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Advice on which linear regulator to get

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buffalow bill:
Though the raspberry pi required a 3.3 volt supply not 5 volts.

Cheers to everybody on the forum, have a merry Christmas, a happy and prosperous new year.

 :beer:  :beer:  :beer:

John Rudd:

--- Quote from: buffalow bill on December 17, 2015, 08:50:02 AM ---Though the raspberry pi required a 3.3 volt supply not 5 volts.


--- End quote ---

Nope 5v @ 2amps....hungry beast it is....

sparky961:
I did a bit more reading today on the subject and it would seem this little thing is quite picky when it comes to power.  I'm guessing good system design went out the window in favour of low cost.  That's not to say you can't still have a good system, you just need to foot the bill for a better power supply - as you've already discovered.

I still like the idea of using a computer PSU.  If it's clean enough for the computer I'd bet its clean enough for your project.  Of course, I always tend to have a few of them laying around so it makes them quite handy.  Not so much if you don't.

PekkaNF:
It's ages when I was at the school and we were playing with A/D circuits, but if I recall right, few points in random order:
* Digital system requires different "good" power supply than analog system. You want pretty exact voltage level and rigid power line i.e. aux. supply capacitor for each circuit and near IC:s that cause surges.
* Analog system requires it's own power line/filtering and same on return line.
* Shielding and grounding must be kept separate. Grounds connected only on one point on chassis.
* ADC supply and signal lines must be considered carefully, Best to shield it too and sow ferrite beads on every analog/power line out. Best to tuck the whole cacadoodle on outside corner of the PC board as far away from processors and their fast busses as possible.

Much of the problems comes from noise that is introduced to power feeds....Circuit board could be right, but all wires tied to same bundle and grounded randomly....

Some of the stuff here sounds familiar(ish):
http://www.analog.com/library/analogDialogue/Anniversary/12.html
Designer might have made some choices allready.

When I don't know what to do with grounds that are connected less than ideal way, I'll toss small resistor in series towards to "dirty system" and ferrite bead over the wire/leg. There will be small voltage drop, but on analog system that hardly never matters and even on digital systems you'll get away of it if you have decent coupling capacitors on IC:s.

This sort of idea:
http://e2e.ti.com/cfs-file/__key/CommunityServer-Discussions-Components-Files/64/8371.Audio_5F00_ground_5F00_isolation.png

There are finer points on ferrite beads, but I don't have need to go there:
http://www.sigcon.com/Pubs/edn/ferritebeads.htm

Only thing I know that this concept has got me out of pickle, maybe basic understanding of the problem and token beads were just enough for me to avoid bigger problems.

Pekka

nrml:
It appears that the iPad charger delivers a constant current but allows the voltage to drift down to 4.4V under load, which I understand is still within USB spec. So the Pi as a USB powered device should work on it without any issues. The noise and ripple measurements of the Apple wart appear to be better than most common branded or generic 5V power supplies.
http://www.righto.com/2012/10/a-dozen-usb-chargers-in-lab-apple-is.html

The annoying thing is that I'll have to buy a separate power supply for the hard disk. The Pi cannot supply enough current to run one off its own USB port reliably.
Would I need super clean  power for an external hard disk (magnetic or SSD) or would a cheaper wall wart suffice? Shame I don't have a decent computer power supply lying around.

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