# lopsisded draw on power supplies

Ken Chase math at velocet.ca
Tue Jul 8 17:15:18 EDT 2003

```So, what's people's experience with PC power supplies and power draw
on various voltage lines?

We have a buncha old but large SCSI drives here that are somewhat hefty, and
we want to power them with as few ATX supplies as possible. We have no
motherboard involved (yes, we have to find a hack to get the power on with a
signal, but I think its just shorting a couple of the pins in the mobo
connector for a sec -- anyone got info on that?).

The thing is we'd only be drawing +5 and +12V out of the thing for the drives.
Im not sure how much of each really, during operation, but the drives are all
listed as max 1.1A +5V and 1.1 or 1.7A +12V (latter for bigger of the 2 types
of drives).

Even the 300W non-enermax cheapo power supply says it supplies 22A of
+12V, which is the limiting factor for # of drives. (It gives 36A of +5V).
The 650W enermax monster we have gives 46 +5V and 24 +12V strangely enough
(strange because its only 2 more amps of 12 for such a big supply.)

Im wondering what will happen if we have a load on only one type of voltage
because of no motherboard or other perifs. Is this a lopsided load that
we should beef up the power supply for? I dont think we should use a 300W
for like 16 odd drives, but perhaps a 400 is enough? Should we go 650?
Is it necessary? We'll certainly use enermax for this, with 2 fans in it.
How close to the rated max should we go? We're looking at 16 drives here,
which is short of the 22 or 24A listed on the supplies.

Thanks.

/kc
--
Ken Chase, math at velocet.ca  *  Velocet Communications Inc.  *  Toronto, Canada
Wiznet Velocet DSL.ca Datavaults  24/7: 416-967-4414  tollfree: 1-866-353-0363

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