Mwyann
Posts: 19
Joined: Thu Aug 02, 2012 12:22 am

Yet another PSU thread...

Mon Aug 06, 2012 1:22 am

Hello,

I'm trying to figure out these PSU problems and I'm really lost here. I tried a bunch of chargers, I know they're not supposed to keep 5V under the load but well, it's supposed to keep it under 1A at least (so reads the label). These are chargers from HTC/Samsung smartphones so I could say it's not china crap, it's more like china not that crap ^^ Never the less, I always get below 4.7-4.8V (between TP1 and TP2) with Ethernet plugged in. It gets worse obviously when I plug some USB devices in or when I run some app/h264/3D game.

Now I took some D-Link Router PSU, rated 5V 2.5A. Should be all good? Well, no. It also "fails" and drops to 4.6V when playing a 1080p video thru Ethernet.

Last but not least: the ATX PSU! This time it's a good switching one so it should have no problem at all. Guess what! Also failing. I couldn't even been able to use Ethernet with it!

What's left... I tried to measure current but the multimeter drops too much power and the RPi fails to boot properly. I used thick enough cables I think, thicker than those found in cheap china PSUs.

And finally: I'd say that, apart from the ATX PSU, every other smaller ones did the trick, they're not totally failing, apart from the voltage drop, for example I was able to plug some USB devices in (keyboard+mouse, Wifi adapter, flash disk...), use Ethernet and HDMI, and run apps. But I think that it's not giving all its power, and perhaps I can get some other problems just because of this PSU problem (for instance: sound is quite sluttered, it's horrible, you can't hear a song with that it's devastating it - in my sense). How hard is it to find a GOOD PSU which can firmly HOLD those freakin 5V all the load long? Did you find one that proves to work good?

Thanks in advance.

twocvbloke
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Location: Co. Durham, UK
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Re: Yet another PSU thread...

Mon Aug 06, 2012 3:37 am

With a little logic, the process of elimination, seeing as you have tried three different types of power supply, would lead to it not being a power supply fault, but possibly something on the board itself, reading through some of the threads here in the Troubleshooting section, there have been power supply issues with the TP1 & 2 voltage being low and people having issues with reliability, so in my opinion, it sounds like it could be a dodgy Pi batch that has made it to market... :|

Though never having seen a Pi myself, I couldn't be certain, just my 2p's worth... :)

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reiuyi
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Re: Yet another PSU thread...

Mon Aug 06, 2012 2:28 pm

Well all I can say is that there's a reason many power supplies from USB hubs were made to deliver 5.2v instead of 5v stabilized (diodes dropping voltages, higher current dropping voltages). Please remember that plus/minus 5% will not affect performance. A reading of 4.7-4.8V won't affect stability at all. After all, that's what the usb specifications say.

Now I must ask you (and anyone else who is doing exactly the same) what multimeter you're using, because the 10 quid meters from your local market will have like plus/minus 1 volt accuracy XD. Cheaper multimeters and voltmeters are truly not meant to diagnose such precise issues with. Measuring the difference between 4.6 and 4.7 volts is very difficult. You can forget what I typed if you have a decent one.

The voltage drop is probably one of the most common raspberry pi issues at this moment. It's not really a fault in the circuit board design, but it's certainly an "oops" moment when you think about it. In modern times you really cannot trust what the tin says any more on consumer devices. When I see 5v 1A on an adapter, the most it'll deliver at 5v will be like 400-500mA if it's an expensive one, 100-200mA if it's a China or unknown brand one. Mike's Electrical Stuff made a brilliant review on this issue: http://www.youtube.com/watch?v=T88ej64aXUM .

About the ATX power supply.. yeah those don't really function properly if the 12v and the 5v rail has no load on it. I used an old salvaged ATX as a bench power supply for many years and I know exactly what they need to function, they're really terrible and unreliable. Please don't ever use ATX to power sensitive devices apart from an actual computer.

All I can advice is that you buy a PSU locally. I know it's outrageous to pay 10 quid for a blackberry or kindle plug, but if it doesn't work you can at least return it and get your cash back. In the end you will always be cheaper buying these things locally.

Mwyann
Posts: 19
Joined: Thu Aug 02, 2012 12:22 am

Re: Yet another PSU thread...

Mon Aug 06, 2012 2:35 pm

I don't think it's a faulty Pi, as it's still working good with low voltages, I'd rather say that I've got quite a good unit there. But, although it's working, I don't think it can be as efficient as it would with decent power. That's why, before testing the capabilities of the Pi, I'd like to be sure it's properly powered. And it's still not.

For example: I could do more with my Samsung charger (1A) than with my D-Link PSU (2.5A). Plugging Ethernet+Wifi dongle+flash card and running Xorg would fail on the D-Link, but with the Samsung charger it worked and I could even launch a movie (well it didn't last long, and finally I had to reboot for Ethernet to work again). But at least it went further. It should be the other way around...

