If you are in to little electronics. then you can build this wonderful circuit. It really works.. it can be used in any device (PC, amplifier etc. with different supply voltages too)

Here is the schematic

http://sound.westhost.com/p42-fig1.gif

It is extremely sensitive, and can easily be set so that a few degrees change is enough to turn on a fan, and reduce the power (or turn it off again) as the PC cools.

Description

The controller uses one or more ordinary silicon diodes as a sensor, and uses a cheap opamp as the amplifier. The design of this circuit uses 12V computer fans. These fans typically draw about 200mA when running, so a small power transistor will be fine as the switch.The one used is a BD140 (1A, 6.5W), but almost anything you have to hand will work just as well.

All diodes are 1N4004 or similar, and Q1 must be on a small heat sink.

The temperature is set with VR1. Operate the PC in Idle, then adjust VR1 until the fan starts. Then back off very slowly until the fan stops again. Any increase over the normal temperature will start the fan, and promptly bring the temperature back down again.

You can test the circuit without a PC, using a diode (or diodes) out in the air. Adjust as above, then hold the diode between your fingers - the fan should start up almost immediately, and stop again when you release the diode. Just the heat from your fingers is enough to operate the circuit. I tested the circuit with 3 standard 1N4004 diodes in parallel, and even without device selection I could hold any one of them and make the fan start.

Resistor Values For R5 and R8
You will need to check the current that your fan draws to calculate the resistance. Connect it to a 12V supply, and measure the current. Calculate the resistance of R8 with ...

R8 = (+ve -12) / I Where +ve is your supply voltage, and I is the measured fan current

Select the closest resistor value larger than calculated. You will also need to work out the power:

PR8 = (+ve - 12)² / R8

Now you can work out the value for R5 (1/2W will be OK here) - here, use the next smaller value if an odd resistor value is calculated. The zener current is nominally 10mA, so ...

R5 = (+ve - 10) / 0.01 Where +ve is your supply voltage.

For example, if you have a 25V supply available, and your fan draws 200mA at 12V ...

R8 = (25 - 12) / 0.2 = 13 / 0.2 = 65 Ohms (use 68 Ohms)
PR8 = (25 - 12)² / R8 = 13² / 56 = 169 / 68 = 2.5 Watts (use 5W)
R5 = (25 - 10) / 0.01 = 15 / 0.01 = 1500 Ohms (1k5)
PR5 = (25 - 10)² / 1500 = 225 / 1500 = 0.15W

Having worked these out, you can adapt the circuit to any voltage, as long as it is less than 30V and 12V or more. For higher voltages, see the alternative version in Figure 2 (below). Note that with the 10 Ohm resistor for R8, the actual fan voltage will be about 10V.

Here is the Link that i found all the details...

http://sound.westhost.com/project42.htm

Caution

I recommend using a separate power supply to power up the circuit and the fans... then you can preserve the watts for peripherals, and also preventing any damage to the PC power supply, due to any error in building the Circuit.

Sweet deal, im going to use this schematic and build one for testing on my client's PCs.

Hi,

Simply awesome, thanks for sharing this with us.

Cheers,
Gabriel.

I'm currently building some other case mods of my own.. I'll post them as soon as they are in correct working order.. these include...

1. Temperature sensor and display circuit (including automatic controlling)
2. Internal case fan RPM display
3. Dual power supply system for PC (Connecting 2 power supplies to the same PC...It saves lot of money on high power O/P Supplies )