On-Off Temperature Control

Posted on Feb 5, 2014

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 boltzma

On-Off Temperature Control
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nn distribuition, causing electrons to pass to the conduction band thermically, lowering the voltage drop accross the diode. 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. 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). THe comparator can distinguish 10uV differences so approx 0. 01G‚ °C 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) 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.

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