24 pin main connector missing pin 20

I just bought a new graphics card, which required more power from my existing PSU. So, I went out and bought a 700w PSU.

However, the main connector, which is a 20+4 pin, is missing pin 20 completely. Now, I know all of the newer PSU's have this. What really screws me over is that my mobo won't start with this crazy 23 pin main connector but the mobo does get the green LED to light up when it's connected, just doesn't spin fans or anything important. When I drop in my old 550 watt 24 pin connector (which has a 20th pin), my mobo boots up fine, and all is well.

So, I would like to know if it makes sense that my mobo won't boot up with this new (23 pin) PSU but it will with my old (24 pin) PSU. Should it be working either way? Also, what do I need to search for if im looking for the older, full 24-pin PSUs?
21 answers Last reply
More about main connector missing
  1. Usually helps if you post the specs of your equipment, including make and model.
    Pin 20 is the -5v line that has been optional for several years.
  2. We probably don't have to worry about the specs, since the only variable here is the PSU. However, for your enjoyment, I'll post the relevant parts

    MB: ASUS A8N-SLI Premium
    VC: Raedon HD 4890
    Old PSU: Enermax EPS 12v 24p / ATX 12v V1.3
    New PSU: Kingwin Mach 1, 700w

    I know it's been optional, but my MB seems to think otherwise. Is there somewhere that I can buy a pin and thread it through my connector?
  3. As far as I know, PC main power connectors come in:
    PC AT power connector - 12 pins
    ATX 12V pc power connector - 20 pins
    ATX 12V Rev. 2.2 power connector - 24 pins
    Dude, sounds like you have a defective 24-pin connector. What was the make/model of that PSU you bought?

    I took a closer look at the spec after reading DellUser1's post and pin-20 is obsolete. In which case, you should be getting full power. So no, it doesn't make any sense.
  4. I've heard that the older MBs require pin-20. Is it possible that a 4 year old MB is that old?
  5. No. The ATX 2.2 standard came out in 2005. Pin 20 was "Reserved" in that specification. See page 37. http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
  6. So I got screwed over by less than a year. Any ideas where I can buy 700+ watt PSUs that still have all 24 pins?
  7. In the old ATX 2.1 specification, pin-20 is +5v. See page 19 http://www.formfactors.org/developer%5Cspecs%5Catx2_1.pdf

    In a 24-pin connector, pin-21 is +5v.

    Hmmm, got a soldering gun?


    I went and looked up your mobo. It specifies a 24-pin extended atx power connector. Except I thought that an EATX power connector is a 20-pin connector. ^^
    http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

    On pin-20, your manual (page 26) http://dlcdnet.asus.com/pub/ASUS/mb/socket939/A8N-SLI%20Premium/e2435_a8n-sli_premium.pdf
    says POWER OK. I assume that means -5 volts, as stated in the standard.

    Ah, you need an ATX 12v 2.0 compliant PSU!

    Try this http://www.newegg.com/Product/Product.aspx?Item=N82E16817153077

    Beware, the 700W ATX 12V 2.3 compliant.

    I am really sorry that I wasn't more help. I went from 20-pin to 24-pin ATX 12V 2.2 and must have skipped that generation.
  8. Don't have a soldering gun, and don't know how to solder. Out of luck.


    But surely someone must have older ATX2.1 spec PSU's to sell... right?
  9. The manual on page 5 http://www.thermaltakeusa.com/Upload/Product/Download/a550224a-4f10-46d9-bd01-764387423a29/W0121_en_080530.zip
    says that pin-20 is N/C.

    What about that 12V 4-pin connector? The latest standards have an eight pin 12V power cable. You have to connect the correct 4-pins! If you look, four of the pins are square and four are round. Make sure that the shapes of the male and female pins are the same. BTW, not all ATX12V 2.0 mobo's can take an 8-pin 12V connector. Some have parts that get in the way.

    If push comes to shove, maybe that Thermaltake is the right answer.
  10. Alright, so you think that 600w thermaltake is the way to go then?

    The picture shows the 20p missing, but I will give it a shot. Already sent back 2 PSUs, running low on options.
  11. Hmm... but these Topower's have all 24 pins and are the 2.0 version. Maybe the thermaltake's picture is off

    http://www.newegg.com/Product/Product.aspx?Item=N82E16817101021
  12. Can't say. Your mobo specifies ATX V12 2.0 and that is all that I have to go on.

    Here is a 680 watt PSU http://www.newegg.com/Product/Product.aspx?Item=N82E16817709014
  13. Doesn't look like any of these have enough pins for my Video card. I need at least 1 x PCIE, 1 x 8pin, and 1x4pin
  14. Actually, just found this guy, says its compatible with ATX 2.0 and 2.2

    http://www.newegg.com/Product/Product.aspx?Item=N82E16817139006&Tpk=Corsiar%20750TX

    I think we might have found a winner, if no one finds anything else wrong with it, and it boots my system...
  15. Rats. One of the changes from ATX 12V 2.1 to ATX 12V 2.2 was that the Main Power Connector was changed to support PCI-Express requirements.

    So we can get you the wattage but not the connector.

    Double check the (ATC 12V 2.2) 8-pin to (ATX 12V 2.0) 4-pin secondary power connector. Are they hooked up properly?

    Maybe your best bet is an external graphics card enclosure
    http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=external+graphics+card&x=0&y=0


    EDIT: OK, missed your post. But how can your board support PCIe X16 and ATX 12V 2.0 at the same time? Seems that you need ATX 12V 2.2.

    BTW, external graphics card enclosures are not yet ready for prime time; probably waiting on USB 3.0.
  16. If your motherboard is modern enough to take a PCI-e video card, it doesn't need that -5 volt line. There's a good chance that your new PSU is simply DOA.
  17. It turned to power-on LED on the mother board on though.
  18. hocjoc said:
    It turned to power-on LED on the mother board on though.

    Was that the Corsair 750TX?
  19. hocjoc said:
    It turned to power-on LED on the mother board on though.

    A PSU contains two independent power supplies: the large main section that provides power to the complete system and a small (10 - 15 watt), always on, 5 volt power supply called the standby power supply. Among other things, that is what turns on the motherboard LED, not the main section.

    All that motherboard LED really means is that the PSU is plugged into a working outlet and the PSU main power switch (if it has one) is on.
  20. Well, I tossed the 700W back in, connected it to the mobo only and it kicked on. Slowly added parts (I should have done this in the start, i know), and got to the new HD4890. It slotted up, and turned on. I tried to connect the 8-pin, and it didn't start. So I took out the 8pin and HD4890 and it didnt turn on again. This can't be a wattage problem, can in?
  21. hocjoc said:
    Well, I tossed the 700W back in, connected it to the mobo only and it kicked on. Slowly added parts (I should have done this in the start, i know), and got to the new HD4890. It slotted up, and turned on. I tried to connect the 8-pin, and it didn't start. So I took out the 8pin and HD4890 and it didnt turn on again. This can't be a wattage problem, can in?

    When you said "connect the 8-pin" you meant "connect the two 8-pin connectors" right?

    As I understand it, the HD4890 has two 6-pin sockets. You connect two 8-pin PCIe power cables to two 6-pin sockets on the HD4890 leaving two pins on each cable unattached. The extra two pins are intended for graphics cards that require extra power. They designed the 8-pin cable to fit both the 8-pin and the 6-pin sockets. I guess they didn't want to make two cable types. Good thing; I didn't want to pay for different cable types. :)

    Check the manual and let us know.
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