computer components & hardware - power supply on new build w old power supply

ed jurewicz wrote:
I am trying to build a computer for a friend on the cheap using recycled and
rebate parts. I have a new winfast motherboard (939) and an antec power
supply with the older connector that has four less connectors. Can this
power supply be used with this motherboard.

Yes, a 20 pin power supply, can be used to power a 24 pin motherboard.

The purpose of the extra four pins, is to provide room for more
3.3V, 5V, and 12V current to flow. The critical one there, is the
12V pin, and the 3.3V and 5V are relatively unimportant on a
modern motherboard (because the power consumption is not as high
as, say, on a dual socket Athlon MP machine - some of those
used to burn their power connectors).

With the 20 pin power connector, you have one 12V pin available. It
has room for up to 6 amps of current flow, before the pin gets too
hot. The 12V pin powers the motherboard fan headers (say 0.5A for several
fans). It also powers the PCI Express video slots, and that is the
determining factor.

Some video cards, draw up to 4 amps from their video card slot. A
20 pin connector can power one video card. But if your motherboard
was an SLI motherboard, and had two PCI Express x16 video card slots,
and you used two of those 4 amp video cards, that would be 8+ amps
of load. So what I tell people, is the 20 pin is safe, as long
as you only have one video card.

There are power measurements on Xbitlabs.com , for various video cards,
and cards that have a PCI Express 2x3 on the end of the card, sometimes
don't draw quite as much +12V from the video slot. So it is possible
to drive two video cards, with a 20 pin power supply, because the
one wire/pin would be sufficient. Of course, the 20 pin supply
won't have the necessary PCI Express 2x3 power connectors to begin
with, so at that point you'd probably buy a decent ATX supply to
replace it.

The 24 pin connector has a total of two 12V pins, leaving room for
up to 12 amps of current to flow. That should be good enough for
two 4 amp video cards plus the fan headers.

Pin 1 of the 20 pin, goes to pin 1 on the 24 pin connector. The
nylon shell of the connectors, helps ensure that you mate them
correctly. (The shell has shapes to prevent mis-mating.) You may
find, depending on the shape of the latch, that the latch
won't engage fully (which sucks). Latching the connectors
together, prevents thermal migration of the connector out of
the socket.

The above rules don't take all possible hardware configurations
into account. I presume it is possible for PCI Express x1 cards
to draw power too. I don't know if they'd be drawing down on the
12V rail, as for a lot of peripheral type functions, the 3.3V
supply which is also on the slot connector, is a better fit
for the application. (A GbE Ethernet card might only need 3.3V
or lower operating voltages, for example.) So at least for the
current time, and until a counter example is provided, I'd use
the video card count as the acid test of safely. One video
card should be OK with the 20 pin.

HTH,
Paul