Help designing a circuit

[MattR83]

Electrical Engineering has never been one of my strong points, I can hook all the parts up, but when I have to figure out what I need I get into some problems. That being said, what I want is a knob that will let me vary the output voltages to some fans. Given +5, +12, Ground, -5, and -12 I should be able to come up with 5, 10, 12, 17, and 24v. Now the part I can't figure out is what I would need to do to be able to control this with one 5 position knob (or if it's even possible).

If anybody could give me a little help with this I'd really appreciate it. Thanks a lot.

P.S. I know it's not a good idea to overvolt the fans, but I'm not worried about it, 17v won't be used for extended periods of time, and 24v is only there because it can be.

[Oni]

You want something called a 'potentometer' (I think). Talk to the forum member named Doctor about it, I know he knows, but I'm sure there are other forum members who would know how to help you implement what you're thinking about as well.

[Gravity Man]

Just get a rotary switch from radio shack. The 2 terminals in the center are where you should attatch the red and black fan wires. to get the different voltages, attatch the wires you will be using to terminals opposite each other (i.e. for 5v, attatch the 5v wire opposite the ground wire).

HOWEVER, this mod will probably not work, and you could possibly damage your computer or power supply, because the -5v and -12v lines are not rated for very many amps!!! My generic 300W's -5 and -12 are both rated at .5A, and on my Enermax 431W, they are rated at 2A and 1A, respectively. ALMOST ALL FANS NEED MORE POWER THAN THIS!!!

[RoadWarrior]

I always wonder why no-one takes 3.3V off their ATX supplies, since a few "12V" fans I've seen are specced from 10-16V, so you could run them in spec at 15.3V .....

Anyway, seems you have a 5 pole one way switch there, 5 possible inputs, one out put. You need either two of those switches or a double ganged switch with 5 poles. Since you have to change the "ground" of the fan from 0 -5 +12 (maybe +5 as well) and also change the +ve from +5 and +12. I wouldn't count on using the -5 as it's not very high output on most supplies, only 0.5A usually. You could probably do it with 2 3pole 1way switches too. Set the gnd and +ve independantly.

Potentiometers are variable resistors, that can reduce the voltage, however, most potentiometers will only handle mere fractions of an amp, you need something hardier, called a rheostat if you do it that way.

regards,

Road Warrior

[mw521]

Look around on here, it may answer a lot of questions.:beer:
http:www.fanbus.com