The said:
And this is what my system basically consists of:
E6420
Gigabyte GA-P35-DS3R
2 GB's Crucial 800 RAM
Corsair 520HX PSU
Evga 7600GT
150gb Seagate Barracuda 7200.10
DVD R/W
So looking over the thread, where we're at right now, is
you press the power button on the front of the computer,
and the power supply fan does not turn. There is no
response from the power supply.
The necessary ingredients -
1) When you switch on the supply, the +5VSB part of the
supply works immediately. On an Asus motherboard, the
green LED on the motherboard, glows in response. I'm not
sure the Gigabyte has a LED for that. If there is no
LED, then a multimeter can be used to check it.
2) The +5VSB powers motherboard standby circuitry, and also
powers the logic that interfaces to the front power button.
It is also part of the interface on the power supply side
(the pullup resistor on PS_ON#).
3) If the motherboard is receiving +5VSB, then it can
"listen" to the front panel power switch. The front switch
is momentary contact, and closes when you press it. That
gives a short pulse to a logic chip on the motherboard.
(The front switch does *not* go directly to the mobo.)
The motherboard latches that pulse, and sends a steady
"on" signal in the form of PS_ON#, a signal on the main
power supply cable.
4) The signal on the cable is open collector. If the power
supply is sitting all by itself on a table top, it won't
run, because there is nothing connected to PS_ON#. That
logic signal has a pullup resistor, to +5VSB. So sitting
on the bench, the power supply by itself, has a 5 volt level
on PS_ON#. When the motherboard wishes to start the supply,
a motherboard driver grounds PS_ON#. That tells the supply
to start, and the fan in the supply starts to spin.
Based on that description, is why someone suggested testing
the power supply, basically testing the PS_ON# response. You
can use a paper clip, between PS_ON# and COM, to ground
PS_ON# and fake the same set of conditions the motherboard
uses.
In a pinch, you can even short PS_ON# to COM, while the
main power supply harness is connected to the motherboard.
(The metal pins are exposed enough to touch with probes or
a paper clip, where the wires go into the connector.)
Since the signal driving PS_ON# is open collector (or should
be), it should be safe to ground it. (Open collector is also
known as wired-OR logic, meaning one device OR another can
ground it at any one time. You can connect multiple open
collector outputs together, and the shorting strap between
PS_ON# and COM counts as just another such driver.)
The downside of manually overriding the motherboard, is you
disable motherboard protections. For example, say the heatsink
fell off the processor. The motherboard detects this as THERMTRIP
and attempts to turn off the power. If a shorting strap was
present between PS_ON# and COM, the power supply continues to
run. So in theory, in that particular situation, the processor
could overheat. So the trick of connecting PS_ON# to COM
is not something I would feel comfortable with, for long
term usage (some users who have motherboard logic failures
on PS_ON#, sometimes use that technique). It also affects
your shutdown sequence, in the sense that, at the end of
the present session, when you select shutdown in Windows,
the computer thinks it has powered off, but is still receiving
power. Maybe it'll have that "it is safe to turn off your
computer" on the screen, but I don't know that for a fact.
You can also test the power supply, by removing it from the
computer and testing it by itself. That would tell you
whether the supply is listening to PS_ON#, but not whether
the motherboard PS_ON# driver is able to drive the
signal to ground. (You should have a small bit of loading
on the supply, to help the supply maintain regulation.
I have my own home made load box for that purpose. Costs
about $50 for a handful of power resistors and an ATX
connector.)
If you used a multimeter, and measured the PS_ON# signal
level, with respect to chassis ground, you'd see +5 volts
(coming from the +5VSB on the supply) sitting on the line.
When you press the front panel button, the level should
drop to pretty close to zero volts. That means the motherboard
logic is doing the right thing. But if the power supply
doesn't start, it may be because the driver is weak and
cannot make it all the way to ground. Or the pullup on
the power supply side has become stronger than normal
(unlikely), and the poor driver cannot possibly pull it
down.
If for any reason, the +5VSB rail gets shorted out, inside
the supply is internal protection against overcurrent.
So checking the power supply harness for +5VSB, gives
some idea whether that is happening. In the case of
Asus boards, the comforting sight of the green LED
on the motherboard, saves you that step, as the green
LED is powered by +5VSB.
Ref: Power supply specs, from oldest to newest. The last
link is suitable for a 24 pin power supply. The wire
colors and pin numbers are in the spec.
http://web.archive.org/web/20030424...org/developer/specs/atx/ATX_ATX12V_PS_1_1.pdf
http://www.formfactors.org/developer/specs/atx/ATX12V_1_3dg.pdf
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
Hope that gives you a few ideas,
Paul
