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Can I know which component senses heat or acts as heat sensor in the following circuit? In the given diagram, it is said that the 1N4148 diode acts as the sensor. But basically it is a zener diode and I don't think that it is able to sense temperature shift. This circuit provides a working model but the sensor remains as a mystery for me. Can you please help it for me?

enter image description here

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    \$\begingroup\$ A 1n4148 is a high speed small signal swiching diode, not a zener. Any semiconductor diode inherently is a heat sensor, as its carrier mobility varies with temperature. \$\endgroup\$ – Anindo Ghosh Aug 26 '13 at 5:51
  • \$\begingroup\$ You should change the reference designation on the diode to the right as your circuit shows two diodes labeled "D1" which may cause confusion since your sensor is "D1". \$\endgroup\$ – Tut Aug 26 '13 at 10:33
  • \$\begingroup\$ @Ghosh: any (normal) diode acts as a Zener as well: just polarize reversely and the breakdown threshold will be there. Anyway, in the circuit the 4148 is directly polarized, so (yes) the Zener effect is out of question. \$\endgroup\$ – Mario Vernari Aug 26 '13 at 14:38
  • \$\begingroup\$ @MarioVernari Firstly, like you yourself noted, your comment is not relevant. Secondly, Zener Effect: "The Zener effect is distinct from avalanche breakdown ..." \$\endgroup\$ – Anindo Ghosh Aug 26 '13 at 17:35
  • \$\begingroup\$ The diagram has two diodes named D1. The one on the left is the sensor. It is not a Zener, it is a fast-recovery signal diode. Its forward voltage drop changes based on temperature -- check the data sheet! \$\endgroup\$ – Jon Watte Aug 26 '13 at 21:01
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D1 is a temperature sensor. Voltage drop developed over a diode will drift with the temperature. Usually silicon diodes have a negative temperature coefficient of -2mV/C.

R5 and Z1 make a 10V voltage regulator. As the temperature increases, voltage drop over diode D1 will decrease making voltage at the "+" input less than voltage at the "-" input, opamp will drive it's output low turning on the fan.

R3 accounts for varying current - when voltage drop over diode decreases, more current will flow thru it. By connecting diode and pot to common point at R3 designers make sure that this does not affect the temperature point set by a potentiometer.

R4 provides hysteresis - it affects the voltage at "-" input a bit so that the output will not oscillate at the specified temperature.

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A common silicon diode like the 1N4148 (it's not a zener) have a very linear voltage versus temperature curve for a constant current. If you keep the current constant the voltage across the diode will drop by 2mV per degree Celsius rise.

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