Serial connected fans?
Thank you for a great site and a very interesting forum. I am in the process of planning my new rig, which I will build in a couple of months. Before I found your site I did not know anything about quieting techniques, but now I am getting ambitious! Basically, I always built my systems under the assumption that noise was a 'necessary evil', so instead of thinking about noise I always looked at getting the most bang for the bucks - literally, SCREECH! RATTLE! SWOOSH! SCREECH! RATTLE! BANG!
I am not fanatic about getting below 20dB total noise and it will actually be impossible to build a rig noisier than the Boeing 747 sitting under my desk at the moment. I have to work within a limited budget, so I don't want to spend money on a fan controller as that would mean buying some other component with more noise/lower performance as a trade-off.
Calling me 'a technical type' would be the exaggeration of the century, but I seem to remember something from my school days (almost two decades ago) about that serially connecting two identical devices on a DC line will make both devices run at half the voltage. As the motherboard I am looking at has a 'case fan socket' with three pins (ground, 12V, rpm sensor), I got the idea to connect two fans in serial and connect the rpm sensor wire to one of the fans (as they are of the same brand and model, logically, they should run at about the same speed). In that way I could run both fans at 6V without a fan controller and I could get the rpm feedback through the MB.
The two fans will be Scythe S-Flex SFF21F with the following manufacturer's specs:
1,600rpm @ 12V - 0.20A - 1.8W (63.7CFM)
I found the following test results at www.stillepc.dk
12V: 1,620rpm @ 31dB(A) (manufacturer's specs: 28db(A))
9V: 1,215rpm @ 24dB(A)
7V: 922rpm @ 19dB(A)
6V: 750rpm @ 16 dB(A)
5V: 588 rpm @ 14dB(A)
They use some laser tachometer gadget for measuring rpm, so they also compare noise level at the same rpm (regardless of voltage). By coincidence, the SFF21F was measured to require 6V in order to produce 750rpm for the rpm based comparison, so that is why 6V gives such a 'neat' rpm number!
The two fans will have to fight quite a bit of back pressure as I need a better air filter than the thin foam thingy normally used, so I was looking for a quiet fan with high rpm when undervolted (I assume that means it has higher torque to fight the back pressure). I was hooked on the Yate Loon, but they are not represented in Hungary, so that is why I chose the Scythe SFF21F.
These are my questions:
1. Am I wrong about being able to run two serially connected fans at 6V?
2. Is there a reason why it would be a bad idea to use the motherboard connector for this (e.g. risk of damage to the MB)?
3. Do you think I would still be able to get rpm readings from one of the two fans?