potentiometer for cockpit/demist fan?

[tvrolet]

OK, so I have the fan in place now that'll do the demist for the SVA and also give a bit of air to the driver. Except that with just an off/on switch it's either zero or a hurricane! So, off to the local Maplin's to get a potentiometer thought I...except that after being baffled by some nerd quizzing me over ohms, linear or logarithmic etc., he concluded none of their stuff would handle a 12v fan without burning out anyway.

So, given it's a 12v fan that's meant to be connected through a 4amp fuse I'm guessing I need something rated at 50 watts (48 watts ). But as for all the rest of the options, who knows? All I want is something that has a switch in it (push or turn/click), that controls the speed of the 12v fan from off through to full power. So what of the 1000s of potentiometers I can find on the net do I need?

[steve_d]

Find something old at the breakers. Newer cars have different ways of dealing with fan control such as multiple windings in the motor. Something like an old Rover SD1 or Range Rover may include the heavy resistors you need to switch through.

Steve

[Avocet]

It will probably be a very small fan if it has a 4 amp fuse so you MIGHT get away with a big wire-wound pot that won't give off too much heat. Most cars have several distinct speeds. Some older ones (as has been suggested, a scrapper is bound to have something like that) just have a big wire-wound resistor with tappings off it so that on the lowest speed, the whole resistor is in series, at the next speed, 2/3 of the resistor is in series, at the next speed 1/3 of the resistor...and so on.

The more modern type are a lot more complex. Some use a "chopper" circuit which interrupts the power to the fan motor several times a second and this can be increased to slow the fan down (or vice versa). A simple 3 speed switch from (say) an old Cavalier and a three speed resistor would be my preferred choice. Remember to mount the resistor somewhere in the cool air intake to the fan to keep it cool!