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[sheddyian]

OK, so if I'm going to run my Sheddy webcam server on a Raspberry PI, AND make it solar powered, which will probably be from a 12 volt car battery because I've got several knocking about (and some of those top-up solar chargers), I'm going to need a way of getting the 12 - 13.8 volts down to a decent 5volt for the PI and 5v for the mini USB hub I've got in mind.

My electronics knowledge is patchy, but I believe a DC to DC converter is the sort of thing I want, rather than a voltage regulator that wastes excess current as heat.

Looking on ebay, I see a number of very cheap modules from China that might well fit the bill.  I quite like the one with the built-in voltmeter!

http://www.ebay.co.uk/itm/Car-Led-Display-USB-Power-Supply-12V-To-5V-3A-15w-Power-DC-DC-Converters-akp-/281077360671?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item417184281f

http://www.ebay.co.uk/itm/DC-DC-4A-4-5-32V-to-5-42V-12V-24V-step-up-BOOST-converter-power-regulator-UK-/221193257271?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3380254137

http://www.ebay.co.uk/itm/Waterproof-DC-DC-Converter-12V-Step-down-to-5V-3A-15W-Power-Supply-Module-/330786910039?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item4d046faf57

http://www.ebay.co.uk/itm/1-PCS-LM2596-Power-Step-down-Module-DC-4-0-40-to-1-3-37V-LED-Voltmeter-WST-/271131377887?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3f20b080df

(last one is the one with a meter)

Is that the sort of thing I'm going to want, or is there a better way of doing it?  Can anyone suggest alternatives or agree with my choices above?

Thanks
Ian

[burakkucat]

Yes, that's the way to do it!    Essentially they are switching mode power supplies, designed to operate with a DC input rather than AC mains input.

Any of them could be ideal. Clearly you will need to know the total load current that the R-Pi / hub / camera / what-not will draw and then choose accordingly.

I like the look of numbers 2 & 4, purely because I can see the PCB. I don't like what looks like someone's attempt to erase the markings on the doings of number 2. Number 4 has a lower output current than 1 or 3.

[asbokid]

The first one sold for just 99p delivered!  Hope that was you 

The second one is as a kind of charge pump, that scratched IC discussed here [5]

Number three seems to work okay [3]

The last one is based on the LM2596, a switching-based regulator described in the datasheet as 73% efficient. [4]

Maybe ask on the eevblog [1] or the edaboard [2] forums:

cheers, a

[1] http://www.eevblog.com/forum/
[2] http://www.edaboard.com/
[3] http://jex.thruhere.net/blog/#param=id_342
[4] http://www.ti.com/lit/ds/symlink/lm2596.pdf
[5] http://www.eevblog.com/forum/chat/why-scratch-this-off/

[sheddyian]

It wasn't me who got the 1st one for 99p! 

I'd noticed the numbering scratched off the component, which made me suspicious, so didn't go for that one.  Likewise the ones sealed in potting compound - I'd rather see what I'm getting.

The one with the meter seemed to be well made from the picture, so in the end I decided to go for that one, as I felt that a meter would be a useful bonus - it can show you voltage in or out, so I can use it to see the charge of the car battery, and to check things are ok for the Raspberry pi.

I like the idea that it's variable voltage, I've read (and encountered myself) some problems with Pis crashing because the USB power supplies aren't quite up to it, and there can be a bit of a voltage drop along the power lead and across the PI itself.

So I can measure the voltage on the PI board and adjust until I get exactly 5 volts there, not 5 volts out of the PSU.

Brief anxiety when Burakkucat pointed out it's rated 2A - I'd wrongly thought it was rated 3A like the others. 

However, I can't see my current consumption being that high.  A 1 Amp PSU is recommended for a Model B Raspberry Pi, and this page here http://elinux.org/RPi_Performance#Power suggests I won't see anything near 1 Amp consumption - peak being 750mA during 1080P video playback, which it won't be doing.  Once running I don't expect to have a keyboard, mouse or monitor plugged in either.

I can't find anything on the consumption of the webcam, but as it's USB it ought not to be over 500mA, and in reality surely should be significantly lower than that.

Having had a think about it, I probably won't even need the extra USB hub - the model B has 2 x USB ports as well as Ethernet, and whilst I might need 3 ports whilst tinkering (cam, keyboard, mouse) I'll  need only the camera when it's all running.

I really can't envisage the whole setup needing more than 1 Amp, so the 2 Amp DC to DC converter I've now ordered should be fine for it.

The same seller on ebay
http://www.ebay.co.uk/sch/work4best/m.html?item=271131377887&pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item3f20b080df&rt=nc&_trksid=p2047675.l2562
has lots of interesting modules and LED related stuff - a 42 LED lamp module that works off 12 Volts, designed for car or caravan would suit my shed rather well for intermittent use, powered by the solar charged battery. £3.67 !  Bargain.

http://www.ebay.co.uk/itm/42LED-DC-12V-Car-Vehicle-Dome-Roof-Ceiling-Interior-Light-Lamp-White-Silver-WST-/390559960664?pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item5aef300e58

Ian