nickodemos said:
Hello all.
Was reading up a bit on power suplies and saw that there were
a few with 2 rails and some with 4 rails. What difference does this
make if any?
I also saw this.
+12V@18A (2 Rails), +12V@30A (2 Rails)
Does this imply that it runs as 18 amps and has the ability to
jump to 30 amps? O is it two rails at 18A and two more at 30A?
A dual rail might look like this:
http://preview.tinyurl.com/bfsrdq
It's just one 12V supply with each "rail" simply being a current-limited
output from the same source. The total current draw for the 12V supply
is sliced up across each rail.
For a PSU to be UL approved, "Under normal or overload conditions, no
output shall continuously provide more than 240 VA under any conditions
of load including output short circuit." See "Multiple +12V Rails"
section in:
http://en.wikipedia.org/wiki/Power_supply_rail
The implication is that the more rails you have then the more current
can be drawn from the single 12V source. The power output of the 12V
source might not change with the addition of more rails, plus rails mean
less power per rail and the user has to figure out how to balance the
loads across those wires on the same rail and between rails (which might
not be clearly identified on the wires). Sometimes a special output is
marked for the video card usage. Gamers might prefer singe-rail PSUs
but they may not be UL approved and could cause fire hazards.
Examples:
-
http://img145.imageshack.us/img145/9131/psuiv2.jpg
600W PSU, one 12V source with 4 rails, each rail limited to 18A.
-
http://eu.shuttle.com/archive/en/prod/xpc/600/sn41g2v2_psu_sticker.jpg
250W PSU, one 12V shource with 1 rail limited to 16A.
Be careful when adding up the total power of each rail. The values are
max peak loads and not max sustainable loads. In the 2nd example, the
wattage for all rails, and accounting for the combined max wattage
across the split rail for 3.3V and 5V outputs, the total just using the
table is 310.6W (12V*16A + 5V*2A + 12V*0.3A + 105W). Yet it is rated as
a 250W unit. It can supply 250W continuously (well, you hope it can)
and not the total max load shown for all the outputs.
Many users way overrate the wattage they need for their host. Part of
this is due to many PSU manufacturers lying about their sustainable
wattage load (and instead quoting the peak load but at which there is
very poor regulation). As a consequence, users have become accustomed
to using about 60% to 70% of the rated value as the usable value, so
typically they look at twice the wattage they need in a PSU. As the
wiki article notes, a *good* PSU of double the wattage capacity (where
it is the sustainable load for that PSU; i.e., they don't lie and may
even underrate their PSU) also means the PSU runs cooler since it was
designed to dissipate heat at its rated wattage or use better design or
components that won't generate as much heat (i.e., higher efficiency).
You need to check how many amperes your video card needs if it is the
biggest consumer of power. Often they list minimal wattage for the PSU
(assuming that the max current for the 12V rails will go up to
accommodate their product) but that doesn't tell you the minimum you
need on one of the 12V rails for the video card itself. I've seen a
couple that require 36A and one that wants 46A
(
http://www.evga.com/products/moreInfo.asp?pn=017-P3-1291-AR).
