Rewiring Power supply. Noob question

[nmpnmp]

hi all,

I am not particularly experienced with electronics and need some help.

My NAS PSU failed and instead of purchasing an exhorbitantly priced replacement from the vendor, I purchased the same PSU direct from the manufacturer. Actually it’s the same PSU but higher wattage rating and has the standard ATX connectors. The NAS vendor has custom pin outs.

The NAS device uses a standard flex ATX PSU with the main board running on the 24Pin ATX connector. All well so far.

The backplane which connects the disks uses a custom 20 pin molex connector. This connector is only providing power to the disks, and has 10 ground (black) and 5 x 12v (Yellow) as well as 5 x 5v (Red) wires. I was aware of this and had planned to strip the standard SATA, Molex and FDD connectors from the PSU and rewire these to a 20 pin Molex.

What I didn’t count on is that the PSU doesn’t have 5 x 5v and 5 x 10v outputs direct from the PSU as I had hoped. Instead it has a single set of wires from the PSU with perhaps 3 Molex connectors attached to it, or two SATA adaptors, etc.

I think I’m correct in thinking that the wire gauge will support up to three devices attached at a time, and drawing power as these cables had three connectors on but just want to confirm whether this is going to be a problem or not.

Also, in order to have the required number of connectors to the 20pin Molex I have looped wires from one terminal to a second. See pictures.

Original Molex Connectors on PSU

Example of looping wires between connectors on 20 pin Molex.






Is this ok? Can anyone foresee any problems with this?

Many thanks for your help, and apologies for the noobie Q’s.

 

[kellys_eye]

Second pin down doesn't seem properly seated.....

Doubling up on the wiring would normally be ok - I've seen this done on ordinary dual HDD power cables many times - but find out the total amperage for all the HDDs in use first. Many modern drives have very low actual power requirements so the wiring can easily handle it.

Let us know the total and we can check from that.

 

[nmpnmp]

Second pin down doesn't seem properly seated.....

Doubling up on the wiring would normally be ok - I've seen this done on ordinary dual HDD power cables many times - but find out the total amperage for all the HDDs in use first. Many modern drives have very low actual power requirements so the wiring can easily handle it.

Let us know the total and we can check from that.

Thanks for the reply. I haven’t finished wiring it yet but thanks for pointing that out.

With regards the disk, there are six in the enclosure and a mixture of Seagate and Hitachi.

The 4 Hitachi have the following ratings:

- 5v - 420mA
- 12v - 700mA


The 2 x Seagate are rated as:

- 5v - .72A
- 12v - .52A

Not sure how to calculate total.

Thank you for your help!

 

[kellys_eye]

4 x 420mA = 1680mA (1.68A) plus 2 x 720mA - 1440mA (1.44A) = total 3.12A @5V

4 x 700mA = 2800mA (2.8A) plus 2 x 520mA = 1040mA (1.04A) = total 3.84A @12vdc

So under 5A total for each supply - seems ok to me using the wiring you have connected.

 

[dave9]

You list 6 HDD ( 4 x H + S x S), but earlier you implied support for 3 HDD?

HDDs can have higher spinup current than that listed, but generally one lead will support 3 HDDs, especially if they have staggered spinup.

You did not need to loop the wires in the 24 pin, it doesn't really do much since they are adjacent pins and bound to be on the same ground or power plane on the board. Granted, the NAS is unlikely to need that many power leads so it was cost effective for them to just design for a standard PSU connector.

Regardless, what I would do, instead of looping that 24 pin, is take one ground, one 5V (just cut the loops and wire from either end), and one 12V from it, and run that to the backplane along with the 1 x of each from the molex 4 pin you're already planning to use.

 

[nmpnmp]

4 x 420mA = 1680mA (1.68A) plus 2 x 720mA - 1440mA (1.44A) = total 3.12A @5V

4 x 700mA = 2800mA (2.8A) plus 2 x 520mA = 1040mA (1.04A) = total 3.84A @12vdc

So under 5A total for each supply - seems ok to me using the wiring you have connected.

Thank you @kellys_eye, really appreciate the quick reply. I’ll go ahead and get this wrapped up.

One other question if I may, I will be looking to upgrade the disks in this enclosure soon, as they are quite old and also I need more capacity. Is there a certain Amperage total that I should look to keep the replacement disks when I go shopping for them?

