Tinue
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Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 10:47 am

I've got a 12W / 2.4A power supply (Apple iPad charger), and yet I see the occasional rainbow icon on the console. I can't remember seeing the rainbow icon on any other Raspberry Pi model, so I wonder if anything has changed in the undervolt detection with the Pi Model 2.

The Pi is not overclocked; It runs the latest, unmodified Raspbian. On the USB ports I have a WLAN adapter, and a Logitech unifying dongle. Ethernet is not connected. The SD card is 64GB.

I have seen the icon multiple times when building bitcoind, and I have seen it just now when doing "raspi-update". So possibly it has something to do with SD card write.

P.S. After having done "raspi-update" my Raspbian is no longer "unmodified", of course, but the icon showed before rebooting.

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pluggy
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 10:50 am

Power lead.

Cheap with conductors made of damp thread ?

The the power supply a real Apple one or one of the millions of fakes ?
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Tinue
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 11:35 am

The power supply is real, so I trust that one. The cable looks flimsy like all Micro-B USB cables. I once cut one open, and discovered really thin cables inside. But I assume these cables are all like this, and seem to drive other Raspberry Pi units just fine.
But I guess it's worth a try. I'll go find another cable ;)

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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 11:48 am

The early B and A models didn't have the rainbow low volt warning, it only came in with the B+. The Pi 2 actually uses less power than the original model B, the multi core processor in the 2 uses less power than was wasted in the linear voltage regulators in the early models.
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rurwin
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 11:55 am

The problem with an Apple charger is that it never comes with a micro-USB lead. HTC or Samsung leads that come with their chargers are good. But all leads drop the voltage significantly, so you should find one as short as possible too.

There's an interesting wrinkle in the USB spec. If the power-supply has a built-in lead it is required to produce 5V at the end of the lead. If it just has a USB socket, it is required to produce 5V at the socket, which will then be reduced by the lead.

Some companies (adafruit springs to mind) produce 5.2V power supplies which, while still within spec, give a bit of headroom for voltage drop in the lead.

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pluggy
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 12:02 pm

The better leads specify what gauge wire they use for the power.

http://www.newegg.com/Product/Product.a ... 6812189234

22 AWG or 24 AWG should be OK unless its over long. Its less crucial with shorter leads.
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Mennims
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 1:43 pm

It might be your cable, my Samsung S4 is only charging at 460ma sometimes because of the faulty cable, wear and tear I guess. I replaced the cable and it's fine. I've also noticed with Samsung chargers it only charges non Samsung devices at 500ma (same as most USB's) and the same may apply with Apple, so you might only be getting 500ma.

Mark_T
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 2:27 pm

You should find a USB lead designed for power only and not data - typically
a power lead will be twinflex, not circular in cross section.

rahlquist
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 2:29 pm

20awg which is the largest size in the usb 2.0 spec ought to handle it.

I am ordering one of these now. Will post test results when I get it.
http://www.amazon.com/gp/product/B00KE3 ... K9LXDRB6PN

TimD
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 4:03 pm

The 12W Apple iPad charger outputs 5.2V to allow for cable drop.

Tinue
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Re: Undervolt on Pi 2, with 2A supply, why?

Sat Feb 07, 2015 5:06 pm

Thanks for the many replies! After reading up a bit it seems clear that the lead is the problem. It is 1.8 Meters long, and not designed for charging. It's still powering my Pi, so I can't measure its resistance at the moment. But even if it is just 0.5 Ohms per line, and even if my Pi pulls just 0.5 A, I'd see a drop of 0.5 Volts.

So I learned that using a reasonable power supply is not enough, one also needs a good lead.

c0lts
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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 9:59 am

I have been looking at the same issue, testing my micro USB leads for the under voltage sign. Not surprisingly (in general) the OEM leads supplied with phone etc come out better than the cheapo leads. On Saturday I bought two leads from Sainsburys both £2.99, the 'Blue Chip' one caused problems with corruption and the rainbow icon, whereas the Sainsburys own brand 1m white lead was fine. Just checked the RPi Verified Peripherals list but these leads are not covered.

