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Topic: Techniques for getting 3v3 from the Uno/Mega 5V bus (Read 2 times) previous topic - next topic

oric_dan

#25
Dec 06, 2012, 07:32 pm Last Edit: Dec 06, 2012, 07:37 pm by oric_dan(333) Reason: 1
Quote
Well then the answer is simple, the best case solution is a switch mode 5 volt to 3.3volt regulator. It will not waste 5 volt power just to turn it into heat, so the total wattage consumed from the 5 volt bus from any given 3.3 volt load will be less. Next question?


I don't think that's the best solution either, as somewhere in the chain you have a
5V v.reg that's dissipating a huge amount of heat. Better to run the 3.3V dc-dc
converter off Vin in the first place. No?

And after all, a 3.3V "linear" v.reg running off of 5V really isn't gonna be generating a
lot of heat anyways. Eg, (5V - 3.3V) * 0.5A = 0.85W, whereas the 5V linear v.reg will
be dissipating much more, just to power the 3.3V device. Eg, (9V - 5V) * 0.5A = 2W,
not to mention any other loads.

So you see, OP, there are probably no universally agreed upon solutions to much of
anything.

Actually, a solution I like a lot is being using by Pololu on their Zumo shield. They use
4 rechargeable AA-cells to power a 1 Amp dc-dc converter that puts out 7.45V.
Then you can connect your Arduino board to that. Even if it's using linear v.regs,
they're not running super hot [except maybe for those tiny SOT-223 parts that I
dislike - I always use TO-220 or DPAKs myself].

http://www.pololu.com/catalog/product/2504

[Edit after the fact: after reading through some of the other posts, and in light of
what I said here above, I like the Due idea of a dc-dc converter for 5V powering
a linear v.reg for 3.3V. Good compromise].

oric_dan

#26
Dec 06, 2012, 07:46 pm Last Edit: Dec 06, 2012, 07:54 pm by oric_dan(333) Reason: 1
Quote
The noise from the switcher isn't an issue for digital designs and easily dealt with for 10 bit A/D conversion with a 5 V reference because even the fastest A/D conversion I did averaged several measurements for accuracy and external noise rejection.


Not a switcher power supply as being discussed, but I once built a board with 12-bit
ADC and opAmps running off +/-6V linear v.regs [78L06,79L06], and with the negative
rail being supplied by a 7662 chip. In the prototype lashup, there was a lot of noise on
the ADC conversions, but that was solved by placing a ground plane under the 7662,
and also moving the 7662 further away from the opAmps.

http://www.maximintegrated.com/datasheet/index.mvp/id/1357

So, having good ground planes and physical separation of the spike makers is 1000%
required.

Quote
In bread boarding I've found that those little breadboard jumpers are totally unsuitable for carrying any current above 100 mA or so, depending...
As they are 30 Ga stranded and with the combination of jumpers and breadboard connections typical are lossy.
I start with 5 V from an external PSU and after 6" of travel through jumpers and pins the voltage ends @ 4.5 to 4.75 V.


The Arduino female headers with the tuning fork inserts, combined with temporary
jumpers using circular endpins, makes for a perfect storm of iffy reliability. OTOH,
the usual 0.1" male headers, combined with suitable female pins, makes for a reliable
1 Amp [or 3 Amp] connection.

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=100766&langId=-1&storeId=10001&productId=100766&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView



pico

Well, since the advice in this thread suggested that the Mega _should_ work, I cajoled a friend into letting try his Mega2560. And guess what? It works. For all the PSUs tested and USB power as well.

So looks like I've got a dud Mega.  :(

But the good news is that the shield design appears to be fine, which is the main thing. And everyone seems to agree that for the 1117 LDO regs, there isn't much to be said beyond using the 0.1uf and 10uF caps as specified in the data sheet. And since that's already what I'd done, I guess I'm done.

But it was useful to confirm there wasn't anything else in the way of design tricks I was missing. So sorry about stress testing the group wisdom here on what should be a simple topic, but all's well that ends well.

Except for the dud Mega... really, the only remaining mystery.  Is is a common failure mode for a voltage regulation circuit to "lose capacity" in this way, e.g., will provide the correct voltage up to a certain current and then brown out, way before it's rated capacity? In any case, that would appear to be what's happening here. Weird that the higher capacity 1A PSU seemed to make a difference.

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