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Power on ATX 2.2 PSU/motherboard without +5V standby

Last response: in Motherboards
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April 8, 2009 5:08:16 AM

The short question would be, is it possible to modify ATX 2.2 PSU(or/and motherboard) so that computer will boot up and work properly without +5V STB?


The background of my question is fallowing:
I found out that my motherboard(MSI MS-7027) has some unnormal voltages when the computer is turned off(but the PSU is ON)- it has ~3.5V instead of 0V in +12V line. It is that probably some component is leaking +5V STB through to the +12. I wouldn't bother, but I have replaced my graphics card(both were nVidia GF FX 5200) twice in two weeks. I think that this ~3.5V, instead of 0V as it should be when the computer is turned off, have something to do with it.


MSI MS-7027

At first I thought that this is caused by some capacitor, but tests with resistors proved that the voltage(~3.5V) is stable and constant. So I thought that there are any other possibility than leakage of +5V Standby. So I cut the +5V Standby wire to prove that I am right/wrong. Check that on the second image:



As you can see, the 3.3V, 5V, 12V, AGP pin#1 and SYSTEM FAN pin#2 are now correct! However, the voltages of the Power ON has dropped from ~5V to 1.7V!?

To prove, that this is not a PSU fault, I tested with two other PSU's. Results are here:


However, computer will not start when the +5V STB is broken. So I made a simple switch to power on the computer when the +5VSB is broken, but there was no video output. Computer completed POST(no beep codes and three lights on the keyboard lit up for a moment), but then there was no video output! When I changed the PSU to the one which has working +5V STB, the video output was back. I have few image as well:





The motherboard is a rather rare OEM microATX motherboard for a multimedia computer in a very nice special custom made case- I would really like to fix this rather than replace.

However, why was there no video output when I used regular on/off switch? Is it possible to modify ATX 2.2 PSU(or/and motherboard) so that computer will boot up and work properly without +5V STB?
April 26, 2009 7:02:06 PM

However, what if I put a relay switching the +5VSB OFF when the computer is turned OFF and back ON when I turn the computer ON? I bought this relay(I could not find exact data sheet, but this is very similar) and it has fallowing pin-outs. The coil is between pins 1&2. When the coil voltage is 0V, the connection is between pins 3&4. When the coil voltage is ~5V, the connection is between pins 3&5. I would use 5V(red wire) as a coil voltage and then I will solder purple wire to pin 5 and GND to pin 3. In this way the +5VSB will be OFF when I turn the computer OFF(so I get rid of leakage problem) and when I turn the computer ON by shorting green wire and GND, the 5VSB will also turn ON. Even probably before PSU sends Power_Good signal to motherboard :sarcastic:  Will it work? Should I consider some kind of protection chain or any other additions to that relay idea?
May 15, 2009 3:13:05 AM

Reminder: I have a faulty motherboard, which leaks 3.51V from 5VSB rail into 12V rail when the computer is turned OFF. In other words, when the computer is turned OFF, there is constantly 3.51V in 12V rail. I have put the PSU on load using resistors and etc, but this 3.51V is there until I remove 5VSB wire. I have tried with 3 PSU's and it is definitely a motherboards problem. This 3.51V has harmed two of my graphics cards(when the computer is OFF, there is 3.51V on AGP pin1 instead of 0V) already.

So the idea would be keep the 5VSB off the motherboard when the computer is OFF, and turn it on just before the computer has to start. Will this scheme accomplish this idea: http://img12.imageshack.us/img12/9495/scheme7.png ? It would "hold back" the signal of the ON/OFF button thanks to resistor and inductor so that the 5VSB will reach motherboard few seconds before ON/OFF button signal. Or any other suggestions?
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