DC to DC Converter

Andrew and Morgaine,

It is kind of easy, but it all depends on how much electronics you're into. The trickiest part is if you want to be able to charge the batteries while using the R-Pi.

So, let's drill into it! Powering the R-Pi:

(1) 12 volt sealed cell battery.

This is the most easy one: a linear regulator (LM7805) with a heat dissipator and 2 bypass capacitors should do the trick.

(2) 2 to 4 cell lithium packs

It depends... with the packs in series  (2S or 2P2S) you have between 5.6V and 7.3V. In that case, the voltage is too low for a 7805.

The circuit is almost the same, but using a LP2954-5V or a LM2940CT-5.0, the last one being more common. A 2S2P pack would drain in around 1-2 hours.

(3) perhaps even a set of AA batteries.

I would go for 4 AA batteries and a LM2940CT-5.0. The problem is that they will discharge very quickly and the system will shutdown.

The most beautiful solution I can think of is a 2AA pack running thru a LT1302 boost converter (see the Minty Boost http://www.adafruit.com/products/14). The main problem is that they would drain in around 1 hour...

Thank you Rafael.  It looks like the AA solution is generally "out", unless it's "just for fun".  But I will check out the MintyBoost.  It sounds like fun.

I do have four packs of 2150mAh, 20C, 3 cell, 11.1v lipo packs.  Would one (or more in parallel) of these work with the LM7805ACT.LM7805ACT.?  Is there a cut off voltage?  I don't really want to kill the lipos on the first discharge.

For the 12volt sealed, it is a small one with 1.4 Ah.  However, I may graduate to something bigger.

I would like to charge the batteries while running the RPi.  I have a small solar unit that I was hoping to use to get me started.  (1.8 watts/ 125mAmps Max, current 125mAmps at 15v, I see 16 to 25 volts on a meter in full sunlight.)

I was going to ask about the solar charger in a separate post, but it seems the overall project should be considered.  For my first project, I want to run an RPi outdoors, with a web cam and motion detection software.  I would use IR at night.  I'd like to run it off battery at night and solar/battery during the day.  Later, I'd like to network it (WiFi, Bluetooth, or modem) with another RPi unit for image storage on a common drive. 

I don't have a clue how to calculate the overall drain.  While I used to be able to read a schematic, calculate loads, voltage drops over cabling, etc. that was MANY years ago.  Trust me, I've forgotten most everything except how to follow instructions :-)

If you will lend some guidance, it will be greatly appreciated.

Thanks in advance,

Andrew

Andrew,

    Dom has suggested:  (actually it was Gert as noted below)

"I plan to remove the on-board 3v3 and 1v8 regulators and replace them with two switch mode supplies. Any other solution is foolish as it will drain your batteries about twice as fast as is required! I have no cost for that solution yet but I estimate it to be the price of about 2-4 sets of batteries."

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=2&t=3261

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=9&t=5371

http://www.raspberrypi.org/phpBB3/viewtopic.php?f=5&t=4784

coder27:  I think you meant Gert, not Dom.

Gert also explained further:

Gert van Loo write:

 

... you put the resulting 5V into the board. There on the board half of it is converted to heat. To prevent that the regulators on the raspberry-Pi board itself must be replaced.

...

... the alpha boards had SMPS but the cost was too high to get a $25/$35 end product. So we had to cut cost and use linear regulators.

So, it looks like Pi v2 is in for a redesign of its power circuitry, which is what we've all been advising.  Now that the boards are being manufactured in high volume and so their BOM cost has plummeted, it shouldn't be a problem anymore to do the power properly and still remain within the desired price niche.

Morgaine.

coder27:  I think you meant Gert, not Dom.

Gert also explained further:

Gert van Loo write:

 

... you put the resulting 5V into the board. There on the board half of it is converted to heat. To prevent that the regulators on the raspberry-Pi board itself must be replaced.

...

... the alpha boards had SMPS but the cost was too high to get a $25/$35 end product. So we had to cut cost and use linear regulators.

So, it looks like Pi v2 is in for a redesign of its power circuitry, which is what we've all been advising.  Now that the boards are being manufactured in high volume and so their BOM cost has plummeted, it shouldn't be a problem anymore to do the power properly and still remain within the desired price niche.

Morgaine.

right, I meant Gert.  Thanks.

