Raspberry Power DC Power Supply

Hi Paul,

Your design on its own looks fine (just an EAGLE thing, if you use the 'Net' icon instead of the 'Wire' icon to draw your wires, then it will automatically add blobs at junctions; the 'wire' icon need  never be used in a schematic, although it works electrically just like the Net icon) and will definitely function. But looks like you've got to grips with EAGLE quickly overall.

The only concerns are (1) If you use a (say) 9V DC supply, then around 2W of power is dissipated if the RPI consumes 5V 500mA since (9-5) * 0.5 = 2W (the RPI may use less power, the 500mA was a guess). 2W is a enough to get a small heatsink hot : (

and (2) lack of schematics for the RPI B+ make me uncomfortable wiring to the pins like this, although in theory it should be fine

and (3) might be worth double-checking the pinout of the 7805, most models have pin 2 as the ground, not pin 3, but I wasn't sure which package you were using, so it may be fine.

So to summarize, (assuming the pinout of the 7805 is correct) in practice this board would probably be fine, as long as a (say) 7V supply was used (although that may be borderline for a 7805) or with a heatsink.

Well... > say 8V input would require a heatsink. And the current that the raspberry pulls would require > 8V to allow for the dropout of the 7805. (Datasheets usually claim about 2V, but that's "typical at 25C", while calculating the worst-case situation you quickly go towards the "rule-of-thumb" that 3V is required).

I have 5V powersupplies with that plug. So I was thinking of simply "dropping the regulator". On the other hand, adding a simple-switching powersupply would also be "nice".

Thank you for that shabaz. I will replace the wires with nets and look into the 7805 heat concerns and pinout.

I agree with all you mention. I too have plenty of supplies that are 5V with that plug. That might be a better board Paul, nice and simple - just a DC socket and the connector for the RPI.

The other option Roger mentions would be pretty cool too, especially if it could handle automotive supply, and had some latching Molex connector or something as well as the DC socket you already have. But then it is a more complex circuit although probably there are some off-the-shelf DC-DC converters that could be soldered on.

Thanks Rodger.

Will look into switching powers supplies too.

Some great suggestions here. Thanks guys!

About 7v is the minimum for good regulation.

Even at this point there will be some heat dissipation issues @0.5A unless you have a decent heatsink.

3 ideas:

Decent heatsink(could be the most expensive part of  the design!)

Try a 78S05 rather than 7805 as it has a 2A operating extending the range of your supply

Try one of those small switch mode modules available from EBAY for £1-2 ea that are variable output at upto 2A.. Cheap and easy

Ha another if you get one of those 5V DC Jack supplies you can easily get 5V @4A  so you won't need a regulator at all

good luck ... John A

Damn I fell asleep before answering the question and you's already done it

....Yawn said Bagpuss

Without stating that it would be an appropriate method to use here or not, let me just mention that sometimes, another method to spread heat around that a VR must dissipate, is to use intermediate regulation potentials. 

If you are really into bread boarding the power supply, I've had good luck with these tiny switched regulators (http://www.pololu.com/category/131/step-down-voltage-regulators).  I mount them onto the bread board when prototyping, or solder onto the circuit board when the design is complete.  I can hardly build one from scratch for the price of these switched power supplies on a breakout.  Also  note, that the input voltage on these is quite high, allowing for use from a car electrical system with all it's over voltages.  There are also step up and step up/down which are great for battery use.  Good luck.