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I have a GTX 770 and a 650w PSU, but I don't know how to power my card.

The card has an eight pin (150w) and six pin (75w) socket, and I'm fairly sure both need to be connected. However, I can only free two four-pin molex plugs. I am using a four pin ATX cable to power the motherboard and two SATA power cables for the CD-ROM and HDD.

What can I do? I could use molex splitters, but how do I prevent overdraw on any individual rail? If I split the two molex four pins, could I have them share the eight pin and six pin power connect ions equally?

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  • Can you post some pics to make it a bit clearer.
    – user88311
    Jun 30, 2013 at 18:43
  • Not really - I'm having to post this from a tablet, for obvious reasons.
    – Jimmy Breck-McKye
    Jun 30, 2013 at 18:44
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    I see, well from what you've posted all I can say is to purchase a couple of adapters to plug into the molex cables, aside from getting a different PSU with the required cables/plugs, which I would recommend doing eventually anyways.
    – user88311
    Jun 30, 2013 at 18:47

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A PSU often come with several connectors. Usually there are plugs for:

  1. The motherboard (20 or 24 pins).
  2. Extra 4 pins plugs to feed extra power to the motherboard (mostly P4 era)
  3. For general devices (molex)
  4. For the floppy drive (getting less common these days as floppies are getting ancient).
  5. Extra pins to feed a graphical card (Either 6 pins or 8 pins) (modern systems).
  6. Connectors to feed SATA devices.

On your case items 3 and 5 are the most important.

Either your PSU has one or more 6/8 pins connectors to feed a graphical card, or you will need to use something to transform the molex connctors into extra PCI-e power connectors.

Such a device could look as simple as this:

Molex_to_PCIe

I could use molex splitters, but how do I prevent overdraw on any individual rail?

Go to the webpages of your PSU's manufacturers webpage. Check the PSU's manual.
If it only has a single +12 rail and it outputs enough power to feed the graphical card (once again, use the manual. How much does the card need, how much does the PSU output?) then there is nothing to worry about.

If the PSU has multiple +12 rails then these should be marked on the PSU, or at least mentioned in the manual.

If I split the two molex four pins, could I have them share the eight pin and six pin power connect ions equally?

This depends on your PSU and its internal wiring. The information who got "a 650w PSU" simple is not enough to answer that,

What can I do?

Either:

  1. Buy a new PSU (which seems rather unneeded. 650 Watt is a lot and should be enough to power your PC)
  2. Convert your two molex connectors to a 6 pin and a 8 pin PCI-e plug.
    This will probably work, though I am reluctant to advice this since most molex to 8 pins PCI-e connectors I found draw power from two molex plugs.
  3. The same as above but also convert the plug going to the CDROM. (when was the last time you used optical media? My last time was when I installed windows XP).
  4. For completeness sake only: You can buy a graphical card which needs less power.
  5. If you do not OC, check if your motherboard really needs the 4 pin connector. If it is a P4 then it most likely does need it. If it is a modern system with a 24 pins ATX connector (basically a 20 pins ATX plug with a 4 pins added to its side) then you might not need it and you can use something like the reverse of this:

enter image description here

4 pin to molex to 6 pin PCI-e is rather kludgy though.

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  • I'm thinking of splitting the two molexes (let's call them A and B) into four further molexes (A1, A2, B1, B2). Then, I'll connect A1 and B1 to make up the 150w eight pin connection, and A2 and B2 to make up the 75w six pin connection. Does this sound feasible, or too much bother? I'm very angry at having to fork out for more equipment when I already paid more for a supposedly 'future' proof 650w PSU when I first bought the computer.
    – Jimmy Breck-McKye
    Jun 30, 2013 at 19:20
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    That might work, though I am no fan of using extra splitters when you draw relative much power though them. The less extra pieces used, the better. If you do go that way, then make sure the connections are solid. You do not want to run 12 1/2 Amp (150 Watt at 12 volt is 12.5 amp) though bad connections.
    – Hennes
    Jun 30, 2013 at 19:31
  • A very comprehensive answer - thank you!
    – Jimmy Breck-McKye
    Jun 30, 2013 at 19:35

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