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Power to system

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July 7, 2009 3:59:15 AM

I have a custom built system that needed a power supply replacement. I replaced a 450W PS with a new 400W. Now with the new power supply installed when I simply plug it into the motherboard (using the 20 pin power connector and the four pin power connector) the system will go to POST. However if I plug anything else in (HDD, CDROM...) the motherboard gets power for a second then all the fans in the power supply and the CPU fan will stop. The board still has power going to it as I can see from the LED light on the board but the system will not power up with everything plugged in. Is it that the PS is not powerful enough maybe?
Why is this? Any suggestions please.

More about : power system

a b ) Power supply
a b V Motherboard
July 7, 2009 4:58:14 AM

Please post your configuration, otherwise all I can guess is "maybe".

Also, what made you get a smaller psu?
a c 144 ) Power supply
a c 156 V Motherboard
July 7, 2009 2:31:10 PM

A PSU has two independent sections, the power supply that feeds the operating voltages to the computer and a small, always on 5 volt power supply (generally 2 - 3 amps) called a standby power supply. The standby power supply feeds the motherboard LED.

Turn your computer off with the case switch and open the case. That LED will be on as long as the switch on the back of the PSU is on and it's plugged into a functioning wall outlet.

Needs specs.
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July 8, 2009 2:11:00 AM

Twoboxer said:
Please post your configuration, otherwise all I can guess is "maybe".

Also, what made you get a smaller psu?



System has a 2.5 GHz Celeron CPU with 40GB HDD, CDROM, and floppy drive running XP Pro. Quite a basic system, used in a training center.

I just had the extra PSU and thought I would give it a try.

Let me know if you need anything more.

Thanks in advance.
July 8, 2009 2:11:42 AM

jsc said:
A PSU has two independent sections, the power supply that feeds the operating voltages to the computer and a small, always on 5 volt power supply (generally 2 - 3 amps) called a standby power supply. The standby power supply feeds the motherboard LED.

Turn your computer off with the case switch and open the case. That LED will be on as long as the switch on the back of the PSU is on and it's plugged into a functioning wall outlet.

Needs specs.



System has a 2.5 GHz Celeron CPU with 40GB HDD, CDROM, and floppy drive running XP Pro. Quite a basic system, used in a training center.

I just had the extra PSU and thought I would give it a try.

Let me know if you need anything more.

Thanks in advance.
a b ) Power supply
a b V Motherboard
July 8, 2009 2:34:51 AM

Well, I guess the mobo doesn't matter for the moment, and I can assume you are using onboard graphics since you mention no vid card. I'll also assume you made no recent change to memory, and whatever memory is in there was working before. Finally, I'll assume there were no other recent changes you thought you might try.

Get the system to post and use a voltmeter to test the 12V and 5V voltage while the system is on. Grab an open 4-pin molex connector, insert the VOM's ground lead into either of the middle two plugs, and the + lead in one end plug, then the other, noting the voltage. One shd be +5V, the other +12V, both + or - 5%. If not, you can be positive that psu is bad. You can repeat the test after a configuration that fails to post, fails. If no or low voltage, its probably the psu.

Or you can put the other psu back in, and if it works, the little psu is bad. Given the assumptions above, 400W is plenty of power.
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