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New PSU on an old MB? (ATX)

Last response: in Motherboards
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September 12, 2009 3:31:22 PM

PSU (CORSAIR CMPSU-450VX):
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Motherboard (KK266PLUS, Iwill co. 2001):
http://www.pcstats.com/articleview.cfm?articleID=801
http://hothardware.com/Articles/IWill%2DKK266Plus%2DKT1...

I wanted to get this PSU as a backup and as a psu for my future system. Since I won't buy everything all at once, I wanted a way to test the PSU so I could determine whether it warrants an RMA or not. Since my current system is using the iwill board, I thought that I could try the PSU on this board for testing purposes. The corsair PSU is more modern than this motherboard, and my concern is that the voltages/currents might not be compatible even though they're both ATX form factor. The corsair is v2.2 ATX. The motherboard was released in 2001. While the corsair psu will fit in the same case that I'm currently using, I was hoping someone could let me know whether the motherboard will spit flames or not turn on when I plug it in to the psu.

The MB, btw, does not have an ATX12v 4/8-pin connector. It would only use the 20(+4 for new MBs)-pin adaptor. I'm not sure whether the psu would like this or not.

Thanks in advance. PS: I noticed that www.iwill.net isn't active anymore.

More about : psu atx

September 13, 2009 5:41:33 PM

I've done some research and this is what i've come up with...

-5 voltage:
The newest ATX standards removed the -5 voltage from the 20+4 pin adaptor since most motherboards don't use it. There're apparently a few that do use it made after the standard was accepted. A lot of older motherboards use the -5 voltage for the ISA slot/card. I'm unsure what the motherboard will do without the -5 voltage on the 20th pin.

Changing voltages in ATXV2.0+:
Instead of putting most of the voltage/current (confused about which term to use) on the 3.3 and 5v lines, it has been moved to the 12v rail. This move towards the 12v rail/line started with p4/xp systems (my MB is an early model of the athlon xp system) and has "escalated" to a point where most of the consumption is on the 12v line. This will cause problems for older systems because they might want more power from the 3.3/5v lines but they won't be able to get it because it's not available. They'll end up requesting too much from the psu. This means that if you use a newer PSU on an older motherboard that you should compensate for this by purchasing a higher output psu. For example, I looked at my old 300w psu and it has 180W output on the 3.3 and 5v lines. 98W output is on the 12v rail, which is clearly not enough for newer motherboards. The CX450 that I"m wanting to get has only 130W on the combined 3.3/5v line, but a whopping 396W on the 12v rail. For comparison, the Cx550 has 140W on the 3.3/5v line and 492W on the 12v rail. So if i'm going to match my 300W psu on the 3.3 and 5volt rails then I"m going to need to buy a very high output psu or otherwise take a chance with a lower output.

The biggest question I have is what will happen if I plug it into the MB and it finds that the -5 volt line isn't present. Will it post? Will it randomly freeze or shutoff or restart as a result? And if it needs more than 130W on the 3.3 and 5 volt lines, what will happen? I know that the VX450 has a much higher output potential than is listed in its specifications - while still maintaing a respectable efficiency.

I've been looking around for a power supply that can work for a new system AND my kk266-plus motherboard but I'm not having much luck because I can't find very many that have the -5 voltage line. The only ones that I've found are here:
http://www.ultraproducts.com/product_details.php?cPath=...

Confusing the issue is reading on a help site that buying the a 2.01 or later atx12v psu is best even if you're buying for an older MB. And then, 5 minutes later, reading that you shouldn't mix new psu's with old MB's. *shrug*
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