power connector 7900GT Vid card

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Got a new (to me) 7900GT PCIe graphics card recently. It was 'slightly
used'. Came with original box and accessories, except no power adapter,
and my PSU does not have a PCIe power connector.
No problem, I had another PSU that had a wiring diagram, showing the 3
yellow +12 powers, and the 3 black commons of the PCIe connector. I
wired one up myself, just adapted the proper pins on a 4-pin molex to
the pins on a spare PCIe connector. I can't imagine it would use enough
power to stress the actual wires.
But, to be proper, I bought, off of ebay, a few 4pin-molex-to-6-pin-PCIe
adapters (2 molex into 1 PCIe). Got them, and first off noticed improper
wire colors, but I can live with that.
It is wired as follows:
+12 on each molex (pin 1) to 2x +12 on PCIe
COM on each molex (pin 2) to 2x COM on PCIe
COM on each molex (pin 3) to remaining 2 pins on PCIe, of which I think
one should be +12. I can't imagine this would cause any problems, but is
it proper? Are all molex-to-PCIe adapters made this way? Really, the
PROPER way would be to make it a 3x-molex-to-PCIe adapter, so each +12
is the real thing, not split.
Am I being picky, or did I get cheated out of my $1.99 for each adapter?


Re: power connector 7900GT Vid card

Mike wrote:
Quoted text here. Click to load it

This is the one I like the best so far.


It has two 12V pins going to three yellow wires on the PCI Express (one Molex
drives two 12V pins). It has three of the four possible black ground wires
going to the PCI Express as well. It falls short of using three Molex
connectors, but still has decent ampacity. (The Molex end is good for 8+ amps
per pin. The PCI Express averages about 2 amps per pin worst case. As far as
I know, they haven't drawn more than 75W through one of those, which is roughly
12V @ 6A.)

The Molex 1x4 is rated for high current. The exact current depends on the
gauge of wire used. 10 amps per pin for 14 and 16 gauge wire. 8 amps at
18 gauge. And 6 amps at 22 gauge. I'd count on at least 18 gauge on a cheap
adapter. Which means, even one Molex pin could do it. The problem is, stuffing
multiple wires into a crimp pin, and doing a good job (which is why they
might not try to stuff three wires into one pin).


I've seen a picture of an adapter that came with an ATI video card,
where a single Molex drives a PCI Express. The Molex 12V drives two yellow
wires on the PCI Express 2x3. A Molex ground wire goes to only one pin on
the PCI Express. Implying that for that particular ATI card (probably not
going to 75W max for the connector), they were happy to pull all the current
through a single Molex on the 1x4 end.

In terms of the PCI Express end, the 2x3 has six pins, and the current
rating depends on the size of the connector (since it is a family of
connectors), the metal used in the pin, and the wire gauge. At the small
size of 2x3 (6 pins total), they're good for 6 to 8 amps, depending on the
wire used. If the connector has a 75W design limit, or about 12V @ 6amps,
the 2 amps per pin is no real challenge for the pins on the connector.
(Even with current hogging, the current probably does not reach 3 amps on
the worst pin.) A single pin, with some 16 gauge wire, could handle the whole


Using two Molex, and a couple pins at either end, should mean clear sailing
as far as the amps of current are concerned.

Just make sure the wiring is correct, as mixing up the 12V side and the
GND side, means shorting something. Since I don't know if the PCI Express
connector is a pcisig.com standard, or a formfactors.org standard, I'll have
to use a page like this as a pinout reference. Pcisig sell their standards,
while formfactors (Intel in disguise) gives theirs away.


Having said all that, the current split on the 7900GT is kind of a joke.
It is listed here as 48.4 watts, and the entire current could have
been drawn through the motherboard connector. Instead, the current
splits, 2 amps through the motherboard connector, and only 2 amps
through the adapter cable. As you can imagine, the 2 amps on the adapter
plug, could be handled by the most sloppily constructed adapter.


Xbitlabs has another page, with a certain 8800GTS on it, and the
adapter plug supplies 72 watts on that card. So that is an example
of using all the (standards limited) capacity of the plug.


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