I am looking for a sanity check on my NAS electrical situation. I see a lot of NAS cases without proper back planes. If the disk bay count is high, this can create some serious cabling & electrical issues to solve. I bought one like this. I hope this posts can also help someone else. 
I must power individual drives directly from the PSU (Corsair SF750). I have 10 HDDs (~8W@12V,.66A) and 4 hi-power SSDs (8W@12V,.66A, 4.5W@5V,.9A). I’ve rigged six more SSDs into the back but we’ll limit this to the reference design first.
The manufacturer-included power solution was 2 x 1-to-5 SATA daisy chain power connectors. I’ve heard these can melt, particularly when molded instead of crimped, if current is too high. The base connector looked molded.
- Assumptions
- SATA connectors can handle 4.5A nominally.
- All power for the chain passes through the base connector.
Not a problem for normal operation, but spinup is double the Amps. That ends up being 9A (double spec) for however many seconds that lasts.
I understand the current at the modular PSU connector would be fine so long as I didn’t attempt to put both 5-sata daisy chains on a single PSU peripheral port (this PSU has three). Apparently any given peripheral port is limited to 5A – 60W @ 12V and 25W @ 5V – a point made with this photo of a burnt PSU. Putting each daisy chain on a different PSU port would put me at 4.25A @ 12V per port during spinup. That should be fine, but the issue at the individual SATA connectors remains.
I don’t think that setup would exactly burst into flames at once, but I’m concerned about things melting or getting brittle over time. Am I being a drama queen or are my math and assumptions sane?
For now, I’m sticking to Molex peripheral PSU cables and splitting off some high quality (18AWG & crimped) molex-to-sata splitters for the HDDs. Each pin is apparently rated to 11A, so it seems more robust. All of the Molex makes for an inflexible cabling nightmare though, so I’m open to a challenge I’m doing this for no reason at all.
Epilogue:
The questions of electrical and heat are especially important for this build, because I rigged an additional 6 SSDs (4.5W on 5V - .9A) to the rear 120mm fan. With the additional cabling I added, the thermals weren’t great even before the extra heat of these six drives…
Fun story: initially I had 4 SAS SSDs in here and a 25Gbe Mellanox NIC. During testing, both low-rpm 120mm x 120mm x 30mm fans (attached to PWM header) stalled in this case for ten minutes before I realized the failure. I’ve never seen so many simultaneous critical thermal warnings. The outside of the case was scorching to the touch, and I burned my hand on one of my SAS SSDs trying to remove them in a panic.
I jettisoned the Mellanox NIC and replaced the SAS SSDs with lower-power SATA ones, and resolved my fan issues by attaching them directly to a PSU connector instead of a PWM header.
And to anyone who asks, I have a Sliger CX3701 as my TrueNAS case. To be fair, the above episode was my own fault, but I still wouldn’t recommend this case to 95% of people – even those who intend to use it as directed. I’m using it because it was my last resort for my disk capacity & physical depth constraints in a rackmount form factor.
Here is the 3d-printed bracket I made to attach to the 120mm fan in the interior of the case. It’s designed for 4 x 15mm SAS SSDs + 2 x 7mm SATA, but if you use it, stick to low-power SATA drives & a high static pressure 120mm fan to avoid turning your case into a space heater. Or don’t use it at all and get a better case…
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bottom_ssd_holder_long-gussets.txt (5.4 KB)
