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Pages such as this discuss wiring ATX PSUs in parallel, in what seems to be a sensible manner, with links to various projects showing details.

Companies with a decent reputation for ATX PSUs, sell cables specifically for that purpose, and many others sell them on Amazon.

I'm interested in running two PSUs in parallel but would like to know how safe it is, if done right.

I'm running a large (20+) HDD array on an underpowered but extremely high quality 750W PSU. I need to upgrade the PSU to ideally get around 5v @ 35-40A and 12v @ 115-130A. Dual PSU with as second 750W - 850W PSU seems to have a lot of advantages over replacing with a 1600W or higher PSU, and perhaps no disadvantages. Both would be same brand, possibly same model, single rail, and very high quality build+output.

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(OP updated: image added as recommended by comments/replies)

Electrically it looks simple enough. Give them both the same shared "soft start/stop" feed, and crosswire 0v to give them a common ground plane. I'd have a stability advantage over the mining users in that I don't want extra power for PCIe graphics cards, my need is extra 5v/12v power with a common ground plane just for external HDDs. So I'd have one PSU handling the baseboard and 1/3 of the HDDs, and the other PSU handling the other 2/3 HDDs.

Parallel PSU also beats a larger monolithic PSU in other ways that benefit me: I need disproportionate 5v, more than even a 1600W PSU would provide (5v @ up to 36A needed for HDDs but not used by PCIe/graphics cards which large PSUs target, so max on even a 1600W PSU is about 24-30A while dual gets me 48A). It doubles my HDD connector sockets so I can reduce current/heating in any given string (5-6 HDD connections per PSU, so only half the number of HDDs on each). It also spreads heat dispersal between them. I don't have resources for a formal dual-PSU case of the kind I'd need, so it's either dual high quality standalone PSU or large PSU. So I'd like to do it if I can.

In theory I'm guessing there shouldn't be much scope for problems due to current loops. Also, unlike wiring in series (see this question), their 0v planes would be connected so even if both PSUs were earthed, I guess the earth would be in common for both anyway, which might help avoid earth loop issues.

In theory, provided it works electrically, I use very high quality PSUs from the same brand/model range (which I would), and I double check the crosswiring and junctions on any crosswiring cable before using it, dual PSU is a far more efficient solution, gives several electrical benefits, and costs 1/4 the price of a monolithic 1600W PSU while guaranteeing that both 5v and 12v peak current capability are met.

But electrically, the PSUs weren't explicitly intended for dual use so I don't know. How risky is it? How safe if done right?

Is this pretty much how any dual PSU works?

Am I missing any other important safety/setup issues that I have to know/do, to dual PSU safely and not imperil my equipment?

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  • \$\begingroup\$ This can't be known for certainty without analyzing the actual circuits used. But since they were not designed for load sharing, my gut tells me that if you do this with two stiff power supplies then one (whichever has the tiniest higher output voltage) will run at maximum current while the second will pick up the remaining load when the first folds back. \$\endgroup\$ – Michael Gorsich Jan 23 '18 at 12:23
  • \$\begingroup\$ Interesting. So instead of sharing, one would ramp up with the other idle, and when it reached its maximum the other would start to take up the further load? But in this case they're powering different devices (one only powers baseboard+some HDDs, the other only powers the other HDDs) and only have 0v crosswired/shared, so I'd have thought they can't current-share, as they never power the same loads or devices AFAIK. Would this apply, or what else...? \$\endgroup\$ – Stilez Jan 23 '18 at 12:34
  • \$\begingroup\$ I have no idea how you are going to wire them up exactly, but two sources trying to get one point to slightly different voltages sounds like an oscillator to me \$\endgroup\$ – PlasmaHH Jan 23 '18 at 12:57
  • \$\begingroup\$ I'm doubting that you array is sooo big, that a 750 W PSU is not able to handle them. With 750 W you should get up to 50 to 70 HDDs. Note that the current requirement given are typically the peak currents during spinup which can be mitigated by staggered spinup. Or is you machine running with other high power components? \$\endgroup\$ – Arsenal Jan 23 '18 at 13:14
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    \$\begingroup\$ "parallell wiring two ATX PSUs" is not the same as "using two PSUs and distributing loads over them". Please be clear which of the two situations you are referring to. \$\endgroup\$ – marcelm Jan 23 '18 at 17:15
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If in doubt don't parallel them - it could end messy. However, you should be able to split the load into two halves and use one supply for about 50% of devices and the other supply for the remainder. You should cross-tie the 0 volt ends of the two power supplies as well.

