sh4d0w0lf
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Re: Power supply

Sat Jul 30, 2011 10:15 am

Hi, I'm interested in purchasing multiple raspberryPI's.
I have a couple of questions first of which is to do with power supply, Do these units come with a power supply or is that something that we would have to purchase seperatly from you or another vendor?

Secondly (this is more of a silly question) is the $25 price tag the same for every country or is this a US only price?

Thanks.

MDC
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Re: Power supply

Sat Jul 30, 2011 10:27 am

How about powering it via a renewable source e.g. solar, the movement of you walking or moving ect

ShiftPlusOne
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Re: Power supply

Sat Jul 30, 2011 10:38 am

Quote from MDC on July 30, 2011, 11:27
How about powering it via a renewable source e.g. solar, the movement of you walking or moving ect
the solar cells, battery and charging circuit would cost way too much for solar to be reasonable. movement doesn't generate that much energy either, it's good for a watch, but it's not going to power a computer.

MDC
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Re: Power supply

Sat Jul 30, 2011 1:13 pm

Movement doesn't generate that much energy either, it's good for a watch, but it's not going to power a computer.

I think your wrong there. I seem to remember some news show going on about a pair of pants with wires through them that powered a mobile phone when the wearer began moving.

Sorry to be so vague.

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Re: Power supply

Sat Jul 30, 2011 2:27 pm

I've looked it up and only found solar-powered pants (never thought I'd say that).

If you have a portable battery pack which can be charged via USB and output over 6v, $1000 solar powered pants you might have some luck, but how would it work practically? You'd still need an external power source for all your peripherals.

'course there are solar powered battery packs out there which might do the same thing in a much more practical way. It could work if you're camping for example, but again, how would you the monitor and everything else? It works great for mobile phones or even laptops, but raspberry pi isn't designed that way.

Same goes for any kind of mechanically charged pants. Think of a hand-cranked LED torch. You have to crank that thing for ages. Now let's be ambitious and say that throughout the day these pants will charge a 2300mAh battery. That would last about 3 days in a phone and about 3 hours (if that) in a laptop.

To sum up, there are some things to consider.
How much do these pants cost?
How much power can they generate over a specified period of time?
What practical application could you possibly have to justify the expense?

Cafe
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Re: Power supply

Sat Jul 30, 2011 3:52 pm

Quote from sh4d0w0lf on July 30, 2011, 11:15
Do these units come with a power supply or is that something that we would have to purchase seperatly from you or another vendor?

Thus far we know the power will be from a coax unit, but I get the feeling that the Raspi will not include a PSU. We will probably need to use a PSU with the right rating.

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Gert van Loo
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Re: Power supply

Tue Aug 02, 2011 8:03 am

The power supply is a set of SMPS plus LDO protected by a diode against reverse powering.
It requires 5V for the HDMI and USB connectors (and the HDMI I2C). The board itself does not requires 5V and can be run of 3V3. The main reason for the wide input range was to keep the system cost low by allowing the owner to use an arbitrary DC 'brick' they may already have.
A lot of people have asked about powering it from 5V USB. Although we have not tried that yet (the boards have still to come in) I think that is possible. But you have to re-jig the power supply yourself: Remove the 5V SMPS and use the USB power directly. Connect the 3V3 SMPMS input to the 5V, all of which of course voids your warranty. Note that it does not turn the board into a USB slave. It remains a master.
For the final design I will see if I can add some 0-Ohm links to make that easy.
One more note about the power.The board may use 1W but if you start connecting a mouse and keyboard the power usage will go up.

AmyS3
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Re: Power supply

Tue Aug 02, 2011 8:13 am

how do you guys measure the power consumption in detail?
in other words: how could a customer lower the usage? like disabling the lan interface.?
its obvious that the usage rise with more external devices. but is it possible to bring it down?

Cafe
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Re: Power supply

Wed Aug 03, 2011 8:57 am

Quote from Gert van Loo on August 2, 2011, 09:03
The power supply is a set of SMPS plus LDO protected by a diode against reverse powering.


Are you saying a PSU is included with the device? Please clarify. Thanks

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Re: Power supply

Wed Aug 03, 2011 10:46 am

Cafe, I think he was talking about the power supply circuit that's actually a part of the board. You still need an adapter to supply the 6-20v.

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abishur
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Re: Power supply

Wed Aug 03, 2011 12:28 pm

Rather than the modifying (and the voiding of warranties) what about a USB -> coax adapter I've gotten a couple of them with small time components nowdays (like a VGA -> RCA adapter) Since I'm using the existing port would I still need to modify things, or is the overall issue here not the power port but the power / kind of power going into said port?
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Re: Power supply

Wed Aug 03, 2011 9:33 pm

Normally power supplies need an input that's a bit higher than the output. Since, there's a 5v switchmode supply on there, it sounds like that would have to be removed either way. Though that seems to conflict with what Eben said earlier about maybe brining the input voltage down to 5v.

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Gert van Loo
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Re: Power supply

Wed Aug 03, 2011 9:35 pm

The issue with the power supplies is that they will be changing from the current one. On the Alpha board I plonked down what was in our parts library. There was no time to look at cost. Therefore I can't promise anything until that is sorted out. Most cheap ones are buck converters which require a higher input voltage then output voltage. With 5V out and a reverse-voltage-protection diode I am not sure we can convert 5V to 5V. Don't want to either because you are burning power for nothing.

