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Enermax 485W PSU Question

A

Atreus

Gawd
Joined
Aug 20, 2001
Messages
641
I got myself an enermax 485w ATX 2.0 PSU. The exact model is EG495P-VE SFMA.

I currently have an older motherboard (Asus P4T-E) which I believe is ATX 1.2. When I replaced my old power supply with the new enermax, the system wouldn't power up. Actually, it did power up briefly, before shutting down, and couldn't get power again. I believe this is to protect in case of any short circuits. Upon further checking the connections, I noticed that the 20 pin ATX connector is missing a white wire (I believe pin 18) on the Enermax. My old Aopen PSU has a white wire on pin 18. The Aopen PSU works, while the new Enermax doesn't. My question is - is the lack of a wire on pin 18 a new design for the ATX 2.0 standard, or is my enermax missing a wire? I haven't dealt much with newer model systems that use ATX 2.0, so I'm unsure if the wire should be there.

Cliffs:
- installed Enermax EG495P-VE SFMA (485W) PSU
- system won't power up
- checking connections, I see that the Enermax doesn't have a white wire in pin 18 (pin 18 is completly empty)
- replace with old Aopen PSU, system works fine.
- faulty enermax? or missing a wire due to new ATX 2.0 standard?
 
the negative 5 volt rail was removed from the ATX12V v2.01 spec
and the white wire would be the -5V in the 18th pin location

but that rail has been unemployed since ISA buses where common
and its very unlikely the cause

far more likely has been a change in the Power_Good Signal
+5VSB or PS_On signal
the first one the most likely
the Power Supply turns on, doesnt get the right Power_Good signal and shuts off
Power Good Signal

When the power supply first starts up, it takes some time for the components to get "up to speed" and start generating the proper DC voltages that the computer needs to operate. Before this time, if the computer were allowed to try to boot up, strange results could occur since the power might not be at the right voltage. It can take a half-second or longer for the power to stabilize, and this is an eternity to a processor that can run half a billion instructions per second! To prevent the computer from starting up prematurely, the power supply puts out a signal to the motherboard called "Power Good" (or "PowerGood", or "Power OK", or "PWR OK" and so on) after it completes its internal tests and determines that the power is ready for use. Until this signal is sent, the motherboard will refuse to start up the computer.

In addition, the power supply will turn off the Power Good signal if a power surge or glitch causes it to malfunction. It will then turn the signal back on when the power is OK again, which will reset the computer. If you've ever had a brownout where the lights flicker off for a split-second and the computer seems to keep running but resets itself, that's probably what happened. Sometimes a power supply may shut down and seem "blown" after a power problem but will reset itself if the power is turned off for 15 seconds and then turned back on.

The nominal voltage of the Power Good signal is +5 V, but in practice the allowable range is usually up to a full volt above or below that value. All power supplies will generate the Power Good signal, and most will specify the typical time until it is asserted. Some extremely el-cheapo power supplies may "fake" the Power Good signal by just tying it to another +5 V line. Such a system essentially has no Power Good functionality and will cause the motherboard to try to start the system before the power has fully stabilized. Needless to say, this type of power supply is to be avoided. Unfortunately, you cannot tell if your power supply is "faking" things unless you have test equipment. Fortunately, if you buy anything but the lowest-quality supplies you don't really need to worry about this.
Click to expand...

the Enermax isnt a supply that would "fake" the signal
but the ATX mobo spec for the signal might have changed
and its not picking it up correctly
that would be my best guess

I guess its possible that the mobo is checking for the -5V rail before it gives the Power_Good signal, in which case it could be indirectly responsible
that would be a mobo or BIOS by mobo implementation
and this would be the first time Ive heard of the issue

you could test this hypothesis by cutting the -5V on the other supply
if it then fails youd have the answer
of course youd likely want to repair it as well
a spot of solder and a bit of heatshrink is the ideal solution

but simply twisting them together and a bit of electrical tape would do
 
Thank you sir for the detailed reply. Before cutting the current -5v line, I will try to test the PSU in another system and see what happens.
 
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