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ATX vs ATX12 Power Supply


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9 replies to this topic

#1 vegassounder

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Posted 30 June 2009 - 10:25 PM

I just swapped my S2440BX Pentium 3 @ 500Mhz MB for a D845WN Pentium 4 @ 2 Ghz MB.
The new MB requires an extra 12 volt input for the Processor which the ATX12 supplies via an extra cable.

My system incorporates 2 standard ATX supplies connected in parallel, so I swapped one of the flat 4 pin molex HD pwr connectors for the square 4 pin connector needed to supply the 12 volts to the processor.

DOESN'T SEEM TO WORK! Can't even get into bios setup. Monitor doesn't even wake up.

The main 20 pin MB pwr connectors are identical so I assume the pin assignments are also the same.

Am I mistaken here?

Are there other differences between these two supplies that I'm not aware of?

If not, I may have a bad MB, in which case I'll need to return it for a replacement, but need to exhaust all other possibilities first.

Thanks for any advice.

Chuck

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#2 fairjoeblue

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Posted 30 June 2009 - 10:41 PM

Not every power supply will run every motherbaosrd eben if it has the proper connectors.

It really depends on how much power is being supplied to the connectors.
Not the 12 volt rating but the amps.

If I am reading this right you used a 4 pin HD type connector to supply power to the 4 pin CPU connector ?
You used a adapter cable to go from a flat 4 pin molex connector to a "square" 4 pin CPU power connector correct ?

The "square" 4 pin CPU power connector HAS to be connected.

Before I pronounced the motherboard drad I would try a power supply designed for Pentium 4 use that has the proper connectors.

I have a Compaq 300 Watt Pentium 4 power supply I took out of a older unit that isn't strong enough on the 4 pin CPU connector to even start many P 4 motherboards !
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#3 vegassounder

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Posted 30 June 2009 - 11:09 PM

If I am reading this right you used a 4 pin HD type connector to supply power to the 4 pin CPU connector ?
You used a adapter cable to go from a flat 4 pin molex connector to a "square" 4 pin CPU power connector correct ?


Not quite. Simply cut off the HD connector and spliced the 12 volt and ground wires to the CPU cable which I removed (cut off) a non working ATX12 supply.

#4 fairjoeblue

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Posted 01 July 2009 - 01:46 AM

The power supply probably simply doesn't have enough "oomph" to start the new motherboard/CPU.

If you are using a Prescott or Pressler core CPU with a old P3 PSU I can almost guarantee it doesn't have enough power to start it.

Edited by fairjoeblue, 01 July 2009 - 11:47 AM.

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#5 High500

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Posted 02 July 2009 - 05:56 AM

Its a dedicated rail normally for the 12v cpu header power. I personally wouldnt risk splicing HD rails and using them. Get a new psu thats 100% compatible.

high500

#6 case.bolt

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Posted 02 July 2009 - 07:46 PM

agreed with high500

playing around with psu's is a good way to fry your system or yourself. as a general rule, only use what your system recommends as far as PSU's go, there's a reason they recommend certain psu's/psu ratings.


did you run a voltmeter/multimeter on these before doing your cutting/splicing jobs? while the connection may be supplying the same voltage, it may not be the same amps or ohms... these things are so easy to vary... the 20-pin connector should be the same for all standard atx mobo's (and atx 12).

#7 vegassounder

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Posted 03 July 2009 - 07:12 PM

fairjoeblue, high500, and case.bolt,

First of all I want to thank all of you for your prompt replies to my problem.

In my original post I should have taken the time to to detail the following:

My system is custom built and incorporates two (2) load sharing ATX supplies. The +12v rails on each are rated at 15amps so current supply is sufficient.

After swapping MB's and trying to boot, when my monitor wouldn't even wake up I panicked and posted this thread. Thought my new MB was DOA.

Upon further investigation I noticed the beep code, (3 long beeps) which indicated the first 64Kb of memory was failing.
I had simply transferred the RAM from my old MB and, as it turned out, it wasn't fast enough for the new MB.

Replacing the RAM with faster sticks solved the problem.

Please accept my apology for being a DUMB ASS.

Chuck

#8 case.bolt

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Posted 05 July 2009 - 05:29 PM

hey, don't worry about it, i think it's an immutable law that says that anytime a new system is built, there has to be at least one thing not plugged in, or not working properly, and it's usually something that makes you feel dumb for not seeing.

the last system i built i spent 30 minutes trying ot figure out why it wouldn't boot, then realized that i had forgotten to plug in the secondary 4-pin psu cable to the mobo :thumbsup: it happens.

#9 fairjoeblue

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Posted 05 July 2009 - 09:11 PM

I build computers every day.
Recently had one come back because it kept losing the picture while a game was being played.
It worked fine otherwise.
I thought the video card was bad so I opened the case to replace the video card.
I discovered I hadn't plugged the power cable in to it !
I was more then suprised that it even worked without the power cable, but it did !
I plugged the power cable in & it worked perfect .

Don't call yourself dumb because you overlook a minor thing.
Everybody overlooks something on occassion.
Anybody that says they don't isn't truthfull. :thumbsup:
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#10 audioAl

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Posted 06 July 2009 - 02:58 AM

To tell the truth, I've got a computer right now I cant get a screen on. I must tear it back down today. LOL, I changed PS and I get it to turn on the processor and PS lights up, but the case fans wont, and no screen. I am using a new Seagate 500 gig HDD too. Just keep fiddling, Ha! Alan :thumbsup:
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