies the load has no influence on voltage. Other reasons might be because they get better deals from the supplier, or you get a USB hub, which you power of the 3A, allowing you to connect a USB HDD, WiFI, USB TUner and the Pi to make a mean PVR. So the Pi will never user more than 1A and the 1.1A polyfuse will be happy!

OK this graph shows the difference between the cheap power supplies you talk about and regulated power supplies. (Yay, a graph!) (The actual curve is more parabolic than linear, where the voltage starts to drastically fall at about 50%~70% load, but it illustrates the problem well)

enter image description here

Power supplies can get really complicated as some devices need extremely clean power to operate properly, like high end oscilloscopes. Personal Computers needs a decent power supply in various voltages too. Really, if you want Pi to work properly you should use a regulated power supply because allot of problems, like poor WiFi is caused by crap power supplies and cheap cables and might as well get at least 5A, so you can power anything else from it. You need to remember that the Pi actually has 3 more power regulators built into it.

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    Sorry for the delayed reply (I thought I had email notifications set up, but apparently not). I understand that a higher-amperage rated supply doesn't guarantee a more stable voltage, if it's still a crappy supply to begin with. However, I wonder if you can still make the general statement that a 2-3A supply is more likely (but not guaranteed, as you said) to provide closer to the full 5V when drawing 1.1A, compared to a 1.0-1.5A supply. There's a great blog post here that shows actual power curves for chargers: righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html – user14384 May 7 '14 at 14:34
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    No. Cheap power supplies, the more the Amps does not mean at all the voltage drop will be less. It is more likley fo the reverse!! the higher the amps the more likely a bigger voltage drop will be- because providing more AMPS requires better circuits, and with cheapo's. It is worth buying a switching power supply or like USB chargers marked with FAST CHARGE, like Apple, Belkin, etc. Because this will save you allot of head scratching. – Piotr Kula May 7 '14 at 14:39
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1.1A polyfuse means "you may draw approximately(!) 1.1A continuously and do not trip this polyfuse". however it does not say anything about the current spikes, and I can tell you, 1.1A polyfuse can safely handle up to 2.0A currents for a short period of time.

So, YES, there's a way to draw more than 1.1A from power supply, and raspberry Pi regularly (during the startup mostly) does use that much, that's why your power supply should be able to provide at least 2.0A or more, especially if you have anything plugged into the USB ports.

  • Interesting - I see "roughly 700mA" quoted all the time as typical Model B power consumption, but I've never actually seen any measurements of spikes in startup power. Did you take these readings yourself, or do you have a link? I'd be curious to read more about it. – user14384 May 7 '14 at 14:38
  • I think lenik meant startup with other peripherals attached, which would create big spikes in power consumption if you turn the switch on. But only briefly, like in less than a second. – Piotr Kula May 7 '14 at 15:06
  • @ppumkin no peripherals, just the raspberry pi. mine has at least one large capacitor next to a power regulator, and I suppose it would easily take a few amps (for a short period of time) to charge it to the nominal voltage. besides that, I've noticed that CPU consumes a lot during the startup, not sure why, maybe some kind of memory check/recharge access pattern. – lenik May 8 '14 at 2:44
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    Thats interesting :) I never measured that before. It would be interesting to hook up the Pi to a good enough oscilloscope and do a side by side video, where the amps are plotted while the Pi is booting. – Piotr Kula May 8 '14 at 8:09

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