PC Fan Speed Controller - For a Low-Noise PC

The fan operates continuously in many PCs, which may not be necessary. A simple controller circuit can adjust the fan speed based on demand. This not only conserves energy but also minimizes noise irritation from the fan. Only three components are required to control the fan speed according to actual demand: one adjustable voltage regulator and two resistors that form a voltage divider. One of the resistors is an NTC thermistor (a temperature-sensitive resistor), while the other is a standard resistor. If the 12-V power supply is not located near the regulator, a decoupling capacitor is also needed. The thermistor has a rated value of 470 ohms, setting the output voltage of the LM317T to approximately 7 V at 25 °C, ensuring reliable fan startup. As the temperature increases to around 40 °C, the output voltage of the regulator reaches its maximum, causing the fan to operate at full speed. The voltage drop across the regulator is at least 1.75 V for a motor current of 300 mA and at least 2 V at the maximum current level of 1 A. Therefore, it may be advisable to use a low-drop regulator, such as the National Semiconductor LM2941CT. Although this increases the circuit to five components, it reduces the voltage drop to 0.2 V at 300 mA or 0.5 V at 1 A. It is important to note that low-drop voltage regulators are not available in a three-lead package. The circuit can be assembled as an insulated free-standing unit or built on a small prototyping board. In either case, it should be secured to one of the fan's mounting holes via the cooling tab of the TO-220 regulator package for free-standing construction. The circuit board should be positioned out of the airflow, but the NTC thermistor must extend into the airflow.

A fan speed control circuit can significantly enhance the efficiency and user experience of a PC by dynamically regulating fan operation based on temperature. The core of the circuit utilizes the LM317T adjustable voltage regulator, which is configured to output a voltage proportional to the temperature sensed by the NTC thermistor. The thermistor's resistance decreases with increasing temperature, which in turn influences the voltage divider formed by the thermistor and the standard resistor, allowing the LM317T to adjust its output accordingly.

The voltage divider's configuration is critical for achieving the desired output voltage. At the nominal temperature of 25 °C, the thermistor's resistance of 470 ohms allows the regulator to output approximately 7 V, which is sufficient for starting the fan. As the ambient temperature rises, the thermistor's resistance drops, causing the output voltage to increase and thereby accelerating the fan speed to maintain optimal cooling.

In applications where the power supply is located further from the regulator, a decoupling capacitor is essential to filter out voltage spikes and provide stable operation. The capacitor should be placed close to the regulator's input to ensure effective decoupling.

For applications requiring higher efficiency, substituting the LM317T with a low-dropout regulator like the LM2941CT is recommended. This alternative reduces the voltage drop significantly, allowing for better performance at higher currents while minimizing heat generation. The low-dropout design is especially advantageous in battery-operated devices or systems with strict power constraints.

The physical layout of the circuit is also important. The components can be mounted on a prototyping board, ensuring that the NTC thermistor is exposed to the airflow for accurate temperature readings. Proper insulation is necessary to protect the circuit from environmental factors. The assembly should be securely fastened to the fan body, utilizing the mounting holes to prevent movement and ensure stability during operation.

Overall, this fan speed control circuit is a practical solution for improving energy efficiency and reducing noise in PC applications. By leveraging a simple arrangement of components, it effectively adjusts fan operation based on real-time thermal conditions.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).

The thermistor has a rated value of 470 . It sets the output voltage of the LM317T to approximately 7 V at 25 C. 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. 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. 🔗 External reference