[Nut-upsdev] How do I approach this problem?

[Robert Woodcock]

On Mon, Nov 10, 2008 at 07:23:58AM -0800, Jeff Cunningham wrote:
> Is there any way to have the UPS restart my computers once it knows its
> back on live power?

Most BIOSs have a setting for this - it's usually set to "Last State", which
unfortunately doesn't really work on some motherboards - "Always On" will
work.

There's (at least) two other hardware routes you can take:

1. On the ATX motherboard power header, short the green wire to a black
wire. This forces the power supply on.

2. Rig up a relay to momentarily short the motherboard's power switch when
power returns, toggling the power on. I have not actually tested the
following circuit in a PC, but it's simple enough, and it should act a
little nicer than the first method:

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


Basically, when everything is off, the N/C and C pins will be shorted
together (like you're pressing the power button). When power returns, this
will turn on the computer. Once the computer's 12VDC+ line puts out power
long enough to charge the capacitor and energize the relay, the relay
switches and the short is removed (like you had released the power button).

If you attempt to turn off a computer with such a circuit attached, the
capacitor will temporarily power the relay (the diode prevents it from
attempting to power the rest of the computer) and give the disks a chance to
spin down. Then the relay contacts will close and the computer will turn
back on.

I did go so far as to test how long a capacitor would power a relay - with
the relay I had (which was 380 ohms across the coil), a 15000uF cap would
power it for about 6 seconds, and a 25000uF cap would power it for about 10
seconds.

The resistor will see about 1.5 watts peak, but since that's only for a
second or two, a 1/4 watt resistor won't even get warm.

If you actually use this, please let me know how it works for you.
-- 
Robert Woodcock - rcw at debian.org
"If an elderly but distinguished scientist says that something is possible he
is almost certainly right, but if he says that it is impossible he is very
probably wrong."
	-- Arthur C. Clarke