JeremyF
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Re: Interesting Thing I found on eBay.

Thu Mar 29, 2012 10:22 pm

So basically I've taken it upon myself to go on eBay everyday and report scams related to Raspberry Pi. Right now, I haven't seen a lot of listings claiming to be boards, however I've seen quite a few cardboard cases, power supplies, and preloaded SD cards. Anyway, this item caught my eye.

http://www.ebay.com/itm/Raspbe.....3373cc7e90

The first thing is I realize that the 4 AA batteries mentioned the FAQ refers to the Alpha boards, which could accept way more than 5V (Most AA Batteries = 1.5V*4=6V=fried pi). However, from some quick Wikipedia searching I've found that certain AA batteries (made from nickel-cadmium or nickel-metal-hydride) typically are rated at 1.25V. (http://en.wikipedia.org/wiki/AA_battery) So, in theory, production boards could actually use the AA battery trick as well.

Second, I know close to nothing about "actual electronics" (i.e. bare wires, circuits, and whatnot) However, with the correct voltage, the eBay item may actually be helpful.

So I have 2 questions:


Could the production boards actually be powered by AA batteries easily? (without a system involving resistors, etc.)
Does the eBay item shown make any sense at all?


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error404
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Re: Interesting Thing I found on eBay.

Thu Mar 29, 2012 11:31 pm

Yes but. Batteries aren't a constant voltage source, when fully charged they'll be somewhat higher than their nominal voltage, and when empty they'll be below it. That changing voltage shouldn't be a problem for the Pi itself (within reason) as it doesn't actually run on the 5V, so its power regulators will take care of that. However the USB ports are directly connected to the input 5V power, and running it this way will probably put it out of spec for periods during the life of the batteries. This could damage or cause to not work things you plug into the Pi's USB ports. My guess is that most things will work okay, but YMMV.

Also keep in mind that the Pi isn't really optimized for low power. It's not really well understood yet what its power draw will be in practice, but I wouldn't expect more than a couple of hours of runtime on AA cells.

mobeyduck
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Re: Interesting Thing I found on eBay.

Thu Mar 29, 2012 11:36 pm

It looks like a guy who wants to earn a few bucks selling some home made power solution.

If you would use 1,25v battery's there wont be a problem, but as you mentioned most battery's are 1,5v so you would need a voltage regulator.

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Jim Manley
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 3:53 am

Battery voltage varies with current drain, and measuring the open-circuit (aka no-load) voltage will not be the same as the closed-circuit (loaded) voltage due to the fact that batteries have internal resistance.  Hence, there is an internal voltage drop within each battery proportional to the load current and the internal resistance, which will lower the output voltage.

The power draw of the R-Pi and anything connected to the USB ports can vary between about 200 and 700 ma and the internal resistance of a single AA battery is around 150 to 300 milliohms, so, the voltage drop across each battery is between about 0.03 (0.2 amps * 0.15 ohms) and 0.2 (0.7 amps * 0.3 ohms) volts.  So, the nominal no-load 1.25 volts provided by each battery will actually be somewhere around 1.22 and 1.05 volts at full charge (which will immediately start dropping gradually until it gets to about 80 ~ 90% of full charge, depending on the battery technology, e.g., NiCd, NiMH, Li-ion, Li-polymer, etc.).  In any case, the R-Pi components provide on-board voltage regulation, and will be able to handle the 4.2 ~ 4.9 volts actually available from the batteries.  Since USB VBUS is specified to be between 4.4 and 5.25 volts, the upper limit of current draw from external USB devices would need to be somewhat less than 500 ma to keep the maximum voltage above 4.4 volts.

So, there shouldn't be any problem using four rechargeable AA or AAA batteries, as long as the current draw from the USB ports is kept within a few hundred milliamps, which is more than enough to supply a keyboard and a mouse/trackpad.
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mole125
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 4:56 am

Jim Manley said:


So, there shouldn't be any problem using four rechargeable AA or AAA batteries, as long as the current draw from the USB ports is kept within a few hundred milliamps, which is more than enough to supply a keyboard and a mouse/trackpad.


For clarity you are refering to running off rechargeable 1.25 Volt batteries, and not standard 1.5V AA batteries - if someone uses a fresh set of these they could very easily damage there RPi*.

*Without any further comfirmation I would be surprised if in this scenario it would be down to the tolerances of individual components, 6V may work fine for some/many/most RPi's and attached peripherals but that would be no guarantee that it wouldn't damage someone elses.

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Jim Manley
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 6:20 am

I seem to recall that the alpha boards could take something like 6 ~ 20 volts, but, I don't know whether they had additional regulation on the board to handle the higher voltage.  When they went to the micro-USB power jack, then 5 volts became the only supply discussed.

James, or anyone else, can you confirm that 5 volts is the maximum that can be supplied through the micro-USB connector?  Thanks!
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rurwin
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 6:37 am

There is no 5V regulator on the board. There are several others for 3.3V etc. IIUC the only thing that uses the 5V directly is the USB. Everything else goes through a regulator.

As Jim said, the maximum that should be offered to the USB ports is 5.25V

As the input voltage went up, there would be more heating of the various voltage regulators on the board. At some point they will either be destroyed or come unsoldered. I suspect that you would need Pete Lomas and a 3D heat modeler to work out where that point is.

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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 7:09 am

If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.

In any case, I doubt these regulators will fail for a such small rise in voltage, and by no means they would come unsoldered. Many of them have internal temperature shutdown thresholds and would just cut off above a given level.

As for the ebay device, I wouldn't recommend it, I wouldn't recommend anything that has no proper stabilizer and protection circuitry (at least a fuse should be required / at most a proper current limiting, and overvoltage shutdown).

mole125
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 7:30 am

ceteras said:


If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.

