Power Supply Question

[Kris Rawlison]

Hello,

I'm getting ready to put a new rig together and I've been looking at what I
plan to put in and how large of a PSU I'll need. I've used some of the
online PSU calculators and all of them are reporting estimates in the
500-550 watt range (this is before my case lights hehe). So what I've been
thinking about is should I just get like two 400W jobbers and have the
second turned on via a relay, and that way I can put say the HDD and
lighting on one supply and the motherboard, and DVD burner on the other. As
far as I can think it's the only way I can kludge it together.

What I'm putting in:

Abit NF7-S w/Barton 2500+
Mushkin Level II PC3500 1GB Dual Pack
Lite-On DVD/CD RW
40 GB Seagate EIDE
2x120 GB Seagate SATA
ATI All In Wonder Raedon 9800
SoundBlaster Audigy Gamer
USR Performance Pro 56k Internal Modem (Controller based)
Mitsumi combination 6-1 card reader and FDD
250 Meg Zip
1x120mm LED fans (intake)
2x80mm LED fans (exhaust)
Some glow wire and CCFLs

I'll also be running a fairly decent amount of USB stuff (PDA cradle,
webcam) everything else (scanner/printer) are self powered.


My other question is if anyone knows where you can get reliable current draw
and volatage specs for specific products? Since I'm probably going to wind
up having to get a second PSU, I want to make sure that I split up the
devices in such a way as to still leave a decent buffer for each PSU.

 

[Dave C.]

Hello,

I'm getting ready to put a new rig together and I've been looking at what I
plan to put in and how large of a PSU I'll need. I've used some of the
online PSU calculators and all of them are reporting estimates in the
500-550 watt range (this is before my case lights hehe). So what I've been
thinking about is should I just get like two 400W jobbers

The online PSU calculators WAY overestimate your actual power requirements.
The reason they do this is that most power supplies are optimisically
over-spec'd. Thus, if you need 400W, you'd better buy 550W, because that
"550W" power supply probably only feeds the equivalent of a balanced 400W to
one rail or another. A single 400W or 450W power supply should EASILY
handle your power requirements. My wife's 2500+ system is using 18-25W less
total power (about 10W each on 5V and 12V) than the system you are planning
to build. It's got mostly the same components, including the USB devices
you listed. The only significant difference is that it's running one hard
drive only, but each hard drive will only draw about an extra 10W . . . and
that's assuming you could get all three to spin up at the same time. So the
system you are planning to build is virtually identical to my wife's system.
It's been rock-solid stable for many months running off a 350W Seasonic
(good quality) power supply. For your system, anything much beyond 450W
would be overkill, assuming the power supply isn't crap. You'll probably be
quite happy with a single HIGH QUALITY name brand power supply of about
450W. I could recommend the following Seasonic power supply. -Dave

http://www.axiontech.com/prdt.php?item=48828
http://www.seasonic.com/pdf/datasheet/01PC/Super Silencer.pdf

 

[jeffc]

Hello,

I'm getting ready to put a new rig together and I've been looking at what I
plan to put in and how large of a PSU I'll need. I've used some of the
online PSU calculators and all of them are reporting estimates in the
500-550 watt range (this is before my case lights hehe).

But keep in mind those calculators usually count all systems pulling at max
at the same time. In reality, that will never happen. I'm pretty sure a
good quality >500w supply would be more than enough.

 

[w_tom]

What system needs more than 350 watts?. However your load
analysis must be done separately for each voltage: 3.3, 5, and
12.

Two power supplies is a bad idea. Perkin Elmer did this
once when they screwed up the design. First power supply ends
up taking all the load until that power supply goes into
current foldback limiting. Then, as voltage drops, the second
power supply picks up the load. Then it too ends up with too
much load; goes into current foldback limiting. To parallel
power supplies means both output the exact same voltage -
generally a function of power supply design. Otherwise one
ends up taking all the load. You are not buying power
supplies that contain that voltage matching function. Two
power supplies will not reliably provide more power compared
to one supply.

Splitting loads creates another problem - ground loops.

Third, it can also result in IC damage. Remember how a
power supply is designed. If one voltage doesn't maintain
limits, then power supply must shutdown all voltages.
Necessary for multi-voltage operation. How do you do this
with separate power supplies not designed to share the load?

A power supply without the long list of numerical specs
should be avoided. Many supplies are sold without specs so
that you will not notice - the supply does not even meet many
requirements of Intel ATX specs. This is how supplies listing
at less than $80 retail are sold at higher profits. No long
list of numerical specs? Then avoid it like a disease. Have
seen inferior supplies sold even in CompUSA. Which one? Look
at the price. That alone suggests functions missing inside
the supply.

 

[Guest]

I've used some of the online PSU calculators and all of them are
reporting estimates in the 500-550 watt range (this is before my
case lights hehe).

Abit NF7-S w/Barton 2500+
Lite-On DVD/CD RW
40 GB Seagate EIDE
2x120 GB Seagate SATA
ATI All In Wonder Raedon 9800
250 Meg Zip

They're wrong, and they're dumb.

One power calculator gave an estimate of over 400W for a certain
system, but in reality it never drew more than about 170W, except at
start-up (typical of 2.5-3.0 GHz systems running a fast video card,
CD/DVD, and a pair of 7200 RPM HDs), and power supplies can put out
much more than their rated power for brief periods like that. This is
typical of most power calculators because they base their figures on
nameplate amp ratings, which are worst-case and typically double the
average. The calculator at http://takaman.jp/psu_calc.html?english
seems to be much more more realistic, but even its numbers on the high
side. Some people say that power calculators give high estimates
because they factor for poor supply quality, but there's never an
excuse to buy a bad supply because Fortron-Source supplies are
available at very low prices from places like Newegg.

Don't use 2 power supplies as this is simply not needed. But if you
ever want to try this, you won't need a relay if both supplies are the
ATX type since the green wire's Power_On\ signal on each supply is
open-collector, meaning the motherboard can safely short out more than
one of them to ground simultaneously. You should also ground the 2
supplies together very well.

My other question is if anyone knows where you can get reliable
current draw and voltage specs for specific products?

The Japanese power calculator mentioned above has figures for many
devices, but if you want to take your own measurements, either make
some adapter cables with current-sensing resistors inline, or get a
DC-rated clamp-on ammeter.

Generally a 350W Fortron-Source supply will run any system slower than
3.0 GHz with less than 5 fixed and optical drives, while the 530W
supplies is best for anything faster or with more drives. Actually
their 400W models are far more than adequate for about anything, but
Newegg charges only $5 more for the 530W model. But don't get the
550W model because its connectors don't fit the usual 20-pin ATX and
4-pin ATX12V connectors found on most motherboards.