Gentle Breeze

Description: Where there is heat, let us bring cooling: a fan will do the job, but unfortunately, fans are usually noisy. In many cases, there is no need for the fan to run continuously at full speed, so it makes sense to control the fan speed in response to the temperature of the heatsink or device being cooled, switching the fan off entirely if it should fall to room temperature. The circuit described here accomplishes this and offers additional features. The low-cost KTY81-110 is used as the temperature sensor in a negative-feedback arrangement with an operational amplifier. The temperature-dependent voltage at the non-inverting input of op-amp IC1.A leads to a voltage variation at the output (pin 1) from 4 V at 30 °C to 4.72 V at 60 °C. The second stage (IC1.D) converts this relatively small swing and inconvenient voltage offset into the range of 8 V to 12 V suitable for the fan. The third operational amplifier functions as a comparator. At room temperature, its output sits at nearly 12 V and pulls the output of the second stage with it, switching transistor T1 off. If the temperature exceeds 35 °C, the comparator switches; diode D1 blocks, and the control circuit can operate normally. The hysteresis of the comparator has been set so that the comparator state will only change again, turning off the fan if the temperature falls below 30 °C. Capacitor C3 ensures that the fan is run at full voltage for about 0.7 seconds immediately after switch-on, so that the motor will start reliably. The fourth op-amp in the LM324, IC1.C, is utilized to create an over-temperature warning indicator. This is necessary in case the fan, even running at full speed, is not able to provide enough cooling, or, due to a fault, cannot reach full speed. This op-amp is also configured as a comparator. If the sensor temperature reaches a value of 60 °C, the comparator output goes high (to nearly 12 V). The output will only go low again (nearly 0 V) if the temperature falls below 40 °C. An LED (with a series current-limiting resistor) can be connected to its output (pin 8); alternatively, a transistor could be used to drive a relay. The circuit is sufficiently accurate without adjustments, but metal-film resistors with a tolerance of 1% should be used. Some of the values used are from the E24 series. The supply voltage is used as a reference throughout, and so it should be well regulated: a 7812 voltage regulator is adequate.

The circuit operates effectively to manage fan speed based on temperature readings, thereby reducing noise and improving efficiency. The KTY81-110 temperature sensor is a key component, providing an analog voltage that varies with temperature, enabling precise control. The use of a negative feedback configuration with operational amplifiers ensures stability and responsiveness in the control system. The first operational amplifier (IC1.A) amplifies the temperature-dependent voltage, converting it to a usable range for further processing.

The second operational amplifier (IC1.D) serves to scale the output voltage to fit within the 8 V to 12 V range required for fan operation. This ensures that the fan receives adequate voltage to operate effectively without running at full speed unnecessarily. The comparator function of the third op-amp monitors the output from the second stage and controls the transistor switch (T1), which in turn regulates the fan's operation based on the temperature thresholds set within the circuit.

The hysteresis implemented in the comparator prevents rapid cycling of the fan, allowing it to remain off until the temperature drops significantly, thereby enhancing the longevity of the fan and reducing wear. The inclusion of a capacitor (C3) guarantees that the fan receives a full voltage pulse upon activation, aiding in the reliable startup of the motor.

An over-temperature warning indicator is an essential feature of this circuit, providing an alert if the temperature exceeds safe operating limits. The fourth operational amplifier (IC1.C) acts as a safety mechanism, ensuring that the user is notified if the cooling system is inadequate. The output can drive an LED for visual indication or a relay for more extensive control systems.

The design emphasizes the importance of component selection, recommending metal-film resistors for their low tolerance, which contributes to the overall accuracy of the circuit. The use of a well-regulated supply voltage, achieved through a 7812 voltage regulator, ensures that the circuit functions reliably under varying conditions. This circuit design exemplifies a practical solution for temperature-controlled fan operation, balancing performance, efficiency, and safety.Where there is heat, let us bring cooling: a fan will do the job, but unfortunately fans are usually noisy. In many cases there is no need for the fan to run continuously at full speed, and so it makes sense to control the fan speed in response to the temperature of the heatsink or device being cooled, switching the fan off entirely if it should f

all to room temperature. The circuit shown here does this and even offers a little more. The low-cost KTY81-110 is used as the temperature sensor, in a negative-feedback arrangement with an operational amplifier. The temperature-dependent voltage at the non-inverting input to opamp IC1. A leads to a voltage variation at the output (pin 1) from 4 V at 30 °C to 4. 72 V at 60 °C. The second stage (IC1. D) converts this relatively small swing and inconvenient voltage offset into the range 8 V to 12 V suitable for the fan.

The third operational amplifier works as a comparator. At room temperature its output sits at nearly 12 V and pulls the output of the second stage with it, switching transistor T1 off. If the temperature exceeds 35 °C the comparator switches; diode D1 blocks and the control circuit can operate as normal.

The hysteresis of the comparator has been set so that the comparator state will only change again, turning off the fan, if the temperature falls below 30 °C. Capacitor C3 ensures that the fan is run at full voltage for about 0. 7 seconds immediately after switch-on, so that the motor will start reliably. The fourth opamp in the LM324, IC1. C, is used to create an over-temperature warning indicator. This is necessary in case the fan, even running at full speed, is not able to provide enough cooling, or, because of a fault, cannot reach full speed.

This opamp is also configured as a comparator. If the sensor temperature reaches a value of 60 °C, the comparator output goes high (to nearly 12 V). The output will only go low again (nearly 0 V) if the temperature falls below 40 °C An LED (with series current-limiting resis tor) can be connected to its output (pin 8); alternatively a transistor could be used to drive a relay.

The circuit is sufficiently accurate without adjustments, but metal-film resistors with a tolerance of 1 % should be used. Some of the values used are from the E24 series. The supply voltage is used as a reference throughout, and so should be well regulated: a 7812 voltage regulator is adequate.