whats the point of pci express 8pin connectors?

[aidynphoenix]

i have a powersupply here with 6/8 pin pci express connectors, checking it out. i see that both the additional connectors are simply ground connections. what is the point of that? do they think that the other ground connections are not ample enough?

[mariushm]

Re: whats the point of pci express 8pin connectors?

They're a way for the video card to "detect" that they can "pull" more current through the cables.

See http://www.playtool.com/pages/psucon...tml#pciexpress

Basically 6pin connectors are designed for 75 watts of power, or about 2.1 A at 12v through each yellow wire.

8 pin connectors are designed for maximum 150 watts of power through those 3 yellow wires, or about 4.2A through each one.

With the 6pin connectors you can use thinner wire, as it's only 2 amps. With the 8 pin connectors manufacturers need to use thicker wire.

If the video card has no way of knowing how much it can pull, it may pull more than 2-3 amps through a thin wire that's used on the 6pin cable and cause it to overheat, melt, burn...

The 8pin cables are an improvement to the older 6pin cable standard - when this was conceived 75 watts was A LOT for a video card, and they thought 2 x 6 pin cables were enough. Later, they made up this 8pin so that you can use only 2 x 8 pin cables to power your monster video card.

The power supply manufacturer simply decided to use the thicker cables (suitable for more than 4a through each yellow wire) for everything but wanted to keep the compatibility with older video cards with 6 pin connectors so they let you split the 2 black pins from the connector.

[aidynphoenix]

Re: whats the point of pci express 8pin connectors?

so those 4pin molex to 6 and 8 pin adapter cables would be quite a hazzard then?

[ratdude747]

Re: whats the point of pci express 8pin connectors?

so those 4pin molex to 6 and 8 pin adapter cables would be quite a hazzard then?

yeah. Not a good idea. They are marketed to cheapskates and noobs.

[Uranium-235]

Re: whats the point of pci express 8pin connectors?

i've gotten them working fine for smaller cards that require 8, but I wouldn't even think about using it for high performance cards. power supplies are supposed to be GUARANTEED 75 or 150, molex are dependent on the power supplies design and general wattage

[mariushm]

Re: whats the point of pci express 8pin connectors?

They're bad for several reasons.

They usually have very thin wire, AWG 20-22, which can't handle so many A without heating up.

Even if the cables in the adapter are thick according to the maximum A they'll have to carry, you don't know if the cables coming from the power supply to the molex are thick enough to handle the current.
Remember, the 8 pin connector delivers 150 watts or 12.5A through 3 wires, but the adapter connects these 3 wires to 2 regular wires.
So, through one molex connector you have one of these 3 wires connected, and you have 2 wires connected on the second molex connector. Therefore, you may load one molex connector with 4A of power while the second has 8A of power going through it.

Even if the power supply was made with thick wires capable of handling 8-10A on 12v through them... there's a question of how those molex connector wires are connected inside the power supply.

Some power supplies have multiple "rails", for example a 600w power supply can have 2 rails of maximum 30A each, but in total only 40A on 12v (480w).

Let's say the user wants to plug a second video card with an 8 pin connector besides the already installed video card on the 8 pin connector the power supply came with.

If one molex connector is on rail 1 and the second molex connector is on rail 2, the video card may hate this as the voltages can be slightly different.

Also, the user would have to know to which rail is each wire connected to.

For example, the 8 pin connector + the motherboard can be on rail one, while all the other connectors (molex + 4 pin cpu) are on rail 2. This is good, as user has up to 30A to use on the second rail, where the 4 pin cpu will only use up to about 8-10 A.
But if the power supply uses motherboard + 4pin cpu connector + 2 molex on rail 1 and pci express 8 pin + 2 molex + sata on rail 2, then it gets messy.
Use the adapter with the 2 molexes on rail 1 and you may overload the rail powering cpu and video card from one rail, you may exceed the 30A limit.
Use the adapter with 2 molexes on rail 2 and you have 2 cards on second rail - again bad...
Use a molex from each rail... not recommended... video card may hate that.