rchapman said:
My PC shutdown and won't turn on. I noticed a little green light on on the
motherboard. It goes out when I unplug the power supply. I tried jumping the
power button/switch to see if it went out but get nothing. What can I do to
find out what has happened etc? Thanks.
On an ATX power supply, the supply is divided into two parts.
One part puts out +5V and that rail is called +5VSB. The
SB means "Standby" and is used to power the computer when
it is sleeping.
When you plug in the power supply, there is no power output
to begin with. When you switch on at the back of the computer,
that turns on +5VSB only. The rest of the power is controlled
by a "soft power" feature. That is controlled by a signal
on the main 20 or 24 pin connector, called PS_ON#.
So when you turn on at the back of the computer, +5VSB
starts to flow. It lights the green LED on the motherboard
(not all motherboards have that LED, but many do). The +5VSB
also powers a bit of motherboard logic. The Power button on
the front of the computer, connects to the motherboard logic.
It is up to the motherboard logic, to decide what to do.
For example, when you turn off a computer sometimes, via the
button on the front, there is a delay of four seconds. That
delay is introduced, to make it harder to accidentally hit
the switch. The delay (time measurement) is done by the
motherboard logic. So the motherboard logic "conditions"
the signal from the switch.
The signal from the switch, therefore, doesn't go directly to the
power supply. There is a "lump of silicon" in between them.
When the computer will not switch on
1) It could be the switch itself. Unplug the switch and
check with an ohmmeter. It will read zero ohms (shorted),
only when the button is pushed. Sometimes the switch is
broken, and that is the source of the failure. The switch is
a momentary contact switch, which is ON (shorted) only when
you press it. It is open circuit the rest of the time.
2) The motherboard can fail. The motherboard logic may fail
to react to the logic signal from the switch. Or the
driver may burn out. The driver that sends the signal
on PS_ON#, is open collector, and what it does is
bring the voltage level on the PS_ON# signal to ground
(zero volts). That is the state that turns on the rest of
the supply, the fans spin and so on.
3) The +5VSB may be out of spec, from the power supply. If
the voltage was low enough, the LED might light, but
the sequence could not be completed. Sometimes, it is
the power supply itself, which is no longer listening
to PS_ON#.
In terms of "jumping something", you can connect PS_ON# to
COM, on the main power cable harness. That will turn on
the power supply. It would override the motherboard logic.
It also overrides one of the safety features though, so is
not a permanent solution for those people who have a motherboard
type failure. If the CPU starts to overheat, it is important
that the motherboard logic be able to turn off PS_ON#.
To debug the problem, you could do a little work with a
$20 multimeter from the hardware store. Check the voltage
level on PS_ON# when the button is pushed on the computer.
(Remembering that 5V is "OFF" and 0V is "ON", and intermediate
values like 2V mean something isn't working right. PS_ON# should
be close to either extreme, but not at a median value like 2V.)
But if you're not a "debugger" person, then swapping in
a replacement power supply, and if that doesn't work, a
replacement motherboard, are other solutions. Not nearly
as cheap, but less skill is required to do it. You do the
power supply first, only because they're easier to get
at a local store. An exact replacement motherboard may take
weeks to acquire.
For more info, this document covers 20 pin power supplies.
http://www.formfactors.org/developer/specs/atx/ATX12V_1_3dg.pdf
And this one is for newer, 24 pin supplies.
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
Note that the "Timing/Housekeeping/Control" diagram in those
documents, is not a complete functional sequence. The
dropping of the voltage level on PS_ON# does illustrate
what happens during turn-on, but at the end of the
figure, they "pulled the plug out of the wall", rather
than using the PS_ON# signal. A diagram showing normal
function, would have showed the PS_ON# signal rising to a
high level again at the end of the diagram. The diagram is
mainly used to illustrate what they mean by T1,T2,...T6
timing numbers.
For example T5 is called the "holdup time",
and is how long the computer is guaranteed to run if the
power goes off for a split second. The holdup time is
16 milliseconds and gives your UPS time to switch over
to batteries and continue to provide A.C. to the computer.
A cheap UPS actually stops supplying power for a
short interval, and the ATX supply has enough holdup
to continue running while that is happening.
One other thing. In terms of failures and collateral damage,
modern power supplies are pretty good. A modern supply
may fail internally, without damaging the motherboard,
RAM, hard drives and so on. But if you have a machine with
a certain Bestec 250W supply in it, the Bestec ruins the
motherboard when it says farewell. So there is a possibility,
if you have a computer with that Bestec in it, that
there is an awful mess waiting for you inside the
computer. Power supplies generally have a label on them,
with power rating and brand name, so you can check it.
A comment on Bestec 250W, here...
http://www.dexplor.com/phpBB2/viewtopic.php?t=7747292
Paul