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abishur
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Re: Power supply

Wed Aug 03, 2011 9:47 pm

Good to know! Thanks guys!
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AmyS3
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Re: Power supply

Thu Aug 04, 2011 2:38 pm

how about powering the raspi with a lipo?
like this one -> http://www.ebay.co.uk/itm/290498140802
with 5,2Ah on 7,4V it should last a while. and for under 10 quid its kinda cheap.
i tried to calculate how long the raspi would run on that lipo but somehow im a bit braindead today XD

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Re: Power supply

Thu Aug 04, 2011 4:41 pm

Since R-Pi uses <1 W, at 7.4V that should be 130mA. 5200mAh/130mA=40 hours?

I am pretty sure I am wrong, 'cause 40 hours sounds a bit too good.

AmyS3
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Re: Power supply

Thu Aug 04, 2011 4:56 pm

well i estimated around 38 hours...
given 7.4V at 5.2Ah would eqal 38.48Wh....?

so 40h sounds good. i just bought 2 of them :-) so it could run for 80h..? nice

if i add a solar panel or rather several cells with an output of 3-5W it would run forever on green power. even on only one battery..

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abishur
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Re: Power supply

Thu Aug 04, 2011 5:49 pm

Yes, you could theoretically get that out of the batteries, however make sure to hook your batteries up in parallel! (Hook the + terminal to the + terminal together and the - terminal to the - terminal). When batteries are hooked up in parallel the provide the same voltage but a cumulative mAh. That means that in the posting you listed each battery is 7.4V with .5Ah. Hooking two batteries together in parallel will give you a the same 7.4V but double your Amp hours. Meanwhile, hooking them in SERIES will double your voltage but keep you Amp hours static (Meaning your batteries would loose their charge just as quickly, but could handle a higher voltage).

Or for you mathematically mind people out there the formula for a number of identical batteries (X) hooked together in a parallel circuit is:
V=V
mAh = mAh*X

and the formula for the same identical batteries hooked together in series is
V = V*X
mAh = mAh

I would advise against hooking batteries of different voltages or mAh. Keep it Simple.

If you do set it up with batteries are you planning on making them a battery backup or just charging the batteries whenever need be?
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AmyS3
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Re: Power supply

Thu Aug 04, 2011 6:19 pm

i´ll definitely putting the batteries in parallel to gain a higher mAh of 10.4Ah in total.
the tricky part will be the charging mechanism. im going to charge the batteries with a solar module which will supply ~7W but while the raspi is running on the batteries.
given the 7W from the solar with a usage of 1W from the raspi should leave 6W to charge the batteries. in best conditions with bright daylight...

best configuration would be to charge the batteries if needed and use the solar module for the raspi directly to conserve the batteries when they are fully charged.

the problem is that i can´t charge the lipo´s from the solarmodule directly so im in need of a separate charger.
and the raspi is not the only device which will use the batteries. since the raspi is going to be the cpu of my uav, i also have to think about the motors, servos, gps, camera and other sensors.
i´m lucky that the uav is helium backed so the motors don´t have to run all the time.

ahh.. the power unit is going to be a tricky pice..
it would be easy if it would be only the raspi on the batteries.. or maybe i split them and use one for the raspi and the other one for the rest.. can´t decide.. help

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abishur
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Re: Power supply

Thu Aug 04, 2011 6:21 pm

Just as a side note, be very careful if you're not using a charger designed to charge this battery. Improper charging of a battery can make it... explode. On the other hand if you've done this kind of thing and could display a good circuit board that would be to charge the batteries while there's power and then run off the batteries during a loss of power event I'd be interested in seeing those schematics :) Otherwise charge it separately and use it whenever you want to be mobile.
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abishur
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Re: Power supply

Thu Aug 04, 2011 6:24 pm

ah, you beat me to the post! That should work, just make sure you have a means of "float" charging the batteries, even solar with a continuous charge will run the risk of overcharging the batteries to... unfortunate ends.
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AmyS3
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Re: Power supply

Thu Aug 04, 2011 6:29 pm

hehe..
well im trying to figure out a shematic for that charging circuit and post it here if someone else is in need of it as well.

speaking of power supply, i need to change the batteries of my wireless keyboard.. its skipping some chars already XD

Cafe
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Re: Power supply

Fri Aug 05, 2011 7:51 am

Quote from AmyS3 on August 4, 2011, 19:19
given the 7W from the solar with a usage of 1W from the raspi should leave 6W to charge the batteries.

I could be wrong, and am probably just nitpicking: The battery's charging current should be greater than the battery's discharging current. For example, if the battery is discharging 500 mA, it is important that the charging current is greater than 500 mA, prolly safe to be about 1.3x. Of course, not all batteries respond well to being charged and discharged simultaneously. LIPO batteries do fare better in this regard.

Cafe
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Re: Power supply

Fri Aug 05, 2011 8:00 am

Quote from AmyS3 on August 4, 2011, 15:38
how about powering the raspi with a lipo?
like this one -> http://www.ebay.co.uk/itm/290498140802
with 5,2Ah on 7,4V it should last a while. and for under 10 quid its kinda cheap.

That's a great find. Here is a list of eBay.com entries for this item (most entries to US merchants, some to Australian). It costs between $16 and $21 depending on the merchant: http://bit.ly/nqvSoR

Des_Zac
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Re: Power supply

Fri Aug 05, 2011 5:30 pm

But wouldn't a battery like that defeat the purpose of it being really small? Because it's supposed to be about the size of a thumb drive, (At least A is) And that battery has to be 2.5x that from the look of it.

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