EtonMess
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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 3:16 pm

I'm getting good results from leads which came from a popular high street discount store that is named after a unit of UK currency, and the leads cost the same.
.G6FIR

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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 3:22 pm

EtonMess wrote:I'm getting good results from leads which came from a popular high street discount store that is named after a unit of UK currency, and the leads cost the same.
99p Shop?

http://newsthump.com/2015/02/06/99p-sto ... ces-by-1p/
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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 3:29 pm

The fact that they are only a foot long helps..... ;)
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Tinue
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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 7:28 pm

pluggy wrote:The fact that they are only a foot long helps..... ;)
I thought that the UK is on metric standards? Or was that Canada? :D

In Switzerland it's hard to find a lead with a clearly specified diameter or quality. Finally I found one called "DigiPower", which is at least advertised to be good for high current charging. But the thing costs 12 CHF, which is about the same in Euro.

P.S. Why is this thing called a "lead"? It's made of copper, right?

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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 7:33 pm

EtonMess wrote:I'm getting good results from leads which came from a popular high street discount store that is named after a unit of UK currency, and the leads cost the same.
Poundland are good for some stuff but don't bother with their cheap cloned BlueTooth dongles.
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rahlquist
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Re: Undervolt on Pi 2, with 2A supply, why?

Mon Feb 09, 2015 9:55 pm

Got my 20AWG 8ft long cable today ( http://www.amazon.com/gp/product/B00KE3 ... K9LXDRB6PN ) and have been testing it a bit. With loads up to 1A the voltage drop @5.28 in is .10-.13v. Seems that this might just be a good reliable cable. Will have to see how it holds up over time.

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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 12:41 pm

Tinue wrote:
pluggy wrote:The fact that they are only a foot long helps..... ;)
I thought that the UK is on metric standards?
The foot is a common term for the light-nanosecond, so is a metric measurement ;)

henryhanselscott
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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 3:04 pm

It seems that wimpy micro USB cables are the norm. Building your own cable might be problematic. Probably been asked before, but if you properly fuse a 5VDC source into the GPIO could you not skip running power through the micro USB?

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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 3:36 pm

henryhanselscott wrote:It seems that wimpy micro USB cables are the norm. Building your own cable might be problematic. Probably been asked before, but if you properly fuse a 5VDC source into the GPIO could you not skip running power through the micro USB?

as long as you can solder then building your own cable is easy.

https://bmsdoug.files.wordpress.com/201 ... 242178.jpg

the next one I do I will take photo's before covering the ends with hot-glue.
Last edited by BMS Doug on Tue Feb 10, 2015 4:37 pm, edited 1 time in total.
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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 4:30 pm

Is it really necessary to post a 5MB image 2976 x 5312 resolution for a snap shot of a cable?
Storage and bandwidth is cheap I guess but this plays havoc with my browser.

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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 4:37 pm

Heater wrote:Is it really necessary to post a 5MB image 2976 x 5312 resolution for a snap shot of a cable?
Storage and bandwidth is cheap I guess but this plays havoc with my browser.
Sorry. dropped it to a url.
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Re: Undervolt on Pi 2, with 2A supply, why?

Tue Feb 10, 2015 6:56 pm

Burngate wrote: The foot is a common term for the light-nanosecond, so is a metric measurement ;)
I'll never look at 29.98 centimetres in the same way again.
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Re: Undervolt on Pi 2, with 2A supply, why?

Wed Feb 11, 2015 12:53 am

I've tried to solder a micro USB surface mount socket onto wires ... and failed. That is not the way to go.

My power supply tester uses a USB to micro USB converter. There won't be much loss in an inch, no matter how thin the tracks are.

And in case anyone is wondering, it's very close, about 1.5% out.
units wrote:[email protected]:~$ units
Currency exchange rates from 2013-07-11
2564 units, 85 prefixes, 66 nonlinear units

You have: foot
You want: light nanosecond
* 1.0167034
/ 0.98357106
You have: foot
You want: mm
* 304.8
/ 0.0032808399
You have: light nanosecond
You want: mm
* 299.79246
/ 0.003335641
You have:

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