What's off when it's off!?

[SpellowHouse]

Can anyone tell me how a modern PC power supply works...what is off when the PC is in standby?

I know that when powered down that some motherboards maintain some functionality, so there must be some power-lines still active, but surely some must be physically off, such as the SATA power connectors?

The reason I am asking is because I want to "sense" the state the PC is in, and it seems to me that monitoring the SATA power lines from the PSU is one possible answer.

 

[SpellowHouse]

I don't think SATA devices should be powered when the PSU is off.

Exactly. So my thoughts are that I will monitor the SATA lines. It's an easy one to do because of course STATA power connectors are easy to obtain, so all I need to do is buy a SATA power extension and clip the male plug off and BAM there are the wires to connect to!