I am in the process of building myself a fancy schmancy Raspberry Pi "laptop", and am trying to power it with a single cord/power supply. My strategy is to put together a small project box with 120VAC inputs, and the innards from a couple wall warts to provide 5VDC and 12VDC power. Before I start wiring crap together, I wanted to run my idea past some more experienced electri-gurus to make sure I'm not missing anything safety-wise. It seems like its the perfect solution in my mind, but want to make sure I'm not missing anything that I'd be expected to know if I wasn't self-taught.

My parts:

  • I have an old laptop power supply that I've gutted. It's basically the cord and an empty plastic shell (2" x 3" x 5"-ish) with the male end for the plug, and the wires that were clipped from the PCB from the male cord receptacle.
  • I've gutted 12VDC/2A and 5VDC/10A wall warts; I'm left with PCBs that have wires leading to the board, and barrel connectors coming off the board.

If I connect the three hots, the three neutrals and the three grounds from the PCB and power supply inputs, this will leave me with 120VAC feeding the box and getting shared by 2 PCBs that, upon testing SHOULD be putting out 12VDC, 2A and 5VDC, 10A.

Am I thinking about that right? In my mind, it's like I've got two wall warts in a power strip, minus the power strip.

Here's my power needs:

  • The Raspberry Pi: 5V, 700-1000mA
  • A powered USB hub: 5V, 1A (based on the 5V, 1000mA PS that came with it)
  • A powered USB WiFi interface: 5V, .5A ('cause it's USB...amperage is a TOTAL guess based on no information whatsoever).
  • A portable USB keyboard: 5V, .5A ('cause it's USB...amperage is a TOTAL guess based on no information whatsoever).
  • A 4" LCD monitor, 12V, 0.53W (which, by my math is way under 2A).

The 12VDC barrel fits the 12V monitor, so that's set. The 5VDC output needs to have the barrel connector replaced, and as long as I'n stripping wires, I'll be adding 3 USB ports to the power supply for device charging, and plan to split the 5VDC output between the individual USBs and the cable that will supply power to the hub--the hub will supply power to all the Raspberry Pi devices, 3A. With 10A supplied, those 3 extra charging ports won't be an issue unless my phone decides to draw 7A+ for charging. Am I good with that concept as well?

Please let me know if I'm missing any fundamental safety concepts for putting this thing together, and if appropriate, an appropriate link for where I can research & read about these concepts.


3 Answers 3


I think this is not a good idea. You should use a 120V power cord attached to a fused power entry module http://www.digikey.com/product-search/en/connectors-interconnects/power-entry-connectors-inlets-outlets-modules/1442743, in turn connected to the right 5V/12V AC to DC converter rated for the currents you're interested in.

Alternatively, just use a 12V wall wart with a high enough current rating and appropriate DC to DC converters or voltage regulators (linear or switching), with the whole deal fused.

  • \$\begingroup\$ If you go with the power entry module/power supply, be sure to buy all the little drawers and things that are usually sold as accessories to the power entry module. \$\endgroup\$ Jan 20, 2014 at 22:37
  • \$\begingroup\$ The inlet I'm using (from the original laptop power supply case is a C6), but I'm not sure it's fused. Excellent idea to add a fused one, and I also like the fused ones with an on-off switch. One question about how DigiKey's catalog. All the C6 Connectors (filtered from the above link) note either "fuse not required" or "drawer required". Does the latter mean "this takes a fuse, make sure you get the drawer?" I can't seem to spot a simple "fused/not fused" sort column. :\ \$\endgroup\$
    – dwwilson66
    Jan 21, 2014 at 13:18
  • \$\begingroup\$ @dwwilson66, yes, drawer required means be sure to buy the drawer, but such warnings are not always guaranteed to appear. The only way to be sure is to look at the data sheets, especially with digikey. Digikey does make this a bit easier by giving you something like "related products" near the bottom of the list of specs on the product page. It was actually easiest in a paper catalog, where everything was on one page, or when digikey still gave you a link to the catalog page, so you could look. \$\endgroup\$ Jan 21, 2014 at 13:32
  • \$\begingroup\$ and don't forget fuses!! \$\endgroup\$ Jan 21, 2014 at 13:33
  • \$\begingroup\$ The fuses are a given. :) But thanks for the reminder in case I order some morning before coffee. I've done that. \$\endgroup\$
    – dwwilson66
    Jan 21, 2014 at 13:34

Areas of concern:

  1. The mains connections between the wall-wart PCBs and the mains wires going to the cord. Ideally these need to be tied together in a mechanically secure manner - soldered and taped would be fine, as would bringing them to a terminal block with ring terminals.

  2. The mechanical security of the wall-wart PCBs. They cannot flop around inside the case - there must be minimum creepage and clearance between primary and secondary which cannot be guaranteed if they can bounce about freely inside the box. Any secondary circuit is not allow to approach any primary circuit else there's a shock hazard.

  3. The environmental security of the new case. Wall-warts are fairly impenetrable. By cracking them open and stuffing them into another shell that was well-sealed, they're not so impenetrable now. What happens if you inadvertently spill water on the case?

  4. Lack of safety approval. Any safety approvals on those little wall-warts are null and void as soon as you crack them open and start attaching wires to them. Should the unlikely happen and your improvised brick goes up in flames, good luck collecting any insurance money.

  • \$\begingroup\$ 1.) I was thinking wire nuts with tape. 2.) the wall-wart PCBs will be secured with a dab of hot glue. 3.) a bead of silicon caulk in addition to the epoxy to glue the shell back together. 4.) I got so many broken-warranty things I've hacked already...but I'm trying to minimize the burst-into-flame probability. :) Thanks for the feedback! \$\endgroup\$
    – dwwilson66
    Jan 21, 2014 at 13:07

Using hacked together power supplies can be dangerous if you aren't sure what you are doing.

Luckily there are tons of off the shelf power supplies thay already do exactly what you want - 5v and 12v in one supply. You will find them for many external hard drives with a 3 (5v, 12v and gnd) or sometimes 4 pin (seperate grounds) cable.

I would simply take the connector off an old external hard drive enclosure and use its power adapter as is - its already been tested by the appropriate regulatoey agencies to not be likely to burn your house down.

  • 1
    \$\begingroup\$ Only if he finds one that does 12v 2A + 5v 3A. Which is alot higher than a typical 3.5" hard drive needs. \$\endgroup\$
    – Passerby
    Jan 21, 2014 at 4:59
  • \$\begingroup\$ @ppasserby most do at least 2A on each output. Some do much higher. And they can often be had on ebay for about $10. \$\endgroup\$
    – Grant
    Jan 21, 2014 at 5:36
  • \$\begingroup\$ ...and I have a whole mess of external drive enclosures in a box in the studio. A number of them have a transformer box with what looks like an S-Video input for the power supply. I like this idea; I can just create a small box with a female power connector that has a couple outputs with barrel connectors/USB Ports for my devices. \$\endgroup\$
    – dwwilson66
    Jan 21, 2014 at 13:32
  • \$\begingroup\$ @dwwilson66 yep, the ones with the svideo like (mini DIN) plug are the ones I'm talking about. \$\endgroup\$
    – Grant
    Jan 21, 2014 at 13:39

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