Any point to a 2A power supply, given the 1.1A fuse?

[bfinio]

I wasn't sure if it was better to post this here or in "Recommended Peripherals", so please let me know if this should be moved. I figured this might be a little more advanced than typical questions there.

My question: it's my understanding, based on the official Raspberry Pi FAQ, the wiki, and numerous forum threads like this one, is 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.

You can see my email exchange with ModMyPi support about this in this thread on Reddit. I also started a new thread there but figured I would get a better answer here.

[GTR2Fan]

You're correct in saying that anything much beyond 1A capacity is a waste of time for powering just the Pi and anything powered via its own USB sockets. I'd have said that 1.5A should give ample overhead so as not to be caning the PSU. It does seem to be a sad fact of life though that hardly anyone (including many major brands) is capable of manufacturing a budget regulated 5V PSU worth buying nowadays.

PS Whoever this Jean person is, they're giving you bum information regarding there currently being no polyfuse.

[ShiftPlusOne]

A 2A supply is 'better' because it is more likely to provide 5V at 1A then a 1A supply. The label only tells you the nominal voltage and maximum current, it says nothing about the voltage regulation. The pi will start acting up when the input voltage drops below about 4.7V. This is usually not a problem for phones and tablets, which such power supplies are designed for.

But yes, you won't need more than 1A, IF the label is accurate.

[MaxK1]

I use the "excess" current to power things like a USB hub (or 2) or a laptop hard drive. The local Microcenter had a good deal on some USB hubs (sold without power supply for $7, or $23 with PSU)
That didn't take lot of thinking about since I already had a couple of 5A and 3A (well) regulated supplies I picked up the PSU's at a local "junk" dealer - The 5A supply was for some piece of medical equipment.

[FTrevorGowen]

I use the "excess" current to power things like a USB hub (or 2) or a laptop hard drive. The local Microcenter had a good deal on some USB hubs (sold without power supply for $7, or $23 with PSU)
That didn't take lot of thinking about since I already had a couple of 5A and 3A (well) regulated supplies I picked up the PSU's at a local "junk" dealer - The 5A supply was for some piece of medical equipment.

That's similar to what I do. My "headless" B1 Pi is powered by a very good (ie. conforms to its spec.) 850mA PSU, my B2 Pi's use either the same or a good 1A PSU and, when I need a powered hub I use a "multi-output" 1.5A or 2A PSU. I've tried a number of "medium-cost" hubs that had a "power input" and found, where a PSU was provided, said PSU was not good enough to provide all (four) ports with enough current to power "high current" USB devices.
Trev.

[iinnovations]

I use a board that pulls off the power from the PS and data from the pi usb for this reason. Then I can use hungry peripherals and unpowered hubs on my USB ports. Works a treat, as china special unpowered hubs are $1 each. I've got a bucket of them.

Colin

[Tarcas]

I use a 2 or 2.5A (can't remember) PSU on my powered hub backpowering my Pi. But it's also powering a small handful of other peripherals and came with it, so there was no added expense. I found the 1A that came with my Pi to be sufficient until I added higher-powered peripherals (USB webcam/mic combo) that the Pi's USB just couldn't handle.

But you're right, if you're only using it to power the Pi, 1A is about all you need. Overdoing it a little (1.1, 1.2, even 1.5) would stress your PSU less, but I think that's going to be the only benefit.

[klricks]

My question: it's my understanding, based on the official Raspberry Pi FAQ, the wiki, and numerous forum threads like this one, is 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.

FYI - The 1.1 Amp shown on the schematic is the trip current. The hold current is 750mA as indicated by numbers printed on the fuse itself. If you attempt to draw more that 750mA the fuse will begin to increase in resistance and the RPi and/or USB devices will become unstable. Of course going over 1A will trip the fuse in a few seconds.
This means even an 800mA supply would be sufficient IF the labeling/ specification is correct.

[bfinio]

Oops - I thought I had email notifications set up for this thread, but never got notified of the replies for some reason. So, a belated thank you to everyone who chimed in. I got an interesting reply on stackexchange from someone pointing out that just because a power supply has a higher current rating does not guarantee that the voltage will be more stable under high load:

http://raspberrypi.stackexchange.com/qu ... 2160_15396

because a cheap supply could still suffer from voltage dropoff. However, I think it's still more likely that a 2-3A rated supply will stay closer to 5V than a 1-1.5A rated supply when drawing the max 1.1A (even if it isn't guaranteed).

[GTR2Fan]

...I think it's still more likely that a 2-3A rated supply will stay closer to 5V than a 1-1.5A rated supply when drawing the max 1.1A (even if it isn't guaranteed).

If both had the same quoted line regulation, then the PSU rated at a higher current should stay well within the quoted line regulation for the same load. The problem seems to be in finding a PSU that quotes any actual figures besides a seemingly imaginary voltage and current rating that may bear very little relationship to reality.

Unfortunately, the budget PSU market is an absolute shambles in terms of sub-standard electronics engineering.

[Tarcas]

Unfortunately, the budget PSU market is an absolute shambles in terms of sub-standard electronics engineering.

No, no, no. It does a GREAT job at sub-standard electronics engineering!


Quality control should not exist. Quality should remain uncontrolled and allowed to run rampant through all products.
Quality assurance, on the other hand, is vital.

[GTR2Fan]

No, no, no. It does a GREAT job at sub-standard electronics engineering!

Well caught!