In any case, I doubt these regulators will fail for a such small rise in voltage, and by no means they would come unsoldered. Many of them have internal temperature shutdown thresholds and would just cut off above a given level.


I agree the regulators will probably be fine, however from my understanding the USB chipset, ethernet chipset and any USB devices aren't behind any form of regulator they are connected straight(ish) to the input voltage, so it is a case of what these can tollerate. My guess is the manufacturers only guarantee them to 5.5v but in reality they can go higher, but there's a chance a bad batch won't be able to go higher and you'll fry this part of your board.

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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 7:48 am

@mole125: I agree, it's risky.

For battery operation, I'd recommend a regulator built on something like the LM3668 .

The image shows the 3.3V variant, it also has a 5V/1A version, it's a very simple circuit to build, if you can get the LM.


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RaTTuS
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 7:59 am

ceteras said:


If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.


see rurwin's comment above - there are no regulators on board - just a couple of resetable fuses
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ceteras
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 8:15 am

RaTTuS said:


ceteras said:


If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.


see rurwin's comment above - there are no regulators on board - just a couple of resetable fuses



I meant regarding this: "There are several others for 3.3V etc"

These regulators might be offended a little if they are supplied with higher voltages than expected, but I'm sure they could handle a little extra. If I knew exactly what make, I could dig the datasheets and say for sure.

Not that important. The end picture is: if you care for your Pi, don't mess with it's power supply.

Harrkev
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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 6:07 pm

RaTTuS said:


ceteras said:


If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.


see rurwin's comment above – there are no regulators on board – just a couple of resetable fuses


Ummm.  Not necessarily.  Assuming that you are running off of four 1.5V batteries, it MAY work — for a while.

The problem is that voltage regulators work kind of like variable resistors.  They adjust their resistance so that the output voltage is the right amount.  However, there is a limit to how low the resistance can go.  If you are trying to output exactly 5.0V, then if the input is too close to 5.0V (say, 5.2V, for example). then it just plain won't work.  If you really want to use a simple linear regulator, then you probably want five batteries (or use four alkalines, and throw them out when they stop working for this application, well before they are truly dead).

There are things called "low-drop-out" (LDO) voltage regulators, but you have to be careful on the capacitance on it, or it can become unstable and turn into a nice little oscillator.

What you "REALLY" want (if you can afford it) is a DC-to-DC converter.  A quick search of Newark turned up this (link to PDF data sheet):

http://us.tdk-lambda.com/lp/ft.....s/cc-e.pdf

Note that this part runs around $26, but can take 4.5V to 9V and output a smooth 5V the entire time.

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Re: Interesting Thing I found on eBay.

Fri Mar 30, 2012 6:50 pm

You can get the LM2596 dc-dc step down adjustable power supply module. I bought 5 off of ebay for about $16. Should work just fine. They can put out up to 2amps. It can take 4 to 40 volts and put out 1.5 to 35 volts. Tested it at a steady 5 volts. It uses a switching regulator. A little bit big in size 1 3/4" x 3/4" x 1/2" (43mm x 22mm x 13mm).
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Re: Interesting Thing I found on eBay.

Sat Mar 31, 2012 1:21 am

error404 said:


Yes but. Batteries aren't a constant voltage source, when fully charged they'll be somewhat higher than their nominal voltage, and when empty they'll be below it. That changing voltage shouldn't be a problem for the Pi itself (within reason) as it doesn't actually run on the 5V, so its power regulators will take care of that. However the USB ports are directly connected to the input 5V power, and running it this way will probably put it out of spec for periods during the life of the batteries. This could damage or cause to not work things you plug into the Pi's USB ports. My guess is that most things will work okay, but YMMV.

Also keep in mind that the Pi isn't really optimized for low power. It's not really well understood yet what its power draw will be in practice, but I wouldn't expect more than a couple of hours of runtime on AA cells.



Yes an no.  It really depends upon how you are using your Pi and how important "stability" is.  If all you are doing is watching videos, and the Pi crashing while you are watching is the"worst case" scenario, then running off of an unreliable power source can be tolerated.  Your choice.

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Re: Interesting Thing I found on eBay.

Sat Mar 31, 2012 3:35 am

I was aware of DC-DC voltage converters (the idea of a step-up converter without a transformer seems positively like voodoo ), but, hadn"t really looked closely at any, and the LM2596-based units for ~$2.50 seem too good to be true, especially with free shipping. I assume that, since these chop up the power and restack it at the desired voltage (it appears they can be configured as constant current, as well as constant voltage!) that they"re pretty efficient with batteries, much moreso than a typical LM318 type three-terminal regulator. This sounds like a winner even when used with droopy-voltage Ni-Cd batteries. I can"t wait to try this out ... of course, I can still wait to order these from Hong Kong the day my R-Pi allegedly ships and the converters will still arrive the next day, while the R-Pi will arrive who-knows-when (even with alleged two-day, space-available air delivery from wherever Newark"s US stock of R-Pi boards will be coming - direct-from-China, I wish, but, possibly from someone"s garage in Newark, New "Joisey" - I grew up ~30 miles from there - "Yo!" ).
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Re: Interesting Thing I found on eBay.

Sat Mar 31, 2012 3:43 am

my advice: don't build your own dc-dc booster modules.

if you insist.. go ahead!

but unless you do electronic circuitry for a living you won't make anything that is efficient.

sorry for the negative attitude, but it is NOT as simple as some people in this thread lay it out to be, well, step-DOWN regulators from mains even a monkey can build, doesn't matter how much power you burn up there.. but when you talk batteries you will need an extremely optimized circuit.

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