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Automating PC Power-Up
Powering off your PC and associated peripherals from the
mains will eliminate standby power usage and save you some
money. It also reduces the PC’s exposure to mains-borne
surges and may prolong power supply life.
However, a recent email from one of our readers points
out that when powered up from the mains, all late-model PCs
simply enter standby, or “soft power” mode. You then need
to push the front-panel power switch to boot up. Not content
with having to perform this extra step, he went on to describe
a simple means of “pushing” the button electronically, so
that his PC powers up as soon as mains power is applied.
On all late-model (ACPI-compliant) PCs, the power switch
is connected to an I/O controller chip on the motherboard.
This chip is powered from the 5V standby supply, so it’s always
powered when mains power is present. Pressing the switch closes a circuit
to ground, signalling the I/O controller to electronically switch power to all
other circuits and thus boot up the machine.
As shown in Fig.1, a large value capacitor can be connected across the
switch to simulate a switch press. At power up, the capacitor charges slowly
via a pull-up resistor internal to the I/O controller chip. If the capacitor is
large enough, the I/O controller sees a valid logic low level on the switch
input line after the power supply outputs have stabilised.
We’ve also included a 100Ω resistor in series with the capacitor to limit
discharge current into the I/O controller at power off. Several motherboards
we’ve examined have a resistor in series with the ground line, which would
achieve the same result. However, as we can’t be sure that they all do, it’s
wise to include the resistor anyway.
The capacitor and resistor can be soldered directly to the power switch
pins. Insulate all leads with tubing as necessary to prevent short circuits. Note
that the polarity of the capacitor is important. Use your meter to determine
which of the two wires from the motherboard is the ground wire. The ground
wire will measure between zero and about 500Ω to case ground, whereas
the I/O controller input will measure much higher.
The suggested value of 470µF for the capacitor should work in most
cases. However, differences in power supply design may mean that you’ll
need to increase this value for your particular PC.
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Important: always disconnect AC power from your PC before opening
the case! Note that old “XT” & “AT” type PCs do not consume standby
power; the front-panel power switch physically switches mains power to
the power supply input.
KALEX
• High Speed PCB Drills • PCB Guillotine
Laser Labels • PCB Material – Negative
or Positive Acting • Light Boxes – Single
or Double Sided; Large or Small • Etching
Tanks – Bubble • Electronic Components
and Equipment for TAFEs, Colleges and
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We now stock Hawera Carbide Tool Bits
Fig.1: a 470µF capacitor and a 100Ω resistor connected to the front-panel
power switch are all you need to make your ATX computer boot as soon
as mains power is applied.
siliconchip.com.au
718 High Street Rd, Glen Waverley 3150
Ph (03) 9802 0788 FAX (03) 9802 0700
Website: www.users.bigpond.net.au/kalex
Email: kalexpcb<at>bigpond.net.au
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June 2004 77
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