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I've got an Intel NUC (NUC8I3BEH3) that's using its own 19V, 90w power supply (pictures).

After a few issues with 4TB 2.5" hard drives, and external USB disks, I'd like to use a 3.5" drive, but the NUC's internal SATA power connector does not support this (doesn't provide 12V).

I'm considering an aluminium case (Hammond diecast enclosure, 1590DBK, 187x119x56mm), that's screwed on to the Intel NUC's 2 mounting holes, with a larger hole in the base to route the power, data cables, and maybe allow some air flow.

For power, could I run a wire off the 19V power supply (internal), in a safe way to power a 4TB Western Digital Red hard drive (WD40EFRX), which needs a 5V and 12V supply, 1.75 amperes (peak), at 4.5 watts.

Maybe using two linear voltage regulators to provide 5V and 12V (e.g. L78S05CV and L78S12CV)?

These can accept an input up to 35V, current up to 2 amperes, and have a thermal resistance of 5 °C/W junction-case or 50 °C/W junction-ambient.

Or, as per @bimpelrekkie's suggestion, a fixed switching regulator (e.g. LM2596T-12 and LM2596T-5.0.

But I'm not sure:

  1. Would this be appropriate for a hard drive? Can it provide a clean enough power supply, and keep running for a long period of time (4+ years).

  2. Would it produce too much heat? I'm considering getting some 20x15x10mm heatsinks, with 3M screws, for the 3.75mm hole.

  3. Is there a good way to connect them to the aluminium case, assuming a heatsink? I don't think solder works, or thermal tape would hold well enough, so maybe an epoxy that can also handle high temperatures (just to be sure.)

  4. Will the 90W power supply be happy with the extra load? The computer currently uses 25W while running both of the CPU cores at 100%.

  5. Where I'd connect the 19V supply to the 12V regulator first, should the 5V regulator connect to the 12V (so there is less of a drop from 12V to 5V, maybe helping with heat/life,) or should I connect it directly to 19V (so there is less current being pulled though the 12v regulator?) i.e. series or parallel?

Is there another way of doing this? Maybe two DC-DC step down buck converters? I've never used these before, they do look interesting, but I don't know how reliable they are.

And is there anything else I should consider with this setup?


Previously I've used a separate power supply, with an external USB hard drive, but this has introduced 2 extra wires, and where this computer will be in an office 4+ hours away, I don't want to do that journey because one of the cables have "fallen out" (at least if the computer only has 1 cable, I can get someone non-technical to check it).

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  • \$\begingroup\$ Linear Voltage Regulators NO, harddrives consume too much current for linear regulators, the regulators will get very hot. Forget about using linear regulators, use a switched regulator and don't build something yourself, instead use a module. Search for "LM2596" based modules, those can do what you need. \$\endgroup\$ – Bimpelrekkie Apr 30 '20 at 12:49
  • \$\begingroup\$ Thanks @Bimpelrekkie; When looking at the step-down regulators, I noticed that you set their output voltage, and I wondered if that might be over-complicating the setup, and wasn't sure if they were good for long term use... and same questions apply (run them in series/parallel? need a heatsink? etc). \$\endgroup\$ – Craig Francis Apr 30 '20 at 12:55
  • \$\begingroup\$ Fixed output voltage are set with a potmeter: not more complex and also not less reliable (unless you re-tune the pot every day). You cannot connect outputs of regulators in parallel period. So just don't. Heatsinks should not be needed. Looking at all your questions: you do run a risk here that something will go wrong. The safe option is the one you don't want: a separate housing and supply for the 3.5" HDD. How about using hot-melt glue to semi-permanently fix the connectors in place. Put colored stickers on the connectors so that it is clear where they go. \$\endgroup\$ – Bimpelrekkie Apr 30 '20 at 12:58
  • \$\begingroup\$ @Bimpelrekkie: The "series or parallel" bit refers to feeding both buck regulators from the 19V source (parallel) or connecting the 12V buck to 19V and the 5V buck to the 12V output (series.) \$\endgroup\$ – JRE Apr 30 '20 at 13:05
  • \$\begingroup\$ @Bimpelrekkie, thanks again, I'm just looking at Fixed Switching Regulators, and just to check I'm on the same page, would the LM2596T-12 be a 12v version you're thinking of... and yes, I have tried coloured stickers, didn't work (glue might), where the place it's going to is a mess (fortunately I've only got one server there, and want to keep it self contained as possible). \$\endgroup\$ – Craig Francis Apr 30 '20 at 13:09
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Let's check the current draw from the HDD documentation:

