Temperature Control switch

This circuit controls a load (in this case a DC brushless fan) based on a temperature compared with a setpoint. The transducer is a diode in the forward polarization regime. In fact, when forward biased, the forward voltage drop across a diode has a temperature dependence, in particular has a negative linear slope. This is due to the Boltzmann distribution, causing electrons to pass to the conduction band thermally, lowering the voltage drop across the diode. This circuit compares a precise voltage reference (Zener) with the forward voltage drop of the diode forward biased with 11mA of current. The comparator is simply an LM158/258/358 working in open-loop mode; the inverting input is connected to the diode sensor, and the non-inverting to the reference voltage. When the temperature rises above the setpoint, the forward voltage drops under the voltage reference, and the comparator output is turning on the transistor and thus the fan. A higher power transistor can be substituted for bigger fans, or a relay, IGBT, MOSFET, etc., can be used to control higher loads (and higher voltages). The setpoint is adjusted with the potentiometer, and an LM3914 LED driver can be used to make a temperature setpoint indicator (needs careful calibrations and the use of Excel to calculate slope and intercept). Many modifications can be done, but the circuit works very well in its basic form. The comparator can distinguish 10µV differences, so approximately 0.01°C differences (carefully adjusting the potentiometer can allow detection of body heat from 1/2 cm from the sensor or ambient heat, making the fan turn on and off continuously). Temperatures can be controlled up to 140°C (150°C maximum diode temperature), but linearity is not ensured. Possible uses include heatsink cooling, computer emergency cooling (though a linear device may be better than an on-off), and metal cooling when drilling, etc. Additionally, this circuit can also be used to heat by reversing the comparator inputs and substituting the fan with a relay controlling the heater.

This circuit operates by utilizing the temperature-dependent characteristics of a diode to control a DC brushless fan based on ambient temperature. The diode functions as a temperature sensor, where its forward voltage drop decreases as the temperature increases. This behavior is leveraged by comparing the diode's voltage drop with a stable reference voltage provided by a Zener diode.

The LM158/258/358 operational amplifier serves as a comparator in open-loop configuration, with the inverting input connected to the diode and the non-inverting input connected to the Zener voltage reference. When the ambient temperature exceeds the setpoint, the voltage drop across the diode falls below the reference voltage, causing the comparator output to switch states. This output is used to drive a transistor, which in turn activates the fan, providing cooling in response to the detected temperature increase.

The circuit includes a potentiometer for setpoint adjustment, allowing users to calibrate the temperature threshold at which the fan activates. For visual feedback, an LM3914 LED driver can be integrated to indicate the setpoint, requiring careful calibration to ensure accurate temperature representation.

The design is versatile, allowing for the substitution of different types of transistors or switching devices to accommodate various load requirements. The circuit can handle temperatures up to 140°C, although the linearity of the response may vary at higher temperatures.

Applications for this circuit include cooling for electronic components, emergency cooling for computers, and other scenarios where temperature regulation is necessary. Additionally, with minor modifications, this circuit can be adapted for heating applications by reversing the input connections of the comparator and controlling a heating element instead of a fan.

Overall, this circuit provides a robust and efficient solution for temperature-based load control, suitable for a variety of practical applications in electronics and thermal management.This circuit controls a load (in this case a dc brushless fan) based on a temperature compared with a setpoint. THe transduced is a diode in the forward polarization regime. In fact when forward biased, the forward voltage drop accross a diode has a temperature dependance, in particular has a negative linear(ish) slope.

This because of the boltzmann distribuition, causing electrons to pass to the conduction band thermically, lowering the voltage drop accross the diode. Anyway this circuit comparates a precise voltage reference (zener) with the forward voltage drop of the diode forward biased with 11mA of current. The comparator is simply a LM158/258/358 working in open-loop mode, the inverting input is connected to the diode sensor, and the noninverting to the reference voltage.

Se when the temperature rises above the setpoint, the forward voltage drops under the voltage reference and the comparator output is vccturning on the transistor and so the fan. Higher power transistor can be substituted for bigger fans, or you can substitute a relay, IGBT, mosfet etc to control higher loads (and higher voltages).

The setpoint is adjusted with the potentiometer, and you can use a LM3914 led driver to make a temperature setpoint indicator (needs careful calibrations and the use of excel to calculate slope and intercept). Many modifications can be done, but the circuit works very well in its basic form. THe comparator can distinguish 10uV differences so approx 0.01C differences (carefully adjusting the potentiometer can allow to feel body heat from 1/2 cm from the sensor, or feel ambient heat, making to turn the fan on and off continuosly) You can control temperatures up to 140C (150 max diode temperature), but linearity is not ensured Possible uses?

Heatsink cooling, computer emergency cooling (but i thint that a linear device would be better than a on-off) metal cooling when drilling etc... Ah! One note: you can even heat with this circuit but you need the reverse comparator inputs and substitute the fan with a relay controlling the heater.

🔗 External reference