/diy/ - Do It Yourself

>>2792280
The easiest thing to do is prevent each source from backflow with a diode as in picrel shown here with two DC input jacks. This is the simplest variation.
This is essentially the same circuit as an automatic battery back-up (like you might see in alarm clocks with a 9 V alkaline battery)
It’s common for two wire DC fans to use voltage controlled speeds, usually 8 V for “low” and 12 V for “high” because the voltage/speed curve is not linear.
Always consider passive cooling before going to fans, that’s an intrinsically fragile design. With dust. Dust is the enemy, probably more things have been killed by dust than anything else.