How Do These Kind of Power Switches/Systems Work?

Question

Just about all power switches in DIY electronic projects, such as Arduino, simply cut off the power source immediately when you turn it off. However, there are devices like 3D printers that when you push the power button to turn off, it does a few last tasks before the power actually cuts off. These tasks include running the motors to specific positions and writing a goodbye message on the display. How do these type of power down buttons/systems works? I don't know the exact name for these types of power buttons as well. If someone could explain what they are and how they work, that'd be greatly appreciated!

Answer 1

As others said, such a button that does not shut down a system (like a 3D printer), it is not a button that is connected to the main power of the system. It is connected to an microcontroller that senses the push of the button. Like this:

^{simulate this circuit – Schematic created using CircuitLab}

SW1 switch is what you press. This is seen by an MCU (microcontroller) and the MCU turns the M1 mosfet off (by outputing 0V to the Q1 transistor) and the "System to be powered" turns off.

A "disadvantage" of this system is that your MCU needs always power to sense when you will press the SW1 to turn off the system, so a little bit of energy is wasted on that MCU.

Answer 2

A power button as you described is not a power button. It is a button that triggers a set of code like any other normal button. It uses the same logic as any other button you include in a project. The microcontroller either polls or has an interrupt that checks for a change in the button and then follows the code you have programmed into your project.

For the most part the code tells things to stop doing anything or send specific commands then puts itself to sleep and the only other thing it should do is wait for another button press to start doing things again. Nothing special about a soft power button.

Answer 3

The power switch probably is marked with the On-Standby symbol, as opposed to the On-Off symbol. To the uC (micro-controller), it is just another input; there is nothing special that separates it from any other digital input. To the uC it is either a polled input or an interrupt. Either way, when the uC sees it change state, it jumps to a designated section of its programming and executes that routine or sub-routine.

I tried to keep the wording of that description as generic as possible, to reinforce that to the uC this is just another thing to do. In this case, the thing to do is turn off the power supply, or place it in a standby mode, or disable a power transistor between the power supply output and the rest of the system.

The reason the switch has a different symbol is because it does not completely turn off everything. Specifically, there still is some kind of connection between the power system and the control system, even if it is only microwatts. Something has to be awake enough to detect when the switch is pressed again, so it can power up the system. The switch marking is a warning to service persons that things still are alive inside.

This small amount of power is actually a big deal. With billions of such devices running across the country, those microwatts (or entire watts in many cases) add up to megawatts of wasted energy while stuff sits in the Standby mode.