Automatic Mains Disconnect

Downloading and
CD-burning programs usually provide the option of automatically shutting
down the PC on completion of their tasks. However, this energy-saving
feature is of little benefit if even after the PC has been switched off,
all of the peripheral equipment remains connected to the mains and
happily consumes watt-hours. The circuit shown here provides a solution
to this dilemma. It is connected ahead of the power strip and connects
or disconnects mains power for all of the equipment via a power relay. A
connection to a 12-V PC fan (which may be the processor fan or the fan
for the chipset, if the latter is present) indicates whether the PC is
switched on.

If you are certain that the 12-V power supply voltage is switched
off when the PC is in the sleep mode, you can use this connection
instead. To switch everything on, press the Start button to cause the
power relay to be energized and provide mains voltage to all of the
equipment. If the PC has an ATX board, its
Power switch must be pressed at the same time to cause the PC to start
up. When the PC fan starts to run, low-power relay Re1 engages and takes
over the function of the Start switch, which can then be released. This
state is stable. If the PC switches to the sleep state, the 12-V
voltage drops out.

The electrolytic capacitor ensures that Re1 remains engaged for a
short time, after which it drops out, followed by the power relay. D1
prevents the electrolytic capacitor from discharging through the
connected fan, and D2 is the usual freewheeling diode. The system is
disconnected from both mains leads and is thus completely de-energized.
Be sure to select components that are suitable for their tasks.
Naturally, the contacts of Re2 should be rated to handle the total
current drawn by all of the peripheral equipment and the PC, and the
relay coil must be suitable for use with mains voltage (6 mm minimum
separation between coil and contacts).

A low-power 12-V relay that can switch mains voltage is adequate for
Re2. The Start pushbutton switch is connected to the mains voltage, so a
230-V type must be used. The circuit board layout and enclosure must
also be designed in accordance with safety regulations. A separation of
at least 6 mm must be maintained between all components carrying mains
voltage and the low-voltage components, and the enclosure must be
completely free of risk of electrical shock. With a bit of skill, the
circuit can be fitted into a power bar with a built-in switch, if the
switch is replaced by a pushbutton switch having the same mounting


  • The circuit is not suitable for use with deskjet printers that can only be switched on and off by a front panel button.

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