temperature control

Discussion in 'The Projects Forum' started by anilsatyakumar, Jul 21, 2009.

  1. anilsatyakumar

    Thread Starter New Member

    Dec 22, 2008
    hi , i am anil
    i am doing the "heat sensor with fan cooling" project. i am submitting the circuit diagram . i donot have enough knowledge on that. so please anybody explain me abt the circuit of all the components & how does it works.i want to know the applications. The diode d1 left to comparator works as a heat sensor.so please help me out and explain in such a way that a non-electronic student should understand.and what tye of questions are asked abt ckt in viva.

    thanks you in advance.
  2. Toughtool


    Aug 11, 2008
    The diode will present a voltage across it's junction depending on the temperature and your OP amp amplifies the voltage to control the fan motor. I built one of these years ago and they were quite accurate and linear over a 100 degree C range. The Zener diode, Z1 is used to provide the regulated voltage and current to the diode. Pot 1 is used to adjust the OP amp's gain and probably to calibrate the measured temperature. My circuit used two pots, one for lower temp and the other (gain) for high temp limit. I sent the OP amp output to an A to D converter and collected data onto a computer. Since diodes are hermetically sealed, you can put them in boiling water and iced water for calibration. The output of your OP amp circuit drives a power transistor which drives the motor. The higher the temperature the faster the motor is driven.
    This excerpt from:
    offers a good explanation.

    5. Diode Temperature Sensors

    The ordinary semiconductor diode may be used as a temperature sensor.[​IMG] Cheap and nasty! The diode is the lowest cost temperature sensor and can produce more than satisfactory results if you are prepared to undertake a two point calibration and provide a stable excitation current. Almost any silicon diode is ok. The forward biased voltage across a diode has a temperature coefficient of about 2.3mV/°C and is reasonably linear. The measuring circuit is simple as shown to the right.
    The bias current should be held as constant as possible - using constant current source, or a resistor from a stable voltage source.
    [​IMG]Without calibration the initial error is likely to be too large - in the order of ±30°C - the largest of all the contact type temperature sensors. This initial error is greatly reduced if sensor grade parts are used.
    One advantage of the diode as a temperature sensor is that it can be electrically robust - tolerant to voltage spikes induced by lightning strike. This is particularly true if power diodes (e.g. the common 1N4004) are used and a second back to back diode is used to limit power dissipation during high peak currents.
    The transistor sensor is used in diode mode by connecting the base and collector together. If this is not done, the sensor is wired between base and emitter and the excitation current reduced by a factor of about 100. The result is a very low power, sensitive and linear sensor. The simplicity and performance of the sensor is under valued...

    Last edited: Aug 7, 2009