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Low voltage power distribution pt.2

A project log for today's assorted project ramble "grab-bag"

Assorted project-ideas/brainstorms/achievements, etc. Likely to contain thoughts that'd be better-organized into other project-pages

eric-hertzEric Hertz 10/24/2019 at 22:423 Comments

OK, I think the conclusion is the renowned star-topology power-distribution idea, oft recommended in PCB design and system cabinets, is not really such a great idea if you've a floor-to-ceiling system cabinet and attempt to run power wires neatly.

Or, in this particular case, also not a great idea.

Which brings us back to the fancy thing [aka main selling-point, to me] about the DC converters I'm using: they've got isolated-outputs. 

Thus: 

A) all the voltage-converters can be powered by a single voltage source [a largish/heavyish bank of series batteries]

B) they can be located anywhere in the system [I'd like to keep the power-system as a single self-contained unit in the overall system. In part because this power-system is multi-purpose; this *particular* setup/use just happens to be the most complex]

C) as long as I run *one* device [rather, "endpoint"] off each converter, there's no concern of "ground" [or other power] "loops" [which, as detailed previously, can be quite a bit more gnarly than the audio-nuts' much-loathed "hum"]

The last point is key to the latest ramblings.

I'll drive the computer off a 15V supply, with a local 5V converter [am actually thinking two; a separate one for USB-powered devices, even though one 5V 6.8A converter should be plenty for both]. [These from scrapped car-usb-chargers rated for both 12 and 24V vehicle systems]. The computer is somewhat mobile, thus the 10ft power cable.

Then hard drives [e.g. backup, large/mass-storage, music, software installers, etc.] can be connected as-needed, minimizing cabling and power-consumption. Each of my IDE-to-USB converter-systems ultimately require 12V going directly to the drive [thus it should be pretty close to 12V, and somewhat stable]. Some have internal 5V converters, others do not. One has a 120V AC power-supply, that will be removed. I'll throw 12V-to-5V car-adapters in, as-needed. Thus, the drives will run off an *isolated* 12V output, again preventing drive/load-current from travelling/"looping" through the USB cables to/from the computer. Then, I'll do the "star" to these guys, as-needed, but keeping them close to the power-bank.

[Which suggests a wise choice may be a long-cord USB hub nearby... to-ponder].

I think that'll do it. There may be some looping between the 12V supply and the various drives through *two* USB cables... I guess these things we just don't worry about. [Seems odd to me]. Again, say one drive is sleeping, the other full-tilt, ~0A and 2A, say the power cables are two feet , even with beefy wire that could be 0.2V difference between "common" at one and at the other, now there's a third path between 'em through the USB cables, the system won't be happy with 0.2V between two different points in the "common" [nor 5V!!! COME BACK TO THIS] circuit, thus the running drive's load-current will actually be split between its main power cable and through the USB cables back down the sleeping drive's power cable. This still seems bad to me, and again, this setup is now hardly different than the ol' two-drive IDE setup in a full-tower PC. Weird.

"Come back to this": we have *another* problem, potentially, in the 5V system... were the drives' 5V coming from the same source endpoint [like common], we'd have the same issues with 5V as with common, since USB also carries 5V.

But, no, it may be even worse, now, as we have two separate converters essentially *fighting* each other *through* the USB cable!

 Imagine one is trying to output 5.01V, the other 5.02V. And, again, imagine one drive runs at 2A, the other sleeps at ~0A... Seems *likely* one converter, plausibly the far one, will take the brunt of the load. Also seems plausible, in the opposite case, one converter will *sink* current, which most are not designed for.

...So, what...? Cut the 5V path in the USB system, maybe...? Fine, maybe even some hubs have diodes on the 5V line... Alternatively, run all drives off a single 5V converter [not ideal, more wires, bigger disconnects] [shit, this *also* applies to my two-converter system at the computer!]... 

and again, what about common?

[I should probably note that a properly-designed USB device probably will take great pains in *not* driving VBUS with another 5V source... but I've seen countless designs where that's not the case. A typical example is a "bus-powered" 2.5in hdd enclosure, which uses two USB connectors [or USB and PS/2, in the early years] at the host-side for power, thus shorting VBUS on two ports... or, even more fun, when the signal connector is connected to the motherboard and the power-only connector is connected to a powered-hub, etc.! Thus, it's kinda hard to *rely* on a device's being properly-implemented when designing a system which might [likely] connect to devices which aren't... Weee! Of course, in my case, it's probably not a bad idea just to 'fix' any I might use regularly... and maybe a simple M-F connector with the VBUS not connected, for those I don't. Still, what about that ground? Nobody cares/d about noisy motor-current going through ribbon cables?!]

Discussions

Starhawk wrote 10/24/2019 at 22:58 point

Oh hi... glad to see you're still around... "You've got mail!" ;)

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Eric Hertz wrote 10/24/2019 at 23:25 point

good to hear from ya!

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Starhawk wrote 10/24/2019 at 23:27 point

Thanks... now go empty your mailbag :P

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