enter image description here

So it consumes 1.75A peak on the 12V line, and apparently doesn't even use 5V since there is no specification. That's high for a linear regulator. The linear regulator will dissipate (19-12)*1.75 = 12W peak. Since we don't know how long the peaks last and what "average" means, it is safer to design the heat sink for peak power. Then the 5°C/W spec on the regulator RthJC is a problem because the chip will be 5*12=60°C hotter than the heat sink, so it's a no-go. You need a buck converter.

19V 90W supply means 4.7 Amps. It'll probably be adequate, although you should measure the power drawn by the Nuc with GPU maxed too.

Unfortunately if you shop online with the usual suspects you will most likely find modules using low quality cheap fake components like "LM2596" ; quotes are a must since many of these modules do not have an actual LM2596, but instead an unknown chip with "LM2596" written on top. Also the last one I tested had the correct inductor value for a LM2596, which was too low for the counterfeit chip, so it saturated on every cycle. Also you will get garbage-tier general-purpose capacitors which can't handle the ripple current and run burning hot. Do not expect any kind of reliability from these abominations. A likely ending for that story is capacitor failure, followed by overvoltage on the output and death of whatever is connected to it.

So I can think about several options :

1) If the HDD does not require 5V.

Check if the Nuc can run reliably from 12V. If this is the case, replace 19V supply with a 12V supply. Done.

2) Use a buck to step down from 19V to 12V (and 5V). Here's one that should be reliable.

this computer will be in an office 4+ hours away, I don't want to do that journey because one of the cables have "fallen out"

3)

In this case a much better option is to buy only commercial off the shelf parts that can be easily delivered if they break and replaced by the end user. Since a 12V power supply for an external hard drive enclosure costs less than a 4 hour trip, I recommend giving a backup one to the customer. If you're afraid of connectors getting unplugged, apply adhesive tape.

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  • \$\begingroup\$ Thanks @peufeu, I’ll check over the details in a bit, but to answer the 5V question, I believe it’s used for the little logic board on it (12V goes to the motor)... and just to check, for the 5V, would you do that from the 19V or on the 12V output? \$\endgroup\$ – Craig Francis Apr 30 '20 at 13:48
  • \$\begingroup\$ Good call on the "PicoPSU-80-WI-32V", didn't know they existed, I've just purchased one, and will see how it goes... out of interest, the fixed switching regulator suggestion from Bimpelrekkie (I found LM2596T-12 and LM2596T-5.0), could they have worked as well? \$\endgroup\$ – Craig Francis Apr 30 '20 at 15:50
  • \$\begingroup\$ Yes they'd work, but you need to design a schematic and PCB, pick the correct components... for a one-off that will take a lot more time than what the ready-made solution is worth. \$\endgroup\$ – bobflux Apr 30 '20 at 17:37
  • \$\begingroup\$ You can also use canned DC-DC converters like these which contain the complete circuit. You'll need to add a couple capacitors though, and if they are PCB-mount you need to mount them. \$\endgroup\$ – bobflux Apr 30 '20 at 17:38
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    \$\begingroup\$ LM2596 is a buck converter chip, but it's just the chip, so it requires an inductor, input & output caps, EMI filter on input probably, a couple resistors to set the voltage, and adequate pcb layout and component choice. The canned DC-DCs I linked are ready-made so everything required is inside, they still probably need a LC at the input for EMI filtering, and a PCB to mount them on, and maybe capacitors if the datasheet says so, but it's a lot less work to do. The PicoPSU is plug'nplay. \$\endgroup\$ – bobflux May 1 '20 at 16:11

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