Hi, I’m modding a mac pro case to a ATX case, and i want to build my psu into the original mac pro housing. I’m using a Corsair RM750. How it fits is not the problem, it’s the cooling. The RM750 has a 140mm fan, that only starts to spin at min. 40% load. The original casing has intake for 2 60mm fans ( i want to place 2 noctua’s there). What i want is to make a small pcb that powers this fans with 7 volts so a have a constant airflow. But when the original fan should kick in, I want to add those volts to the 7 volts to get more airflow, with a max of 12 volts. Anyone knows how i could do this? I have a good soldering skill, but i don’t really know what components i need to make this work. Another solution is to let the front fans constantly spin at 5 or 7 volts, and mount 2 fans extra in the back so when the original psu fan should spin, the back to start to spin. But there i have a concern that the airflow isn’t great when the back 2 fans aren’t spinning.
I'd be very careful of touching stuff inside a PSU as even when switched off the capacitors can hold a change for a while. Your second option is perhaps your 'safer' one I'd guess. Perhaps have front and back in sync so they're always going to keep a good airflow?
I wouldn't even begin to dream about doing that. Why not have the fans just come on when the PSU gets too warm? That's how it works normally. The PSU doesn't produce much heat until it is put under load, so it just idles as the airflow around it is often enough to keep it within limits. You're thinking too much about the airflow. As long as it is in a closed chassis, which I think it is based on how you're describing it, the airflow from the front will be more than enough as it is constantly pushing fresh air through the unit and 'hot' exhaust air out of the rear.