 

[nmpnmp]

You list 6 HDD ( 4 x H + S x S), but earlier you implied support for 3 HDD?

HDDs can have higher spinup current than that listed, but generally one lead will support 3 HDDs, especially if they have staggered spinup.

You did not need to loop the wires in the 24 pin, it doesn't really do much since they are adjacent pins and bound to be on the same ground or power plane on the board. Granted, the NAS is unlikely to need that many power leads so it was cost effective for them to just design for a standard PSU connector.

Regardless, what I would do, instead of looping that 24 pin, is take one ground, one 5V (just cut the loops and wire from either end), and one 12V from it, and run that to the backplane along with the 1 x of each from the molex 4 pin you're already planning to use.

Sorry for the confusion, when I was referring to three disks, I was referring to how the PSU came from the factory with a single lead with three Molex connectors on it, so assuming that the cable was high enough gauge to support the draw of up to three devices. I won’t we doing that, at most I will loop only once.

Do you mean the 24 pin connector going to the Mainboard or the 20 pin going to the backplane? The 24 pin connector works fine. It’s the 20 pin I have to customise.

What you suggest is probably better overall, but I’m not particular knowledgeable about this sort of stuff so don’t really want to start soldering or connecting directly to the mainboard, purely because I doubt I could do a good enough and safe enough job. Hence why I’m using Molex connectors and crimping directly to the cable. On which note, do I have to solder the cable into the Molex connector before crimping or is crimping sufficient? I’ve found i’m getting good connections and the mechanical grip is good too, just with the crimp tool.

Thanks for the suggestions.

 

[kellys_eye]

I wouldn't trust a 'single core' of the wiring to the molex connectors to be good for anything more than 5A. This is why they 'double up' on the numbers of wires. For a PSU with a rated 5V output of 50A (for example) then TEN wires isn't unreasonable....

Look at any PC power pack and the cables used to feed HDDs rarely have more than TWO power connectors on each 5V/12V/GDN loop-out. The cables feeding the mobo are quite different!

I always err on the side of caution though so the 'ten' wires I quoted could be less in practise - the deciding factor would be the c.s.a (cross-sectional area) of the wire cores as they have tables listing the maximum current carrying capacity for wires of given c.s.a.

 

[nmpnmp]

I stand corrected, you’re right, the PSU came with only two connectors per set of cables, not three as I said earlier. Regardless, I’m only looping or chaining over two Molex connectors anyway.

The PSU has the following output specs.

Output Specs:
3.3V @ 16A
5V @ 18A
12V1 @ 16A
12V2 @ 10A
-12V @ 0.8A
5Vsb @ 2.5A

Based on this, I could support safely up to 18A on 5V and 16/10A on 12v. Does that sound correct?

Thank you once again.

 

[kellys_eye]

5V @ 18A = 90watts
12V @ 10A and 16A = 120watts plus 196watts = 316watts.

Total is 406 watts - lots of those PC power units state the MAXIMUM TOTAL power they can supply which is different from the volts (times) amps totals they also list! e.g if the power supply is rated at 350 watts you can't draw the full total of those supplies from them.

Keeping within the total power limit as stated on the power unit is recommended and even then I wouldn't go anywhere near it for safety sake i.e. stick to a maximum of (say) 80% of what they claim it can deliver.

Of course, if your power pack is rated at 650 watts then this easily encompasses the TOTAL of the 5V and 12V outputs so .... no problem.

 

[nmpnmp]

Thanks for that, Its actually only a 270w rated PSU, its this one

https://www.fspgroupusa.com/ecommerce/fsp270-60le.html

Clearly overstated the total output.

I'll take a look around at some new disks and keep as close to the old power ratings as possible.

Many thanks for all your help. I managed to get the crimping done and the NAS is up and running once more, and I have access to all my data.

Many Thanks

 

[kellys_eye]

You can take the full 5V output at 18A (90W) + whatever wattage (at 12V) is 'left over' (and from 270W - the derated maximum at 50^oC this is another 180W) which would be 12V @15A.

I'd think that 5V @ 18A and 12V @ 15A would be more than enough to power any HDDs you fit!

 

[nmpnmp]

That’s great to know. Thank you for your invaluable help. Much appreciated.