I've got 4 Enermax thermistor controlled fans (which are, by the way, on sale at newegg -shipping included- for 1@$12, 2@$19, 3@$25.50, 4@$28.00) that cool my case. There are 3 undervolted panaflos cooling my CPU, HDD in suspension, and VGA and northbridge via a Zalman FB123 fan bracket. There is an 80mm fan in the PSU, along with a 92mm that I have disabled (and need to remove one of these days). Right now the chief noise source is the PSU. I intend to fix that by getting a Silenx.
The 4 Enermax fans are great 90 percent of the time; very, very quiet at normal temperatures. They kick up usually only under stress, like when I've been playing a game for an hour or so.
On occasion, I have disconnected 2 of the fans (the ones monitoring the PCI bus and under the PSU). The system stays quieter still under most web-surfing/word processing conditions. The two remaining fans kick up a little faster when gaming. This is what I want to mitigate.
Here's what I'm wanting to do: I want to rig the two fans that I deem to be necessary only occasionally to an automated process wherein they stay still under most conditions and only turn on at low RPM under system-stress. The fans start at about 5v (1800RPM), thanks to the thermistor. The voltage increases as the resistance in the thermistor decreases due to increasing temperature around the thermistor. 12V is reached at 85C (3100RPM). Since the resistor can't be in stages, it must be linear (the ear test supports this conclusion, as the speed and noise increase increases steadily as the temp does). At 45C, it spins at 2100RPM; at 65C it spins at 2600RPM. Voltages should correlate linearly.
Adding a 56 ohm resistor to the line ought to reduce the initial voltage at start-up to 2v (unless my calculation is off, which I think it probably is), which shouldn't start the fan spinning. The fan shouldn't start (at 5v) until it hits about 50C where the thermistor is located in the case . Since that is free air, it means a pretty high case temp (the 2 main fans will be spinning at about 2200 each). The fan would max out at 12v at about 95C, which will never happen. It should achieve 7v at about 65C.
So the questions are these: Are my calculations correct? If not, somebody please tell me what I can realistically expect.
Also, does anybody see any problems with this set-up. I repeat, I'm no electrical engineer. Am I going to fry or render useless anything I need (for instance, the fan or its PSU line)? Do you think that this will work? Is there anything that I haven't considered?
Thanks for your help guys! Long live SPCR!
