pcchips m760v - restore on ac power loss

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My old pcchips m760v motherboard would not Restore on AC Power Loss
no matter how I configured the bios.

I started building a circuit using a small 5 volt relay, but then
found a better solution on the web:

http://www.expresshosting.net/howto /

I ended up simply putting a 100 micro-Farad capacitor accross the power
button contacts, pins 21-22 on jumper 11 on my motherboard.

(Be sure to check the polarity with a volt meter, as most 100 micro-Farad
capactors are elecrolytic and do NOT like to be connected the wrong way...)

For my motherboard pin 21 is +5 volts and and pin 22 is ground.

This "fix" might also work for other ATX motherboards.
And where the power button switch is broken or missing, but you have a spare

Happy Hacking!

comet at transbay dot net

Re: pcchips m760v - restore on ac power loss

Gonzo wrote:
Quoted text here. Click to load it

There are a couple test cases to be considered.

             X-------+---------------> To logic IC
             |       |
          \       + ---
           \        --- 100uF
            \        |
             |       |
            GND     GND

When the capacitor is fully charged to 5V, and if you then push the button,
the 100uF capacitor is dumped through the switch. The contacts on the switch
have to be able to handle the current surge.

The second concern, is if the Logic IC is powered off. Many ICs have clamp
diodes to VCC and GND. If the unit is powered off (say the AC just failed),
that brings VCC to zero volts. The capacitor, on the other hand, has a full
5V across it (because it is fully charged). In this case, that means the energy
in the capacitor, is potentially dumped through the clamp diodes in the Logic
IC. Clamp diodes are rated for perhaps 10 mA or so. A potentially much larger
current can flow in this case. What would be needed, is what is referred to as a
"fail safe input pad", one where the upper clamp structure is modified or

The solutions involve resistors, and resistor ratios. You can add
a resistor in series with the power switch. That limits the current
when the switch is called upon to drain the 100uF capacitor. You can
also add a series resistor leading to the Logic IC. To make the circuit
work properly, at that point, then you have to analyse what voltage
levels result from the use of the resistors, and whether noise margin
is affected or not. The analysis may be different for different motherboards,
so you may want to again, modify your design concept, to make it more
independent of situation.

Just some caveats for the Google archive, in case someone finds the
original post... If the capacitor wasn't present, then the two added
resistors would no longer be needed.

             X-------+----- resistor ----> To logic IC
             |       |      (protect
          resistor   |       clamp
             |       |       diode)
             |       |
          \       + ---
           \        --- 100uF
            \        |
             |       |
            GND     GND

If you want a concept for generating a pulse, this chip
has existed for an eternity. If the active high output is
a problem, it can be used to drive a transistor that
grounds the power switch terminal. Such a setup would be
more compliant with the interface, but more work to assemble.
There are undoubtedly better chips for the job, but it
would take a while to find more examples. This chip is
so versatile, a whole book was written with sample
circuits in it.



Re: pcchips m760v - restore on ac power loss

Quoted text here. Click to load it

I'm not too worried about the current surge throught the switch as it has metal
contacts and the voltage is low.

(I don't need the switch, that is the whole point of having the capacitor.)

The RC time constant when the system is powered off appears to be in the
neighborhood of one second.  

Time = Resistance times Capacitance  or T = RC

This is good because R must be in the neighborhood of

1/(100*10e-6) = 1k ohms   ( T/C = R )

And 5 volts running through 1k ohms is about 5 mA,  under the 10mA of a
possible clamp diode.  ( I = E/R )

(I think that the logic circuit for a switch must be robust anyway as it
must handle a very dirty switch which generates a very jagged signal.)

This RC time is bad because if the system powers off and then powers on in less
than one second, the "automatic" switch will not switch...

It might help to experiment with lower capacitance.  Maybe 50uF or 25uf might
work.  This would lower the RC time.  

Fortunately newer motherboards do not have this problem  and don't need this
capacitor "fix".

This capacitor solution is not perfect, but it is simple and good enough for

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