Well underpowered electronics such as motherboards, are more prone to failure when powered below there requirements, then if over powered, because there are fuses and things in place to stop overpowering or overheating issues, but fail safes to shutoff devices when underpowered arent so common.
From what I have been reading, you can indeed power the system without harm by only using the 20 pin connector. If I understood what I read so far, the 4 pin connector is for the PCI-E slots. I know that usually cards plugged into PCIe slots are GPU an have thier own connections comming from the PSU.
Have you tried booting the system without the video card inserted? Using the on-board video VGA port to run a monitor?
I have also read that the newer Intel boards require the extra 4 pin connector to work properly, and perhaps could cause problems like the one
you are having.
EDIT BELOW(found this very informative)
This is copied off a forum from Anandtech...
Poster's name... flexy
(Q) the new "xyx" motherboard has a 24pin plug - so do i need a new 24pin PSU ?
Not necessarely. All the upcoming new boards (eg. Nforce 4) SHOULD be capable of running with an "old" ATX 1.3 20pin-plug equipped PSU as well as with a new ATX 2.0 24pin PSU.
The question here is rather whether the PSU itself is a good brand name PSU with sufficient wattage. Dont expect a recent high-end system with pci-express card(s) and A64 CPU to run flawlessly off an old 350W noname PSU.
(Q) but what about the extra 4 pins on the PSU plug/motherboard connector ?
These four extra pins on the newer ATX 2.0 PSUs are solely for providing power to the pci-express port . I guess the specs demanded a separation of the power-rails for CPU/motherboard and high-end graphics.
Therefore they put the additional rails on the plug/motherboard which provides +12V, +5V, +3.3 and GND to the pci-express slot.
(Q) So..but i got/plan to get a pci-express card !
pci-express cards will MOSTLY have an external power-connector (6 pin pci-express). You can connect two unused molex connectors from your PSU to this connector on the graphic card via an adapter. If you dont have this adapter - newegg has one
pci-express power adapter
(!) a single connector/wire on a PSU/connector is usually rated at a max. power draw of 6A. Wattage = VOLTS*A.
The maximum wattage for ONE 12V connector is therefore 72A. The two molex connectors are combined since a high-emd graphic card usually uses MUCH MORE than 72W..therefore it uses two molex which should be ok for up to 144 Watts.
THATS why the pci-ex graphics card SHOULD be able to run fine even WITHOUT the 24pin motherboard connector and WITHOUT a newer PSU with 24pin plug. The card gets enough power already and does not REALLY need the power from the PSU plug.
Thats why the newer boards are USUALLY comaptible with either 20pin PSUs or 24pin PSUS.
[As a sidenote] A pci-express card with NO external power connector which would get the power solely from the pci-port can (therefore) only draw a max. 72W. The current high-end cards all draw much more power. Eg. a Geforce 6800 Ultra is listed as using as much as 110 Watts. There is no way to do this with just the power coming from the 6A/72Watts rated pci-slot
(Q) But wouldnt it be just better to get a newer ATX 2.0 24pin PSU which is "pci-express ready" ?
First..."pci-express" ready is just a stupid market term. See above. It doesnt need to bother you if the graphics card has an external connector and you got a good strong 20pin PSU.
"pci-express ready" will usually only mean that the PSU will have a 6pin pci-express power plug. Which is not bad and saves you the hassle to get the adapter in case the card does NOT come with one.
Another advantage of the newer ATX 24pin PSUSsis that they USUALLY have dual, separate 12V rails. (One 12V rail provides power for eg. CPU, the other rail is separate and provides power for the pci-express card.)
This is not necessarely bad In other words: You cant go wrong with a new 24pin PSU for various reasons. But this does not mean that your older 20pin one is obsolete. Especially not if its a good brand name one with suffcient power.
(Q) I got a 20pin PSU and i will get the "xyx" motherboard which uses a 24pin connector. So...i can get one of these PSU "20pin -> 24pin adapters" to make my old 20pin PSU 24pin compatible ?
See above. I dont THINK you really need one. But..there are certain *concerns* regarding this "20pin to 24pin" adapter. Do NOT confuse with the "24pin -> 20pin" adapter which often comes with new ATX 2.0 motherboards.
The 20pin->24pin PSU adapter will split your ONE rail/wire (eg. the 12V rail) coming from your PSU in two and create the "artificial" extra 4 pins.
The result is that your PSU will draw much more power from the one rail than it was originally intended to. While a real "dual rail" PSU will provide two seperate rails the solution with the adapter will draw twice the power now from one 'wire'.
What was rated at 72W max. before (remember ? 6A * 12V = 72 W) will now all of a sudden draw up to a max. of 144 watts.
Also..the thermal load (eg. heat !) will increase 4 (four !) times.
The most strain wil be on the adapter/plugs itself. You will get some nice, hot wires and plugs...
I cannot see this as an elegant solution - HOWEVER it might work.
But remember: You probably wont even NEED the adapter.
Just plug your 20pin old PSU plug in the new 24pin connector on the mobo (its downwards compatible !) the right way...leave the four extra pins out. This SHOULD work and you wont really need an 20p -> 24p adapter !
(Q) My older 20 pin PSU has this extra 4pin connector with the two yellow and two black wires ? Can i use this to "fill up" the missing 4 pins on the 24pin connector on the motherboard ? Isn't that what it's for ?
NO NO NO..DONT EVEN THINK ABOUT IT !!!
This is the 12V CPU connector which you use to provide juice to the CPU, there is another 4 pin connector on the motherboard especially for this.
Dont even think about using this (YES, i saw people considering this option!) - except you need a sure way to fry your board !
There is a smaller (?) number of newer 24pins PSUs out there who actually *do* have
detachable 4 extra pins on the main connector - so you can attach it to the 20pin connector and have a 24pin connector.
But even if it might look about the same....do NOT confuse this with the 4pin 12V CPU connector !
the reason why many people might run into problems with their older 20 pin PSUs is that MANY (even brand name) PSUs are rather weak on the 12V rails.
15A or 18A PSUs with only ONE rail are (still) quite common.
Now picture a graphic card using 110 W under load which would be a power draw of 9.2 A alone for the card ! (Assuming it would all come off the 12V rail).
If your PSU has only 15A on the rail...well...subtract 9.2A and this doesnt leave MUCH for the rest of the system, ESPECIALLY if you overclock !
You will run into problems, but not primarly because you dont have the 24pin connector.
A 20pin -> 24pin PSU adapter would not "magically" provide more power/amperes and not solve your problem.
Either consider a new PSU with dual rails with TWO 12V rails with *at least* 15A on EACH rail...or a STRONG 20pin PSU with plenty Amps on ONE rail...at least 25A or more !
Edited by s1lents0ul, 29 October 2010 - 08:46 AM.