Raspberry 3B+ undervoltage?

I found little correlation between cable length and cable resistance when I measured lots of them. Indeed, two of the three best ones I had were 1m cables, and most of the rest were shorter.

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Andrew Gabriel 
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Reply to
Andrew Gabriel
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Well it will be a function of conductor size. And whilst some thick cables will be all insulation and no conductor, on average thicker will have more conductor and hence less resistance and voltage drop.

I have many cables and in my statistical valid sample size of about 25, I have one thin, very flexible cable that works well and many thin cables that are not good. The thin one that works is branded Nokia and came with a Nokia E71. The poor ones show consistently poorer transfer rates and slower charge times when used with phones etc.

I have a few flat, rectangular cables that are good and just recently bought stripey cotton covered cables (looks like miniature iron cable) that are very good. Performance and feel and fit of the connectors is

cables are more durable and work better including several 2m long ones.

So, a rule of thumb, thin cables are in general worse for high current use but it depends on who made it and from what.

Reply to
mm0fmf

I purchased my RPi2 from Vilros and have been using their included power supply with no issues for about 4 years now. I was actually going to suggest that you check to see if you could get a PSU from them, or from Canakit.

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 * SLMR 2.1a
Reply to
Mike Powell

I am sure the electricity supply companies will be delighted to have this evidence for the repealing of Ohms Law.

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Reply to
The Natural Philosopher

TNP,

I'm sure they will be delighted to have a stooge which just off-handedly discards empyrical data because he hasn't got the slightest clue about cables, and thus how Ohms law applies to them.

Kiddo, not every cable has got the same size conductors in them, and not all conductors have got the same resistance-per-meter.

Regards, Rudy Wieser

Reply to
R.Wieser

And also, it's important to measure the voltage *under load*. Not only because of contact/cable resistance, but also because of supply regulation. A well-regulated supply should maintain the supply voltage between suitable limits for a wide variation of current being drawn. I'd expect that a 2.5A supply (as stated on the "plate") should be able to maintain the supply within the normal tolerance that USB specifies for all currents up to that value.

Obviously there will be greater voltage drop for greater current (due to cable resistance), even with a perfectly regulated supply, but I think some cheaper supplies can't keep it up (!) even at the PSU end of the cable as more current is drawn.

I'm always amazed at how small switched-mode power supplies are these days: the ones that Amazon supply for the Kindle fit the mains transformer, the diodes and the PSU control circuitry into the size of a normal-pin plug.

Reply to
NY

Dont get your knickers in a twist dear boy.

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Reply to
The Natural Philosopher

Dont be.

Understand that a transformer is, for the purposes of power transfer, able to store so much energy in its magnetic feild per cycle of the frequency it is operated at.

A 5W transformer at 50Hz needs to be 1000 times the volume (or ten times thh (liner) size) of one operated at 50KHz...and as power FETs get better and better at HF switching so the frequency goes up.

Because the ferrite transformer and its windings are a significant part of the cost.

As are the ferrite cores needed to screen he HF rubbish out of the mains.

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Religion is regarded by the common people as true, by the wise as  
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Reply to
The Natural Philosopher

TNP,

Ah, trolling I see (not giving a reasoning for your lashing out, nor explaining yourself when pointed to your "mistake"). A disgrace to the group you claim to be a "natural" member of than.

Regards, Rudy Wieser

Reply to
R.Wieser

I presume the switching is done at mains voltage (to avoid the need for a step-down transformer at 50 Hz) and then the 50 kHz switched, regulated voltage is then stepped down to 5 V where it is smoothed.

When I made a SMPSU as a university project, we did the stepping down first, so we had an enormous transformer. This was mainly because the regulator IC that we used needed low voltage (probably TTL 5V). If you can drive the regulator circuit from rectified and crudely smoothed mains, it avoid the needs for such a big transformer.

