need electronics guru help w/ fanbus resistor wiring

[sanaka]

My fanbus plan:

Leave cpu and psu fans on full all the time. Three other fans controlled as one on a 2 position switch - cool&loud or notascool&quiet - stone simple. I'm not interested in "dialing in" w/ a rheostat (also can't find like a 10 watter).

For reasons I won't bore you with here I don't want to do the 12v/5v-psu-wire-switcheroo to get 7v. Instead I'd like to put a fixed resistor on the 12v line to acheive that result.


Mr Ohm sez V=IR
The total spec'd fan power is 4.8W
so math shows that:

>My total (parallel wired) fan load @ 12V is .40A x 30.00 Ohms

>at 7V (which would acheive my target fan rpm), current would be .23A x 30.00 Ohms

>to retain current .23A @ 12v, I need 51.43 Ohms

>requiring an approx. 22 Ohm resistor (20 or 25 would seem fine) in series w/ the fan grouping.

I had to bang my head on a high school physics book to get this far, so I have no idea whether this setup would conform close enough to textbook ideals that I'm within a resonable tolerance? Or in this instance would voltage freak relative to resistance or something, throwing my math out the window? Basically would this work about the same as doing the PSU wire switcheroo?

Sorry for a dang long post, but hey, you're my friends right?

 

[cockatiel]

I don't have time to redo your math but it looks to me like you did it right. Don't forget that your resister must be able to dissapate at least (i squared * R) at least 1+ watts.

 

[sanaka]

thanks, cockatiel.

Yeah, I'm asking with the assumption that my calculations are correct. Just wondering if there's any fundamental flaw with the real life application of overall concept.