So I know I shouldn't complain as I got a working unit and there's people out there who cannot even boot theirs, but at least I try ;)

Mwyann
Posts: 19
Joined: Thu Aug 02, 2012 12:22 am

Re: Yet another PSU thread...

Mon Aug 06, 2012 2:49 pm

Thanks reiuyi for your answer.

That's right, the D-Link adapter shows 5.20V with no load. But it drops quite fast, and this is disturbing to me... How would it even start off the router with such a drop? Mistery ;)

4.7-4.8V is the global reading with nothing plugged in apart from power. When I plug Ethernet I often lose a bit so I never get more than the 4.75V limit with simple network, and it keeps dropping of course when I run some load on the CPU or some other processing, or if I plug some USB device. I've seen my Pi run with 4.4V, and I don't think it appreciates that...

I'm using a decent multimeter, and I've got another one to confirm: both agreed. So I think readings are OK.

Apart from that, I'm currently using 5V from a running computer to run a wifi router, as its PSU died. It did the trick and it's still working. And a switching power unit should maintain 5V with any load it can support (but perhaps it doesn't works when load is too low...), that's why I tried.

I'm not outraged to pay the price for a good PSU, I totally agree with that, but you'd have to test them, and if it's not goot you can't tell the vendor hat it's not working: he will plug it in and see it's working. You can't tell him that your Pi works but doesn't get 5V under load, he will not (try to) understand and tell you that it charges phones and that's all, no refund.

Do someone have a known working PSU out there? That's the final question :)

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reiuyi
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Re: Yet another PSU thread...

Mon Aug 06, 2012 4:59 pm

The NCP1117 3.3v linear regulator has a quite high drop-out voltage of 1.2v. Meaning the absolute minimum input needs to be 3.3v + 1.2v before serious issues of rail-regulation start appearing. So it's not uncommon to see raspberry pi running stable at just 4.5v. I've seen mine boot even with like 4.0v input voltage, though the instability is terrible! Perhaps the D-link adapter you have at hand comes from a device that has lower requirements (say for example.. the router works fine even at 3.8v, due to a lower drop-out voltage for its internal 3v3 rail).

So I did a quick skim through some datasheets and there are 3v3 regulators such as NCP59150 with just 300mV dropout! It's very likely this thing is more expensive than the NCP1117 on the raspberry pi (I'm convinced it's more expensive and needs more external parts), but with such an ultra low dropout, the raspi would boot even at 3.6v input. But anyway, the regulator on the raspberry pi is fairly typical and using a good psu is just basic 101.

Switching mode psus (smps) don't need to maintain a stable voltage across their entire range. Such a feedback system is desired, but it's unnecessary and expensive. I now have a proper 70A (yeah seventy amps at a max of 15v) bench supply that does drop almost half a volt when you pump it to the max. Voltage drops are unavoidable, appliances are built to deal with them (raspi input: 4.75 - 5.25v)

As said by Eben himself; the Kindle Fire and any Blackberry charger are known adapters to work. Other users have said a genuine iPad charger (the 2a one) works good as well in combination with a good micro usb cable.

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computerpie
Posts: 57
Joined: Wed Jun 20, 2012 10:55 pm
Location: USA

Re: Yet another PSU thread...

Mon Aug 06, 2012 5:08 pm

Keep in mind these PSU's are really meant to be used as battery chargers for phones and tablets. A battery looks like a huge capacative load to the psu so it can have a very sloppy (ie noise and ripple)output and function (as a charger) very well.

I tried a few chargers to power my raspi and found out quickly just because it said it could supply 1 A or more didn't mean is would work. I tried one of those dlink psu's 5v 2.5A that you tried and the raspi brings it to its knees quickly. The best one I found was a samsung 5v 700mA psu. I run headless and don't have a kb and mouse connected.

I tested a few psu's with a 5 Ohm 5 W resistor and found they hold their output voltage with a 1A load on them. Even the ones that the raspi would load down. I actually stressed a couple of 700mA psu's with the resistor and the outputs of those stayed within tolerance.

The best thing to do is use the approved list here> http://elinux.org/RPi_VerifiedPeripherals and pick what you need. If you plan on connecting other usb devices to your pi then I'd recommend buying a powered hub and not worry about a psu at all. If you do get a psu make sure it has a name brand on it like Samsung or HTC. People seems to have the best luck with them.
Pi1>OS: Rasbian Pi2>OS:Xbian .63
1:HDMI>>DVI>>Flat Screen Monitor
2: HDMI>>Toshiba 37" LCD
1:ASUS Wired USB Keyboard and Mouse
2:Logitech wireless mouse, CEC remote
SanDisk SDHC C4 8GB SD

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