Ian said:
Once or twice I've got away with splicing the extra 4 pin connector onto
the existing wires - so why not just patch wire the extra pins on the
print side of the MOBO?
I did have a spare 24 pin PSU - but it looks like some inconsiderate git
has 'borrowed' it!
You can plug a 20 pin, into a 24 pin. This site has a nice web page
that reviews ATX issues.
http://www.playtool.com/pages/psuconnectors/20in24.jpg
All that's needed from the user, is some simple math.
The only wire under stress in the main bundle, is the single 12V yellow
wire. If you use a 20 pin ATX supply, the single pin on that wire,
carries the motherboard 12V load (the CPU is handled separately by the 2x2).
That yellow wire is connected to the PCI Express slots. And to the fan headers.
If you have a single video card, the PCI Express standard pretty well ensures
you won't overload the single yellow wire. And typical video card designs,
limit 12V flow on the video card slot, to around 4.1 amps. That is less than
the 6 amp limit for the Molex pin on the 20 pin connector. (You should allow
a bit of current for fan headers as well.)
About the only configuration you have to watch, is running a couple 6600 or
7600 family cards, from the PCI Express slot supply. Two cards times 4 amps
each, is a bit much for the single 12V pin and wire.
Much higher consumption cards, they actually trend in the downward direction,
with respect to slot power. A high end card, uses 12V @ 2A from a video card slot.
The rest of the power comes from the 2x3 or 2x4 connectors on the end of the card.
It's only the mid-range cards, the ones without a PCI Express power connector
on the end, that are tempted to draw all their current from the video slot.
So in many situations, it's perfectly OK to operate the 24 pin motherboard
from a 20 pin supply. Just total up your amps and use your best judgment.
Video_slot Video_slot Fans
---------- ---------- -------
6600 4.1A fans=1A total = 5.1A < 6A on 20 pin, single yellow wire
6600 4.1A 6600 4.1A fans=1A total = 9.2A, need 24 pin connector (two wires, 12A max)
8800GTX 2A 8800GTX 2A fans=1A total = 5A < 6A on 20 pin, single yellow wire
Xbitlabs has measured power numbers for many video cards, and only recently
have they stopped doing those measurements. That's where I can find lots
of numbers when needed. So if I need to check my own personal video card,
for current draw, that's the first site I look.
In this example, the 6600GT draws 4.07 amps from the slot, at nominal clock.
And 4.35 amps when overclocked.
http://www.xbitlabs.com/images/video/geforce6600gt-oc/6600gt_power_table-b.gif
On modern motherboards, the other rails aren't close to the limits. And the
existing paralleled sets of wires, are enough. The last time the 5V pins
on the main connector burned, was back in the dual socket Athlon era.
(There was one motherboard, with an actual design error in the 5V wiring,
just around the main power connector itself. The error caused current
hogging, and more current flowed through some pins, than the others.
The pins with the most current, tended to char and burn after a while.)
Some motherboards, came with an extra 1x4 Molex on the motherboard, and the
purpose of that, was to supplement any yellow wires already connected to the
motherboard. So that's another way they would beef up the 12V rail, and prevent
any pins from getting burned. Occasionally, you still run into a motherboard
that includes one, such as motherboards that are covered with "all x16 slots",
tempting the user to fill the machine with video cards. If you think there's
a way you can exceed the 12V @ 6A limit of the 20 pin, or the 12V @ 12A limit
of the 24 pin, then look for ways to supplement the path for the current flow.
Paul
