treczoks
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Re: Powering the Pi via the expansion Port?

Sun Apr 01, 2012 8:06 pm

Hi!

Is it possible to power the RPi via the expansion bus instead of the micro USB?

I thought about a simple, small board attached to a (3|4)x(AA|AAA) battery holder, using a DC/DC like the LM1302 to provide power via the expansion bus' 5V/GND pins (which could also provide a shutdown signal for the LM1302 via GPIO)

kaos
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Re: Powering the Pi via the expansion Port?

Sun Apr 01, 2012 8:30 pm

As I understand it, yes, but you should provide your own protection device (fuse/polyswitch/etc.), as you are bypassing the Raspi polyswitch.

A quick google didn't turn up any datasheet for LM1302. I assume it is a buck/boost switchmode regulator? Not sure why you would need a shutdown signal for it; do you intend to make it double as a power switch?

--

Best regards,

Kári.

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abishur
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Re: Powering the Pi via the expansion Port?

Mon Apr 02, 2012 3:42 am

Kári said:


As I understand it, yes, but you should provide your own protection device (fuse/polyswitch/etc.), as you are bypassing the Raspi polyswitch.

A quick google didn't turn up any datasheet for LM1302. I assume it is a buck/boost switchmode regulator? Not sure why you would need a shutdown signal for it; do you intend to make it double as a power switch?

--

Best regards,

Kári.



While I have no advice to offer about the LM1302 itself, Kári is correct in stating that by bypassing the R-pi's electronic protection you need to make sure to include some of your own.  Also I am guessing that by using the expansion bus you are voiding any warranty that might come with the board
Dear forum: Play nice ;-)

treczoks
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Re: Powering the Pi via the expansion Port?

Mon Apr 02, 2012 7:00 am

Kári said:


As I understand it, yes, but you should provide your own protection device (fuse/polyswitch/etc.), as you are bypassing the Raspi polyswitch.

A quick google didn't turn up any datasheet for LM1302. I assume it is a buck/boost switchmode regulator? Not sure why you would need a shutdown signal for it; do you intend to make it double as a power switch?


Yes, indeed I'd like to use it as a power switch. It is there, so why let it waste? And adding the polyswitch went without saying.

Abishur said:

While I have no advice to offer about the LM1302 itself, Kári is correct in stating that by bypassing the R-pi's electronic protection you need to make sure to include some of your own.  Also I am guessing that by using the expansion bus you are voiding any warranty that might come with the board
Well I'm not sure if it will be the LM1302 or some other DC/DC, but it looked as if it was the right one for the job. If someone has something better (maybe with some more mA), I'd like to hear about it.

And regarding the warranty: Don't worry. I earn my money with this kind of job. Thats "Embedded Systems Developer" to you

hippy
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Re: Powering the Pi via the expansion Port?

Mon Apr 02, 2012 9:35 am

Abishur said:

Also I am guessing that by using the expansion bus you are voiding any warranty that might come with the board
It would be worth the Foundation making a definitive statement on what warranty there is and what voids it.

IMO just using the GPIO connector shouldn't void warranty, but it's fair that damage caused through incorrect usage would not be covered by warranty. That would normally apply to all connections.

Who's actually responsible for any failure or damage when neither party accepts responsibility is always a matter of argument and evidence and usually has to be treated on a case by case basis. One can add wide-sweeping 'voids warranty' clauses but that can stifle use and may be over restrictive.

It's also best to have any warranty statements published before boards are in people's hands because it will not sit well if people aren't warned up-front what will void warranty. Likewise having electrical characteristics and limits documented is desirable because it's somewhat unfair, for example, blaming an end user for drawing too much current if the amount of current which can be drawn is not documented anywhere.

Having product and user manuals ( in whichever languages ) may already be part of the compliance issues so it may automatically fall out as part of that.

treczoks
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 7:09 am

My coworker told me to just use a stepup-regulator instead of a steput/stepdown, and go for two AAs.

One question remains: Which polyswitch does the RPi use, and how does it cope with getting power at the output pin, while having or not having a power input? Someone might decide to just plug in the micro USB while the thing runs on battery...