Reply to
NY

The regulation is usually applied by measuring the 5V output to derive the feedback to the switcher which does indeed run at rectified mains voltage. High voltage switching transistors (FETs) are far smaller and cheaper than low frequency transformers and efficiencies can run well into the 90s mainly due to storing far less energy in the PSU which means there's less to lose.

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Steve O'Hara-Smith                          |   Directable Mirror Arrays 
C:\>WIN                                     | A better way to focus the sun 
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Reply to
Ahem A Rivet's Shot

Ah, trolling I see

No sense of humour and ignorant of the English language too. Must be a millenial..

Google 'natural philosophy' and educate yourself.

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"Nature does not give up the winter because people dislike the cold." 

? Confucius
Reply to
The Natural Philosopher

Sort of.

Yep. Thats what is done.

You rectify and smooth mains - to get about 400V DC here in Europe, probably about 200V DC in the USA and other 110v regions, chop that at around 50Khz+.

If its an unregulated wall wart chances are that is all you do - typically centre tapped primary with each end fed by a high speed high

The the turns ratio dictates what comes out the secondary end and that is rectified - usually high speed shottky diodes - and smoothed, although at 50KHz+ you dont need much of a capacitor.

You can take feed back from the secondary final DC and use that to drive the pulse *width* timing that each FET is on, and get regulation that way, but that introduces a problem of isolation between mains voltage and the output. Optical isolators are probably the cheapest way to achieve that.

There are probably chips that just need some power FETs, diodes and capacitors plus some RF filtering to do all of that, in mass production.

Back in the day I worked on kit that had a PSU capable of running off anything from 40V DC (a heavily loaded 48V aircraft battery) to 240VAC (typical UK mains voltage) without needing to be set up in any special way.

I think the first half dozen were destroyed in a nuclear weapons test:-)

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Reply to
The Natural Philosopher

Mm. The main sources of loss in such a beast are

(1) the diode losses in the HT rectifiers. A good power diode will lose about 0.6-1V at say 400V (European voltages) so with a bridge that is one in 200 - say 0.5% there.

(2) ON resistance in the power FETS. this is again pretty low. Similar to the diodes. So lets add anpother 0.5%.

(3) Switching losses in the power FETS. This is a very variable thing depndoing not jost on the device used - the actual one, just not what is on the can - and how fast it us chopping. The actual losses are high here, because at the time when its not got either high current but low (

Reply to
The Natural Philosopher

NTP,

Yes, I already said that.

Riiiight. It was *humor* when you responded that way to Andrew Gabriel.

Ofcourse. That must be it.

So, what joke(s) *did* you try to make either Andrew, or to me with that underwear quip. I'm fully deferring to your mastery in the language ofcourse. Must be some "upper class" kind of thing though. Never understood those gouv'nrs.

Yep, that must be it. I'm sure of it. I mean, as you said it how can it /not/ be true ? :-)

Regards, Rudy Wieser

Reply to
R.Wieser

did you check voltage between the 5V and GND pins on the IO connector? I suggest you measure it there.

Reply to
Big Bad Bob

1A at 0.2 ohms is 0.2V, meaning it drops to 4.8V. Get a better cable.
Reply to
Big Bad Bob

Kenny,

The phrase is. NTPs translation of it ? Not so much: "getting emotional" has a distinctive different meaning than "getting worked up" (about something).

Rather possible, especially with people who's mothertongue isn't English. I didn't have a problem with it though.

Regards, Rudy Wieser

Reply to
R.Wieser

Not heard from the O/P for a while, and wondered if they'd actually got a proper voltmeter and checked the supply at the Pi itself.

Also, overvoltage off-load that high - although not right - is no real indicator of what it would be on-load.

--
W J G
Reply to
Folderol

That'll be where the voltage is being lost. Such useless cheap USB current monitoring devices almost universally drop a ludicrously large voltage in their current measuring circuitry. They are useful only for checking the voltage of an otherwise unused USB port or for checking no significant current is being drawn by some USB device.

--

Brian Gregory (in England).
Reply to
Brian Gregory

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