[hampi]

Well if you want to blow the 1.1 A fuse you need a power supply that can put in a current that is clearly more than 1.1 A. I think a 2 A power supply is a good choise for that.

[evilkitty]

Your best bet is to get a reputable PSU, sadly labels mean nothing, i have had a desktop PSU that was rated for 580W that fails to deliver even below 300w without tripping a safety feature
i am sure some manufactures slap 3 and 5 amp labels on them to make them sound good and they lied
you should use a regulated adapter, most are unregulated and finding out is near impossible without testing the unit yourself
one known good unit is this one
https://www.adafruit.com/products/501
it does not include a cable, cheapest place i have found that has a quality cable is monoprice's 24 awg cable (from there web site)
using that adapter i get 5.07-5.1 v using my Ethernet cable based micro usb cable
i was able to find that adapter with less shipping on ebay with some searching

If you were to find a real 2amp psu you could benefit if you splice into the power cable it has to run additional hardware or a second pi (2 rPis on 1 adapter)

[pluggy]

The Pi will run a display keyboard and mouse with a lot less than 1 amp. Way back, a 700mA or larger was recommended for a 'B' Pi and it is plenty. BUT and its a big BUT, many power supplies won't supply what they say and a bigger one gives more leeway for 'cheap crappy gizzards' to actually get a realistic 5v and 700mA out of it. Very often a 5v 1000mA (1 amp) power supply will provide 5v and 1000mA but not at the same time. A cheap one might provide 5v at 150mA but when its supplying 1000mA the voltage has tailed off to 2v (or often worse). The label is sort of accurate in that it will do 5v and 1000mA but its entirely unsuitable for a Pi that will want probably 500mA in a typical setup but it wants it at close to 5v when its doing it. A good quality one will (or at least should) provide 5v at maximum current.

Realistically you aren't going to get much above 1000mA through a micro USB connector, there just isn't enough of it to provide serious current.

Added to that if you're using a cheap USB lead on a supply with seperate leads the voltage drop in the lead will make a mockery of whatever the power supply can do.

A standard 'B' Pi will consume around 380 mA at idle and not much more at high CPU power. It gets greedier if you use the GPU which many setups never will. Most USB peripherals consume no more than a few 10s of milliamps including many wifi adaptors (a heavy duty one being stressed will get to 100 mA maybe). Bus powered hard drives will use an amp or more. A model A Pi consumes a lot less.

[bfinio]

A cheap one might provide 5v at 150mA but when its supplying 1000mA the voltage has tailed off to 2v (or often worse).

This is a great blog post where someone actually tested a bunch of different real and counterfeit adapters and shows the power curves - it's amazing (and disconcerting) how bad the knockoff ones actually are:

http://www.righto.com/2012/10/a-dozen-u ... le-is.html

[GTR2Fan]

It ought to be sufficient to buy one that states "5V@1A" as that clearly indicates that it's able to supply 5V at 1A. I've got a few power supplies kicking around here that don't respect the usage of the "@" symbol though, so it's still no guarantee.

It's not hard to find a decent power supply if you up the budget to something more sensible. Opting for manufacturers who make 'medical grade' power supplies (Ansmann spring to mind) should be a fairly safe bet, assuming you can find one with the right connector on the end.

Expect to pay upwards of £10 to buy a PSU worth having. Anything less and you're playing the budget lottery.

[boyoh]

I wasn't sure if it was better to post this here or in "Recommended Peripherals", so please let me know if this should be moved. I figured this might be a little more advanced than typical questions there.

My question: it's my understanding, based on the official Raspberry Pi FAQ, the wiki, and numerous forum threads like this one, is 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:


Picking the right power supply is science on It's own
A one for all might seem a good Idea, but It depends
On what you are going to feed from It

High current o/p power supply's contain highly inductive
And reactive components, these can generate noise
Spikes and transients,( basically all the same )
These can cause problems with the Pi

My advice is have a 5v 1.5a good quality well filtered
Power supply just for the Pi.

Pick e power supply at the voltage and current
Of your choyse, for your peripherals.

Don't cut corners with the power supply for the Pi

• 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.

You can see my email exchange with ModMyPi support about this in this thread on Reddit. I also started a new thread there but figured I would get a better answer here.

[piglet]

Expect to pay upwards of £10 to buy a PSU worth having. Anything less and you're playing the budget lottery.

...and anything more and you may just be adding to their profit margin with absolutely no guarantee of greater safety or quality...

There was talk of an "official" psu from the foundation that we could trust. I'd love to hear more of this.

[GTR2Fan]

Expect to pay upwards of £10 to buy a PSU worth having. Anything less and you're playing the budget lottery.

...and anything more and you may just be adding to their profit margin with absolutely no guarantee of greater safety or quality...

You've quoted that out of context. See my preceding sentence. If you're not even prepared to trust a manufacturer of medical grade PSUs that people's lives may depend on then you might as well give up on the search for a good PSU.

[DougieLawson]

IOpting for manufacturers who make 'medical grade' power supplies (Ansmann spring to mind) should be a fairly safe bet, assuming you can find one with the right connector on the end.

http://www.ansmann.de/en/contact/partne ... t-britain/
The usual suspects (RS Components, Farnell & CPC) turn up on that page.

[GTR2Fan]

http://www.ansmann.de/en/contact/partne ... t-britain/
The usual suspects (RS Components, Farnell & CPC) turn up on that page.

Ta muchly.