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  • \$\begingroup\$ I've added a diagram to the OP, does this help to clarify? I think it's roughly what you are describing. \$\endgroup\$ – Stilez Jan 23 '18 at 18:12
  • \$\begingroup\$ Yes this is what I'm suggesting but please make it clear in your question that this is an edit following recommendations so that people don't think I'm just reiterating (in my answer) what you said (ostensibily) originally in your question. \$\endgroup\$ – Andy aka Jan 23 '18 at 18:22
  • \$\begingroup\$ Done - added a note under it \$\endgroup\$ – Stilez Jan 23 '18 at 18:29
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As one of the biggest players in storage pods, Backblaze is one good source of information around these systems.

As they mention, they use two power supplies not for redundancy but for splitting the load between them. I haven't found the details on my quick search but they should be around as they provide all the information for a rebuild of their storage pods.

From that article above they seem to do it like Andy aka suggests - put one half of the drives on one PSU and the other half on the other PSU.

The linked article also describes that they were trying out a single PSU solution with a 750 W PSU - which seemed fine even in the advent of all drives starting at the same time. That scenario can further be mitigated by staggered spinup, so that only a set number of drives spin up at a certain time.

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  • \$\begingroup\$ I've added a diagram to the OP, does this help to clarify? I think it's roughly what you are describing \$\endgroup\$ – Stilez Jan 23 '18 at 18:14
  • \$\begingroup\$ I found their design for it (under "pod 1", thanks for the link!). But they don't discuss the electrical side of having 2 PSUs as asked in the OP. For that, I think this place is better to ask. Backblaze's post (as well as many mining posts) certainly helps to confirm it's doable - but they may have magic or grounding I know not of! \$\endgroup\$ – Stilez Jan 23 '18 at 18:33
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Someone gave a link to this question when asking another question on a third party site, so I just wanted to provide some clarity.

Companies with a decent reputation for ATX PSUs, sell cables specifically for that purpose, and many others sell them on Amazon.

The only thing these cables provide are a convenient way to start and stop a second PSU when turning on your computer. You could achieve the very same thing by shorting the black and green connections that the adapter for the second PSU utilizes. The cable is simply sharing the state of this connection across the two PSU's.

Is this pretty much how any dual PSU works?

The term "Dual PSU" is ambiguous. You typically have two types of ways two or more PSU's are configured:

  1. "Load Sharing" or "Combined". This seems to be what you're describing in your post. You're utilizing the combined capacity of the multiple PSU's. For example, you would use the Molex/SATA from one PSU to power one device, and a Molex/SATA from another PSU to power another device. If a PSU failed, you would still have the other running.

  2. "Redundant". This is what you typically see in server configurations. The individual/separate power supplies are sized so that they can power 100% of the load by themselves. These PSU's are typically "hot-swappable", and don't have any cables coming off of them, they just have metal contacts to touch the rails of the system that manages the redundancy.

In either case, it's not technically what I think most people would consider truly parallel from an electric perspective. That would look more like physically combining all the 3.3V, 5V, and 12V sources from both of the power supplies to increase their amperage, and running new Molex/SATA from that.

Dual PSU with as second 750W - 850W PSU seems to have a lot of advantages over replacing with a 1600W or higher PSU, and perhaps no disadvantages.

At least one disadvantage I could think of with details you provided would be if the PSU that was powering the 2/3 of the ~20+ HDD's failed. You would have have the OS running, and whatever was managing the state of the ~20+ HDD's ( providing they're not running as 20 independent disks ). Whether that was a RAID controller, or something like ZFS, it might start to trigger a process that would make unnecessary work to restore everything where as if all the HDD's were running off of the same PSU the restoration of the disk would be significantly easier.

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