Author Topic: Advice on which linear regulator to get  (Read 3973 times)

Offline nrml

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Advice on which linear regulator to get
« on: December 16, 2015, 01:29:28 PM »
Santa is bringing me a raspberry pi 2 and a GPIO to SPDIF adapter for christmas :ddb:. I am planning to use this along with a USB SSD hard disk as a digital music player / storage solution to replace my CD player and CDs. My research leads me to believe that to be on the safe side, I need a clean 5V 2A power source for the Pi and I need to power the hard drive separately.

I am a bit confused as to which linear regulator would suit my needs best. I see two commonly sold versions on ebay; one with LM317 and LM337 regulators and the other with LT 1083.
http://www.ebay.co.uk/itm/271923226377?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
http://www.ebay.co.uk/itm/271891627271?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
There seem to be smaller versions of these around too.

I already have a 9V output transformer that I am going to feed the regulator with.
https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504690/?searchTerm=10-5843&relevancy-data=636F3D3126696E3D4931384E4B6E6F776E41734D504E266C753D656E266D6D3D6D61746368616C6C7061727469616C26706D3D5E285C642B5B2D5C2E5C732F5D292B285C642B293F247C5E5C642B2426706F3D313326736E3D592673743D4B4559574F52445F53494E474C455F4F525F4D554C54495F4E554D455249432677633D424F5448267573743D31302D3538343326

Which regulator would be best for my little project?
Thank you for all constructive advice.

Nirmal

Offline Bluechip

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Re: Advice on which linear regulator to get
« Reply #1 on: December 16, 2015, 01:47:46 PM »
The LM337 is a negative adjustable regulator.  :thumbup:

Look at the schematic, it shows you there.

Do you want a negative supply as well as the positive.

The LT1083 appears to have 2 separate outputs, not sure I really believe the 10A bit though ..

However, if you're only pulling a max. of 2A you should be OK.

http://cds.linear.com/docs/en/datasheet/108345fh.pdf

Dave
« Last Edit: December 16, 2015, 02:15:26 PM by Bluechip »
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Offline nrml

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Re: Advice on which linear regulator to get
« Reply #2 on: December 16, 2015, 03:13:40 PM »
I don't think I need a negative voltage. Just a reasonably cheap clean power supply to keep jitter in the digital audio output as low as the pi will permit. Having very little knowledge of electronics, I was unsure if one was vastly superior to the other in performance as a 'clean' power source.

Offline shipto

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Re: Advice on which linear regulator to get
« Reply #3 on: December 16, 2015, 03:26:29 PM »
personally I use lm317 for most regulator needs but thats only good for 1.5 amp but the lm338 is wired exactly the same and can handle 5 amps.
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Offline John Rudd

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Re: Advice on which linear regulator to get
« Reply #4 on: December 16, 2015, 04:40:18 PM »
Think personally I'd be using switching regulators....more efficient..less heat.

But, your choice :scratch:
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Offline nrml

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Re: Advice on which linear regulator to get
« Reply #5 on: December 16, 2015, 06:16:28 PM »
My (limited) understanding is that, linear regulated power supplies provide a cleaner output than switching power supplies which is why I was considering a linear power supply. The module is fairly cheap (12 including shipping) and I have a suitable transformer lying around, so costs are not prohibitive.

It is probably a bit of an overkill for a raspberry pi music player, but I didn't want to penny pinch and end up regretting it later, as the DAC I have has a very low noise floor and the equipment downstream is very transparent.

Offline grg12

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Re: Advice on which linear regulator to get
« Reply #6 on: December 17, 2015, 02:10:34 AM »
9V trafo will, after rectification generate about 11V withot load - how much with load depends on particular trafo and capacity of smoothing capacitors.  You need 5V so 6 will be dropped on regulator chip (this applies to all linear regulators) you will draw 2A. So linear regulator will be heated with 2A*6V=12Watts. That's a loot , you will need quite a big radiator and some ventilation

Offline nrml

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Re: Advice on which linear regulator to get
« Reply #7 on: December 17, 2015, 04:50:39 AM »
Thank you. That sort of practical information is exactly what I was hoping for.

Time for a re-think.

Offline sparky961

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Re: Advice on which linear regulator to get
« Reply #8 on: December 17, 2015, 06:11:26 AM »
Why not use a 7805? No external components needed to set the voltage. I am surprised the board doesn't have anything built in though.

It has been mentioned but I'd use a switching supply too. Add some decoupling caps if you have detectable noise. All computers and many other critical applications use switching supplies without problems. I use old computer P/S's to power many a project. You even get a selection of useful voltages already there.

Offline nrml

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Re: Advice on which linear regulator to get
« Reply #9 on: December 17, 2015, 07:21:05 AM »
Perhaps I am reading too much into reports of SD cards getting corrupted by poor power supplies. I thought it might be a cheap and easy thing to throw a ready built fleabay module onto a transformer I already had and avoid this potential problem and also reduce jitter in the output. I didn't take into account heat dissipation and the additional cost and complexity this brings in.

Given the low cost of the entire project, an OEM 12Watt USB apple wall wart might be the best value for money vs output quality compromise. They seem to measure reasonably well in independent tests. At 15, it costs very little more than my original plan. If the output is as good as a half decent CD player I will be happy.

Offline buffalow bill

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Re: Advice on which linear regulator to get
« Reply #10 on: December 17, 2015, 08:50:02 AM »
Though the raspberry pi required a 3.3 volt supply not 5 volts.

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Offline John Rudd

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Re: Advice on which linear regulator to get
« Reply #11 on: December 17, 2015, 09:06:16 AM »
Though the raspberry pi required a 3.3 volt supply not 5 volts.


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

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Offline sparky961

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Re: Advice on which linear regulator to get
« Reply #12 on: December 17, 2015, 08:30:46 PM »
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.

Offline PekkaNF

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Re: Advice on which linear regulator to get
« Reply #13 on: December 18, 2015, 06:06:26 AM »
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

Offline nrml

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Re: Advice on which linear regulator to get
« Reply #14 on: December 18, 2015, 06:45:24 PM »
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.