Phil Spiegel
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 8:57 am

Would Paralleled pairs of AA"s offer a useful solution – with more current capability – but still requiring UP conversion?

With regard to "Warranty" and/or consumer rights legislation for consumer sales…

If a problem is caused by the Add-on board making the Host board fail, isn"t that the responsibility of the Add-On board supplier: they need to have ensured their product was compatible and non-damaging ie fit-for purpose.

It is not the Foundation"s responsibility to make the Pi compatible with countless independant add-ons. (including your own designs for which you are responsible!)

However, particularly for School-type purchasing, it would be advantageous for add-on boards to gain some form of "Approval" and/or central publication (Note – some may only be offered locally by small-scale supliers):

As Distributors, it would be in the remit of, and advantage to, Farnell and RS to distribute and promote (volume) Add-ons – particularly those featured in any school course publications.

hippy
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 9:47 am

Phil Spiegel said:

With regard to "Warranty" and/or consumer rights legislation for consumer sales…
If a problem is caused by the Add-on board making the Host board fail, isn"t that the responsibility of the Add-On board supplier: they need to have ensured their product was compatible and non-damaging ie fit-for purpose.


Add-on manufacturers need to make their boards compatible / non-damaging to the R-Pi computer when used as specified. Any user using the add-on outside its specification would be responsible for any damage caused through doing that.

Add-on manufacturers may not be able to guarantee compatibility / non-damage to the R-Pi computer unless the definitive R-Pi computer electrical specifications are published. Hence why having those electrical specifications is so important to commercialisation of the R-Pi computer.

treczoks
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 9:56 am

Phil Spiegel said:


Would Paralleled pairs of AA"s offer a useful solution – with more current capability – but still requiring UP conversion?


Connecting batteries in parallel is IMHO a big No-No, as the lowest batt draws power from its siblings. Placing an empty and a full batt side-by-side might lead to damaged batteries and dripping acid, or even fires.

OK, one could (to be precise: SHOULD) add diodes, but that would waste too much power.

Another idea (as I thought about connecting the RPi via serial) was to use a CAT5 for the serial line, and just provide a 48V source via the unused lines, and then just step down from 48V to 5V, or use the daughter board to connect to the TP ethernet pins 4,5,7,8 to draw from PoE, and skip the UART altogether.

Phil Spiegel
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 10:08 am

Batteries in Parallel is a common occurance – BUT YES they must be matched!

(Examples are vehicles – admittedly with rechargeable batteries – but that is a sensible choice anyway)

Our camper van has electric cooking with 2 Optima 75Ah batteries in parallel and capable of supplying 3kW for a considerable cooking time (Each individually fused at 400A but usually less than 100A each)

I"ll agree that NON_RECHARGEABLE cells should not be placed in parallel - because there is an inherant risk of one trying to recharge the other .

Admittedly I was thinking of "permanent" battery installations such as used for Off-Grid Solar Panels (of which our camper van is an example – with 2 solar foils on the roof)

Simply stacking batteries for increased voltage is not always an appropriate solution!

plugwash
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 12:21 pm

It's much easier to design a converter that only converts in one direction (up or down) than it is to design one that can convert in both directions (up AND down)

Also higher voltages tend to mean higher efficiencies.

Given that and the Pis relatively high current consumption if I had to run one from AAs I would go for 6 of them in series (9V when new dropping to arround 6V when virtually flat) and a step down converter.

Sets of batteries should always be matched regardless of whether you are running series or paraell. Unmatched batteries in paralell can lead to cross charging while unmatched batteries in paralell can lead to reverse charging (especially with larger sets).

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Gert van Loo
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 2:52 pm

All of this has already been extensive discussed in about a dozen posts about power.

treczoks
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 10:42 pm

Gert said:


All of this has already been extensive discussed in about a dozen posts about power.


Not exactly. All the posts about power topics I've found in the forums supply the power via the USB connector. I'm specifically asking about feeding the RPi via the expansion port.

