Too many drives on a rail?

[cpnhowdy]

I had to get a new PS and I got the Antec TP-II 550. Now I didnt see anywhere on Antec`s website about the number of molex connectors but I didnt think Id have a problem but I do. One rail has 3 molex connectors and the other rail has 2 molex. There are 2 other rails with SATA connectors but my PC doesnt use SATA....

So I was left with connecting 5 hard drives and 1 dvd drive on one rail via Y-splitters, and 4 drives on the other rail with y-splitters. Im not quite sure if 6 drives on one rail is ok.

What exactly would be the maximum number of hard drives on a rail? This PS handles 19A on both 12V rails. The drives are constantly in use; it`s a file server.

On a side note does anyone make a Sata-to-Molex converter? So I could use the other two rails to power my non-sata hhds.

Thanks

 

[Ice Czar]

we generally call that a strand or a lead in here

reserving "rail" to describe the +3.3V +5V & +12Voltages (the three "main" rails)

I limit my boxes to 3 HDD on a lead (approx 6A +12V or 75W)
Opticals arent spinning up at the same time as the HDDs typically would (unless its in the boot order)
so adding an intermittent load isnt that big a deal they average about the same as a drive

HDDs will all hit their maximum draw at spinup, generally about 2A or 24Watts per drive (average)
and since thats happening concurrently most of the time it adds up quickly
then they drop to roughly a quarter of that once they have overcome inertia (down to 0.5A or 6 Watts per drive average)
drives that are in the same RAID array will also perform sychronous seeks (all move the armetures concurrently) that adds a bit more of a dynamic draw over say independent drives, the 0.5A isnt a static load just an average, HDD manufacturers are typically really good at documnting exactly what the draws are, including seek draws

exactly how far you can add drives Im not sure but with 6 HDD thats double what a current PCI-E graphics card is drawing
works out to 150Watts (12A) at spinup. More than Id do.

if some portion of the drives are attached to a RAID controller, it might have a delayed spinup option, common in RAID5 capable cards that would
stagger the peak draws with drives that arent on the array

software monitoring doesnt show spinup draws since the software isnt loaded at that point
in a storage box I often recommend taking digital multimeter reading to observe rail stability

here are some old logs (these are all within "spec" 5% of the baseline) but obviously not all the same stability
http://terasan.okiraku-pc.net/dengen/tester/index.html
http://terasan.okiraku-pc.net/dengen/tester2/index.html
the instability at spinup is pretty obvious
hmmm timed out, wayback alternative if the above arent working
http://web.archive.org/web/20040619112748/http://terasan.okiraku-pc.net/dengen/tester2/index.html

Ideally you dont want to add HDDs to the same lead as your graphics card as they generally arent great when it comes to sheilding and potentially could add noise to the power, but more importantly they are a dynamic load and can induce greater voltage "local" fluctuation on the lead than many cards like

thus many manufacturers suggest dedicated leads for their cards

one last word of caution unlike your mobo that has a voltage regulation modual that affords at least a little bit of a buffer, HDDs lack any buffer and are pretty suceptible to voltage fluctuation and are common victims when things go wrong, running that many HDDs on the same lead likely increases that potential, rather than overshoots (surges) which would fry most drives, it would favor undershoots, which could cause data corruption


Corruption 101

 

[cpnhowdy]

we generally call that a strand or a lead in here



reserving "rail" to describe the +3.3V +5V & +12Voltages (the three "main" rails)

thanks, I wasnt really sure what to call it and misunderstood what a rail is.

HDDs will all hit their maximum draw at spinup, generally about 2A or 24Watts per drive (average) and since thats happening concurrently most of the time it adds up quickly
then they drop to roughly a quarter of that once they have overcome inertia (down to 0.5A or 6 Watts per drive average) exactly how far you can add drives Im not sure but with 6 HDD thats double what a current PCI-E graphics card is drawing works out to 150Watts (12A) at spinup. More than Id do.

Ideally you dont want to add HDDs to the same lead as your graphics card as they generally arent great when it comes to sheilding and potentially could add noise to the power, but more importantly they are a dynamic load and can induce greater voltage "local" fluctuation on the lead than many cards like

thus many manufacturers suggest dedicated leads for their cards
Corruption 101

So if I understand correctly, at startup each drive will use about 2A per drive * 6 drives gives me about 12A total? If the max load on the lead is rated at 19A can I assume that it will be able to handle it? And after all is started up, draw should drop down to about .5A per drive or about 3A total on a lead?
The motherboard is a Asus PS800D-E deluxe so im not powering the vid on any lead or such ( just running in console mode anyway with an old gforcemx) so hopefully that wont play a big part.
I really appreciate your response, I always learn something new reading through your posts.. even if I dont quite understand all of it at first