Green Computer Switch

by bkubicek, published

Green Computer Switch by bkubicek May 17, 2010

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This thing goes between your power outlet and the extension cord to which all your computer gear is connected. The ecological dilemma is that you want to be able to switch on your computer conveniently by one switch, but most devices draw a standby power that can be between 1 and 10 W/device. This adds to your monthly bill.

Master/Slave extension cords or ones with switches are an alternative. However, after shutting down the PC is quite easy to forget to turn the switch, or there is still the standby current of the computer.

Hence, this thing is a (partial) remedy. It basically disconnects all the devices from power. By pressing the big push-button, the computer/devices get connected to power. Most PCs have a BIOS setting "Automatic start on power loss". If this is enabled, the computer will boot up while the button is pressed. Of course also the 12V bus in the computer will be powered, and this voltage is connected to my device again, basically holding the button automatically. So after there is power in the PC, the push-button can be released and will be "pressed" automatically.
Of course, until the PC shuts down. Then, the computer and all other stuff will be disconnected again from all power.

Approximate building costs: 12€.
Approximate cost of all PC devices' standby power:8-15€ per year.
Build time: 2:30h

The only drawback is that you cannot use the standby mode any more, as the 12V power will be cut to stop the hard drives. One would need to draw a voltage from the big ATX motherboard connector for that...


In the schematic an the board, there is an alternative 5 V USB connector that also holds the current, however, in my case, the 12 V from the normal "hard drive" connectors works much better. USB is always powered on my motherboard, for some strange reason.

The circuit
The holding voltage is buffered by an capacitor, and decreased by an resistor to feed the LED of the MOC TRIAC-OptoCoupler. The Optocoupler has zero crossing detection, it will switch on and off only at current-zero crossings.
The switch acts as an override for the MOC.

This is not a project for beginners!
If there is a false connection, you might damage you, your health, and your devices!!
Only create this thing if you have knowledge on working with 220V!!!

Please act carefully and respect isolation distances between the 220V wires, especially via the 220V side and the low-voltage side of the board.

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Interesting idea. Some questions and remarks tho:

Why did you build that complicated design with TRIACS and not something a lot more simple with a relay? You could still use the 5V or 12V supply for the primary side of the relay, so it would stay switched on as long as the supply in the PC delivers the voltage and switch off the secondary side as soon as you shut down your PC. The pushbutton would be used to bridge the relay contact on the 230V side until DC voltage kicks in on the primary side to hold the relay position. You just have to make shure that the relay you use is built for 230VAC on the secondary side and the ciol on the primary side works for the DC voltage you use.

Something else to consider, you should add protective devices for the secondary side to prevent overvoltage spikes induced when switching inductive loads! Even tho your triac only switches at 0V, if you have an inductive load the current and voltage are out of phase, so you would cut the supply of
f at the time some amount of current is flowing thru your load, what happens next is that the energy stored in the inductor wants to go somewhere and creates some pretty nasty voltage spike that could potentially destroy your triac or some other equippment. So, you should get a Varistor or other tra
nsient supressing device on your secondary side.
Another good idea would be to include a fuse on the primary side so you dont go over the specs of your triac and circuit board.

The circuit you describe gave me the idea to create this circuit. I personally do not like relais, and I had a surplus of MOCs. The problem with relais is that you switch on anytime, having a possibly high inrush-current, while the moc has no such problem, I think. Switching off, the electric arc and the degradation of the contacts are not such a big factor, as only the standby current needs to be switched off.

The Triac I used was a so-called "snubberless" triac, that should not worry about inductive loads. BTA16-CW this might not be mentioned in the circuit, as I only used a pin-compatible eagle part....

The triac shuts of at zero-current, hence there is no problem with switching off any device.
If I
understand the data-sheets correctly, the switching on can only be done at zero-voltage crossing, as there is no current yet. Correct me if I am wrong: The phase shift between voltage and current has to develop, so the initial current will also start from zero.

Thank you for your input, I am not a
pro when it comes to power electrics. :)

From what I read, this is just a power strip without the protection. Using the automatic startup from power loss is a bad way to start the computer because it means a hard shut down "ie pull the plug" which is bad for the hard drive. Look into Wake on LAN (WoL) if you want a safe way to start up the computer and a "remote desktop" program if you want to shut it down.

Also the computer cant be on the same circuit, otherwise it wouldn't switch the circuit on.

The button isn't used to shut the computer down. The button applies power to the computer, turning it on. The computer's power supply holds the triac switch on, and power is provided to the computer and peripherals. Then, when the computer is turned off (using the shutdown menu as usual) the computer's power supply turns off and the triac switched turns off, removing power form the computer and peripherals.