Strange screen artifact

It's a low-power warning. It shows up when the 5V supply goes below 4.65 if I remember correctly.

http://www.raspberrypi.org/forums/viewtopic.php?f=29&t=82373

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Ragnar

Thank you very much Ragnar!!!!!

That's exactly what was happening.

I've had issues many years ago with crappy USB leads, so I started buying one at a time from different ebay vendors until I found one that had cables that could carry 2,000 mA and then I bought a dozen or more from him, tested them each and gave most of them to my friends who were complaining about slow charging times.

(I also bought a bunch for myself)

So, when I saw your message, I collected a few of my 5V, 2,000mA power packs and tested them (as well as 2 different cables) with the Pi2.

Voila!!!

One substandard cable in the bin later and the Pi is happy.

Thanks again,

Nick

I have new information to add on the cable front.

I purchased 1m 'Charging and Data' cables before my previous test. The new cables were wimpier than the old ones despite the illustration suggesting otherwise, but actually worked somewhat better. After the test, I purchased 0.5m cables and 0.25m cables all sold as 'Charging and Data' cables.

I have now tested their loop resistance. The values are at the limit of my tester and so are somewhat of a guide.

Old 1m cable    0.6Ω

New 1m cable   0.6Ω

0.5m cable        0.2-0.3Ω over a number of measurements

0.25m cable      0.2Ω

The wire gauge of the old cable was not shown, but all three of the new cables were marked 28AWG. I suspect the cable came from the same cable factory even if the assembly factories were different.

The quality of my short-circuit was not great (a screw-driver gently inserted into the micro USB plug). I know that this could easily have added up to 0.05Ω if not more and so the accuracy of the results drops quickly with the length of the cable. All the measurements were taken several times and only the 0.5m cable showed any variation.

The conclusion is that a 1m USB power cable is likely to be on the verge of causing the rainbow square voltage warning when wireless and wifi USB dongles are used. Use a powered hub if using a cable of 1m or above, although you might just get away with a good 1m cable with some devices.

I am using a 0.5m cable without any signs of problems and without a powered hub. You might well be able to use all four USB ports as long as power hungry devices are not involved. A 0.25m cable does not give a significant advantage over a 0.5m cable, but might still be useful in some circumstances as when running off a mobile device charging battery. As the battery pack discharges and the output voltage drops, the shorter cable might give you a useful extension in operational time.

If you look carefully at USB cables you will find that they are marked with the gauge of wire they use. Although they vary slightly it is usually in the form of something like: 28AWGx1P+26AWGx2C.

This means they are using 28 AWG wire for the data connections ( 1 pair) and 26 AWG wire for the power pins. One thing I have noticed, and it seems to be fairly consistent is that longer cables (2 m)

tend to use smaller gauge wire (30AWGx1P+28AWGx2C is typical) than shorter (0.5 m) ones. This means that shorter cables have two obvious advantages when they are used for providing power, they

use larger wire which can handle more current, and they have less resistance because they are shorter.

I have found in my investigation and as published in my post that most cables don’t even bother to mention what the wire gauge is, these ones tend to be the under rated ones

I found the best way to get a good cable is to use a USB “A” type connector and replace the end with the micro connector of choice, these older cables have proven so far to consistently have better gauge wiring in them

A point of interest is the typical resistance and current rating of the popular gauges

This is from the American Wire Gauge Standards + some math of my own

1Amp Load

Current

1ft USB Cable

1M USB Cable

6ft USB Cable

AWG

Ohm/M

Ohm/Ft

Chassis

Transmission

V Drop

LOAD Volts

V Drop

LOAD Volts

V Drop

LOAD Volts

22

0.05296

0.01614

7

0.92

0.03228

4.96772

0.10592

4.89408

0.19368

4.80632

24

0.08422

0.02567

3.5

0.577

0.05134

4.94866

0.16844

4.83156

0.30804

4.69196

26

0.1339

0.04081

2.2

0.361

0.08162

4.91838

0.2678

4.7322

0.48972

4.51028

28

0.2129

0.0649

1.4

0.226

0.1298

4.8702

0.4258

4.5742

0.7788

4.2212

32

0.5383

0.1641

0.53

0.091

0.3282

4.6718

1.0766

3.9234

1.9692

3.0308

For a 2A Load there are no 6 foot cables that are suitable

Current

1ft USB Cable

1M USB Cable

6ft USB Cable

AWG

Ohm/M

Ohm/Ft

Chassis

Transmission

V Drop

LOAD Volts

V Drop

LOAD Volts

V Drop

LOAD Volts

22

0.05296

0.01614

7

0.92

0.06456

4.93544

0.21184

4.78816

0.38736

4.61264

24

0.08422

0.02567

3.5

0.577

0.10268

4.89732

0.33688

4.66312

0.61608

4.38392

26

0.1339

0.04081

2.2

0.361

0.16324

4.83676

0.5356

4.4644

0.97944

4.02056

28

0.2129

0.0649

1.4

0.226

0.2596

4.7404

0.8516

4.1484

1.5576

3.4424

32

0.5383

0.1641

0.53

0.091

0.6564

4.3436

2.1532

2.8468

3.9384

1.0616

And this assumes the cable is truly as advertised and of good quality, some I have inspected have shown severe corrosion on the wires inside, sometimes not tined or thinner that advertised

One other point to note too is that if your run the wire close or above its rating it WILL become warm and its resistance WILL increase further

ONE REALLY INTERESTING POINT IS THAT NON OF THESE ARE RATED ABOVE AN AMP FOR TRANSMISSION LINE WHICH IS WHAT A USB CABLE IS.

Hope this helps clarify the situation for everyone

Btw, the math is correct, if our thinking the volts drop is too much, you probably forgot that there are two wires involved, one out and one back so the drop is doubled for the USB values.

Twenty eight USB cables I have are marked. There were two clear insulation/shielded cables which were supplied with USB peripherals I bought that are not

marked. I believe in the US at least that the wire gauge and voltage ratings (this is on all of the marked cables also) must be on the cables for regulatory approval.

All of the cables I purchased were bought on the Internet from reputable companies in the US although most (all?) were made in China.

Only thing I could say here Peter is any one who had the parts and to some extent the skills to solder that horrid micro USB connector  probably wont be  asking the question.

You are right though that a bit of decent copper sorts out many problems on the RPI especially the older ones!

While that is true, what I have done on occasion it cut the last inch from the crappy cable (Including the connector) and soldering it to the end of the good cable replacing the big connector. (I did not have any wire able connectors to use)

1” of crappy wire on 6’ of excellent wire is way better than 6’ of crappy wire.

I scanned many of the cables on the E14 sire and many of the ones there do not advertise their AWG so there is no way of knowing before you buy. This does not mean their crappy, just you have no way of knowing

Peter

I was figuring that was what you were intending since wireable  micro USBs are just about as rare as unicorns for general high street use. Even so you need to do a reasonable job not to end up with 3 crap bits of wire

True but I am able to do that (Most of the time )

John Alexander wrote:

 

Only thing I could say here Peter is any one who had the parts and to some extent the skills to solder that horrid micro USB connector  probably wont be  asking the question.

Adafruit has a nice DIY Micro USB plug: http://www.adafruit.com/product/1390

I haven't tried it yet, but it looks a lot easier to solder than normal Micro USB plugs.

johnbeetem

John

Nice find, I'll have to add some to my next order.

Mark

johnbeetem

John

Nice find, I'll have to add some to my next order.

Mark