Reasonability check for the safety of a home-hacked power supply

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Reasonability check for the safety of a home-hacked power supply

Postby vivitern » Thu Nov 16, 2017 1:04 am

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 (like the ESW105-ML:
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 from philips (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.

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Re: Reasonability check for the safety of a home-hacked power supply

Postby Xenocrates » Thu Nov 16, 2017 12:54 pm

The biggest issues I see will be keeping the two isolated from each other properly, and heat accumulation inside the case. I would put a thermal fuse such as this one. Additionally, the monitor may not be matched in polarity with the wall wart, as some are center pin positive, and others are not. Beyond that, I might also heatshrink wrap the PCBs, or kapton wrap them. It is a good idea to test that it will not overheat by connecting loads equal to around 80% of the rated load, or at least as much as the expected load, while keeping it on a non-flammable surface, or with a fire extinguisher nearby, and watching the temperature using a probe. Be aware, if you use a metal surface, it will conduct heat away, and skew your results somewhat.

But, I'm not an EE, and be careful, it could burn your house down.
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