I"m pretty sure Gert was talking about modifying his personal RPi,

not a redesign for Pi v2, although anything is possible.  The only thing

Eben has mentioned for Pi v2 is the FCC class B compliance changes

to the PCB, although I'm pretty sure PeteL wants to make some

changes such as moving the big capacitor to a safer place.

btw, I'm not so sure their BOM cost has plummeted.  They claimed to already

have negotiated a really good price on the parts for the first batch on account

of being a charity, basically I think they were getting the 100K volume price

even for 10K volume.  So maybe not a whole lot of room for further reduction.

And now it isn't the non-profit charity doing the negotiating, it's the for-profit

distributors, so there may be less willingness to cut prices to the bone.

coder27 wrote:

 

They claimed to already have negotiated a really good price on the parts for first batch on account of being a charity, basically I think they were getting the 100K volume price even for 10K volume.

 * morgaine looks out for flying pigs ...

If RPF as a non-profit and with no paid employees only just managed to squeeze in under the $35 bar through 100k pricing, then it is likely that Farnell and RS wouldn't be interested in the deal at all, since there would be very little room left to them for economies of scale.

It's also unlikely in the extreme because it would mean that Farnell and RS are currently making a loss on every board after factoring in site and support costs, which RPF did not have.  So no, pigs will fly first IMO.

Farnell and RS reportedly weren't interested in the deal for the profits.

The Register reports:

"The effect of the Pi on RS Components' overall business is mixed. Edward said the company “won't make money, but might get famous.” He also said that the advent of the Pi has dented sales of other single board computers."

http://www.theregister.co.uk/2012/05/16/75000_pis_by_august/

p.s.

  the foundation has said their break-even point was about 30K units,

so even they were expecting to lose money on the first 10K.

Perhaps 30k, aye, that would still work for the companies.

From 30k to the 300k which Farnell and RS are selling just on preorders alone is an order of magnitude rise in volume, and "plummet" is the right word for the BOM cost under such change of numbers.  Plus, the educational market beckons, and that could quite easily result in yet another order of magnitude in the UK alone (with help of government), and certainly across Europe.  Worldwide ... the numbers are staggering.

So no, don't believe the "“we won't make money" statement, that's just part of the company PR.  The PR person in the report was just doing his job, and it seems very effectively. :-)

Farnell's quarterly results come out Thursday, so maybe we will get

some indication of profitability.

Morgaine,

  the education market may have taken a bit of a hit after Eben's quote today:

"There's nothing specifically educational about this device. The only reason it's applicable is it's cheap."

http://www.pcpro.co.uk/news/education/375106/founder-no-raspberry-pi-for-every-student

Haha, good ol' Eben.

See, you can tell he's an engineer, he's being accurate rather than strategic.  PR is basically about lying, no matter how generously you try to slant it, and engineers are not good at lying.  In an important sense, it conflicts with the engineer's "religion" or core principles.  This is why engineers should never do PR.  The lying is best left to those who lack the engineer's respect for honesty.

That said, he could have been accurate while still promoting Pi for education, simply by following his "There's nothing specifically educational about this device," with "except in the sense that it is mostly open source and hence inherently educational."  That's still accurate.

And, if his audience showed signs of wanting more, he could have added "However, what does have a strong educational element is the motivation that brought this device into existence, because Raspberry Pi was developed for the specific purpose of providing a widely accessible platform for education in computing."

That might have been a bit more encouraging, without descending into manipulating reality for PR.

Morgaine.

When I wrote "This is why engineers should never do PR", I did of course mean "if you want to preserve the current social order".  In contrast, if what you want is a better world, then only engineers should ever do PR.

- "These shoes were made by slave labour with the cheapest materials we could find.  They have a year's guarantee by law, but we really don't expect them to last that long.  And you really shouldn't be wearing shoes with these ridiculous heals, they're bad for your health and my employers should be ashamed."

Morgaine,

   Sometimes engineers can be very effective at marketting because

they are believed even when they should be challenged, just like a discount

store can be very profitable because shoppers tend to mistakenly assume that

all their prices are equally discounted.

   Eben is asked what the greatest challenge currently is, and says its ramping

up volume.  This may be true, but if unchallenged, tends to lead one to believe

that he thinks the design is solid and all that's necessary is more production. 

But I think this theory ought to be challenged.

Farnell's Q1 financial results are out (covering Feb-Apr)

http://www.premierfarnell.com/content/results-first-quarter-financial-year-ending-03-february-2013

RPi is mentioned under "Strategic Highlights" as bringing in new customers, but no mention of

bringing in profits.

• "Continued progress as a digital enterprise, with 2.7m visits in the quarter to our element14 community as it played an important role in the global launch of Raspberry Pi which will add in excess of 60,000 new customers for the second quarter."