PC Fan Speed Controller – For a Low-Noise PC

The fan runs constantly
in many PCs, which may not even be necessary. A simple controller
circuit can regulate the fan speed according to demand. This not only
saves energy, it also reduces irritation from the fan noise. Only three
components are needed to allow the fan speed to be controlled according
to the actual demand: one adjustable voltage regulator and two resistors
that form a voltage divider. One of the resistors is a NTC
thermistor (temperature-sensitive resistor), while the other is a
normal resistor. If the 12-V power supply is not located close to the
regulator, a decoupling capacitor is also required (see Figure 1).

Circuit with LM317

Circuit diagram with LM317

The thermistor has a rated value of 470 Ω. It sets the output
voltage of the LM317T to approximately 7 V at 25 ºC. This should ensure
reliable starting of the fan. If the temperature rises to roughly 40°C,
the output voltage of the regulator reaches its maximum value and the
fan runs at its maximum speed. The voltage drop across the regulator is
at least 1.75 V for a motor current of (for example) 300mA, and in any
case 2V at the maximum current level of 1 A. You thus might want to
consider using a low-drop regulator, such as the National Semiconductor
LM2941CT.

Circuit with LM2941

Circuit diagram with LM2941

To be sure, this increases the size of the circuit to a full five
components, which are arranged as shown in Figure 2. However, this
approach reduces the voltage drop to 0.2 V at 300 mA or 0.5 V at 1 A. By
the way, low-drop voltage regulators are not available in a three-lead
package. The circuit can be constructed as a well-insulated
‘free-standing’ assembly, or it can be built on a small piece of
prototyping board. In either case, it should be fixed to one of the
mounting holes of the fan body (via the cooling tab of the TO-220
regulator package for the free-standing construction). The circuit board
should be mounted out of the air stream, but the NTC thermistor must extend into the air stream.

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