SPST switch to change fan voltage

I want to make a circuit to switch the fans in my computer from 12v to ~7v (using 2 53.6ohm 1/4W in parallel for each fan) but it has to be controlled using a SPST switch. How can I make the circuit switch from 12v to 7v using only the SPST switch? Thanks!

Connect the resistors from +12 to the fan, always, and use the switch to short them out.

And it will take resistors with a much larger power rating than 1/4W, more like 2W or 5W.

Connecting a resistor in parallel with a fan will just increase the power supply load current without reducing the voltage across the fan.

Connecting a resistor in series with a fan will reduce the voltage across the fan.

Be advised that most boxer fans do not run well with reduced voltage. It is a fine line between them slowing down and just stalling.

I used a calculator (found here) and input 12v to 7v, 0.16a (my fans amperage) to get ~53.6 ohms and 0.46w, which is close to 1/2w. I'll try for 1/2w resistors but if I can't I'll run 2 in serial.

PS: Big noob here, so do I connect the SPST switch to each end of the inline resistor?

ASCII art...

........ Switch
.........|...|
.........|...|
+_____R______+Fan
-.........................-Fan

Yes, you want the switch to short out the resistor to cause the fan to run at full speed.

I find that it's much easier to use diodes instead of resistors to reduce the voltage to a fan. I use bridge rectifier assemblies.

My reasoning is that the diodes provide a better starting current. For high current, bridges with mounting holes are plentiful and cheap.

And most appealing is that there's no formula or resistor to worry about if you change, add, or remove fans.