I actually don't need speed control on the fans - I just need them to come on and off
So with that in mind - I take it I wire it like this :
Mains power -> 100 watt single output power supply -> 8 channel relay -> LED and Fans.
My question here, is where would the Pi sit in all of this? I take it I can hook the Pi to the relay module and turn those on or off as needs be?
Also - Regarding the other reply about the MOSFET - would that be a better solution for at least the LED? Then use the relay module separately just for the fans?
The 12V output of the power supply will need to be wired to 9 places........ The relay board power connection AND to the common terminal of EACH relay.
(I am assuming that the mains will be connected to the power supply directly and always on or controlled by a manual switch),
The fans / light would be connected to the NO relay contacts.
Grounds from each all tied together and connected to the power supply ground.
8 GPIO's of your choice will be connected to the IN1-IN8 relay board terminals plus a ground wire from RPi to relay board,
Then you will need to write code to turn each of the relay's on or off according to when you want the light and fans to be turned on. For example you might want turn a fan on at a certain time of day or maybe add a temperature sensor and have the RPi turn on a fan(s) when the temp goes above a certain set point etc.
What is your project about and why do you need 7 independently controlled fans?
Forget the MOSFET's unless you are prepared to do soldering.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.