[SLL] Hard reboot on poweroff (Re: limit on number of files in a directory and hashed dir vs. flat dir file access time)

[Robert Woodcock]

On Wed, Nov 07, 2007 at 04:04:28PM -0800, Robert Woodcock wrote:
> Maybe for an alternate approach, turn the PC off using ACPI, feed the relay
> coil using the 12VDC line off the power supply, and have the relay's normally
> connected and common lines feed to the motherboard's power on pins. Power
> goes off, relay makes continuity between the power on pins, power comes on,
> relay breaks continuity between the power on pins (just like you'd pressed
> and released the power button). Putting a small-value (say, 100 ohm)
> resistor in series with the relay coil and a capacitor in parallel with the
> coil would provide some time delay here.

One more note on this: put a diode in series with the resistor so that the
capacitor does not attempt to supply the PC with 12V power.

This might be clearer:
                                          ____________
                      100ohm             |  12V relay |
12VDC+ ------->|------/\/\/-------+------|+           |
(yellow)     diode   resistor     |      |C       N/C |---------- To mobo
                                  |      |o           |           soft
                      capacitor =====    |i         C |---------- power
                                  |      |l           |           switch
GND    ---------------------------+------|+       N/O |
(black)                                  |____________|

I just did a bit of testing - the relay I have measures 380 ohms across the
coil. A 15000uF capacitor powered it for about 6 seconds, and a 25000uF
cap was good for about 10 seconds. So if you want your drives to have time
to spin down before they spin back up, use a very large capacitor.

I also tested to see if the peak 1.5 watts through the 100ohm resistor would
fry a 1/4 watt part - it won't - the peak wattage doesn't last long enough to
get the resistor warm.

Note that I haven't actually tried this in a computer yet - if you decide to
try it, let us know how (if) it works.

This circuit might also be useful for remote machines that you really don't
want to accidentally shut down. :) It should also force the computer to
power back up after any power disconnect, no matter what, although most
BIOSs have a setting for that anyway.
-- 
Robert Woodcock - rcw at blarg.net
"When you can measure what you are speaking about, and express it in numbers,
you know something about it; but when you cannot measure it, when you cannot
express it in numbers, your knowledge is of a meager and unsatisfactory
kind: it may be the beginning of knowledge, but you have scarcely, in your
thoughts, advanced to the stage of science."
	-- William Thomson, Lord Kelvin