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I am trying to build a mini computer that contains a Raspberry Pi and a 3.5" HDD, with built-in PSU so it requires only a single 120v input.

I'm thinking about a PSU like the MeanWell PD-25A, which provides 2.1A @ 5v and 1.2A @ 12v. I would solder a Micro USB cable to power the Pi, and solder a 15-pin SATA power connector to both 12v and 5v.

Will this work, and are there any safety implications?

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Depends how much power the HDD needs. Some may even need more than 1.2A @ 12V so you need to check those specs.

My feeling is that it will probably be ok, but keep the 5V leads to the Pi as short as practical. (If they are long you may need to make them thicker to reduce the voltage drop)

You don't need to use the micro usb. You can instead use the GND and +5V on the GPIO header, or the full size USB ports to supply power. This has the advantage of bypassing the polyfuse on the main power input which is more trouble than it is worth in my opinion.

  • You're right, I might need a larger one to handle the hard drive. My main concern is putting two devices on the same 12v output line, is that ok? – Elliott B Jan 10 '14 at 12:18
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    Could you explain why bypassing the poly fuse is an advantage? In my understanding, the micro USB input has the most protection. – Elliott B Jan 10 '14 at 12:19
  • @ElliottB, Protection from what? Polyfuse are too slow to protect semiconductors in most circumstances. It's hard to imagine a circumstance where it would offer real protection. All it does is add an extra impedance to the power supply that may cause instability problems – John La Rooy Jan 10 '14 at 20:42
  • over-current protection. I've never heard of these stability problems, do you have any reference for that information? I'd love to read more about it. I do see a minor benefit to using GPIO, that is a smaller footprint by not using the micro USB connection. – Elliott B Jan 13 '14 at 0:45
  • @ElliottB, If you check the specs of the poly fuses you'll see that they can take seconds or even longer to operate. They are an inappropriate part to use for protection here. It's up to you whether you choose to or not. I'm not going to google it for you, there are plenty of places (including this site) where it has been discussed – John La Rooy Jan 13 '14 at 0:49
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There's no problem putting both of these devices on the same voltage rail, PROVIDED that you have enough power to keep them within spec. Most hard drives spike their power on spin up, but anything modern should also have a limiter that keeps that constrained. The hard drive you choose will matter a good bit, but just as a reference point I have a 1TB "standard" WD 3.5" HDD here in my desk that is labelled at 0.66A @5V and 0.55A @12V. There are definitely other drives that use more power, but I think you'll find that these values are pretty common for most "general purpose" drives that are out there today. If we add up the power rails assuming 0.5A for the Pi itself, we get: 5V rail: 0.5amp for the Pi + 0.55amp for the HDD -> 1.05amp 12V rail: 0.0 for the Pi + .66amp for the HDD -> 0.66amp

My personal experience is that most things like hard drives are conservatively labelled, because manufacturers want to re-use the same motors and other equipment and it's perfectly fine if they over-state the power on the label (whereas under-stating it gets you in trouble)... so given that you're only at 50% load on the 5V and barely over 50% on the 12V rail, I'm pretty comfortable saying you're OK with this.

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