The how and where to get the 5V from is not the primary issue. Battery is one option, PoE just another, or it could be some unused cables on a otherwise RS232 connection.

Lynbarn
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 11:31 pm

Ttreczoks said:


Gert said:


All of this has already been extensive discussed in about a dozen posts about power.


Not exactly. All the posts about power topics I've found in the forums supply the power via the USB connector. I'm specifically asking about feeding the RPi via the expansion port.


Try this, one of the latest on this subject: http://www.raspberrypi.org/for.....nsion-port

plugwash
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Re: Powering the Pi via the expansion Port?

Tue Apr 03, 2012 11:33 pm

treczoks said:


One question remains: Which polyswitch does the RPi use,


Dunno the exact model but afaict it's trip point is somewhere arround 700ma-1A (i've seen conflicting reports).


and how does it cope with getting power at the output pin, while having or not having a power input? Someone might decide to just plug in the micro USB while the thing runs on battery…


A polyswitch is just a fuse substitute that is self resetting (AIUI under overcurrent conditions they heat up and break the circuit, then when they cool down the circuit is restored), nothing too fancy and not polarised. So if you connect a 5V supply to the GPIO header then 5V will appear on the power input connector. If you connect 5V supplies to both the GPIO header and the power inlet connector you will be connecting those two power supplies together (via the polyswitch)

What exactly will happen if you connect two power supplies together is an interesting question that depends completely on the design of the two power supplies. Most power supplies are designed not to sink current so most likely what will happen is that the two power supplies will have slightly different set voltages and the one with the higher set voltage will power the Pi while the one with the lower set voltage will sit there doing nothing.

However if the power supply with a slightly lower set point CAN sink current you may get  current flowing from one power supply to the other. If this current is high enough then the polyswitch will interrupt it (regardless of which direction it is flowing in) but you could still have a pretty substantial current flowing without tripping anything.

TL:DR version: plugging a PSU in while the battery system is connected probablly won't break anything but may well drain your batteries in a hurry.

treczoks
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Re: Powering the Pi via the expansion Port?

Wed Apr 04, 2012 7:07 am

plugwash said:

A polyswitch is just a fuse substitute that is self resetting (AIUI under overcurrent conditions they heat up and break the circuit, then when they cool down the circuit is restored), nothing too fancy and not polarised.
Self-resetting fuses come in many shapes and sizes. And technologies. There are bi-metal mechanical types as well as complex electronic circuits with embeddeed controllers, shutting down via MOSFETs or relays (OK, those are a bit larger than what I'd expect on a RPi-like board). It might not be fancy, but it still might react badly when fed backward. As long as the type is unknown I would not assume it to be non-polarised. If it is indeed a simple non-polarised kind, then we are fine. But thats the piece of information I'm looking for.

plugwash said:
So if you connect a 5V supply to the GPIO header then 5V will appear on the power input connector. If you connect 5V supplies to both the GPIO header and the power inlet connector you will be connecting those two power supplies together (via the polyswitch)
Thats not what I intend to do. My idea is to solely power the RPi via the expansion port. Nonetheless it is important to know what WOULD happen if someone WOULD plug in a power supply while the batteries run - just for safety reasons.


plugwash said:

What exactly will happen if you connect two power supplies together is an interesting question that depends completely on the design of the two power supplies.


Exacetely, as the caterpillar would say.


plugwash said:


TL:DR version: plugging a PSU in while the battery system is connected probablly won"t break anything but may well drain your batteries in a hurry.
OK, so we agree on that. The polyswitch and the regulater on MY side will limit the current drawn from the batteries, so in the unlikely event that someone plugs in a PSU while the RPi is fed by my board it will fry neither the PSU nor the battery.

But the main question still remains: Which kind of fuse/polyswitch does the RPi use?

treczoks
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Re: Powering the Pi via the expansion Port?

Wed Apr 04, 2012 7:10 am

Lynbarn said:


Try this, one of the latest on this subject: http://www.raspberrypi.org/for.....nsion-port


Dang! You got me there

But you should check your calender, it is three days off.

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