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Help with fan controller using LM35

Discussion in 'General Electronics Discussion' started by eng, Feb 1, 2015.

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  1. eng


    Feb 1, 2015
    hi,i just want to make an automatic fan controller,based on this circuit:
    This project uses IC LM35 as a sensor for detecting accurate centigrade temperature. Output voltage of this sensor is linearly proportional to the Celsius (Centigrade) temperature. This sensor use the fact that, as temperature increases, the voltage across a diode increases at a known rate. Output of IC is 10mv/degree centigrade for eg if temperature is 45 degree then the output of sensor will be 450mv or 0.45V

    Output data of sensor is applied to a current amplifier circuit and feed to a low power DC motor. We have made current amplifier by using a general purpose NPN transistor. I have used BC548 as shown in the circuit diagram
    But my question is:How could i take the value of transistor parameters (R,I) to make this circuit working good?Thanks
  2. Bluejets


    Oct 5, 2014
    Don't know what the use of the 2 resistors in series with the motor are supposed to do.
    Other than that I think you will need to use a comparitor in your circuit as I don't believe your present setup will work as you expect it to.
    Also when you supply a circuit, is best to include as much info as possible about all the devices used as otherwise people are just guessing.
    For example "low power DC motor" does not have parameters people can work with.
    eng and davenn like this.
  3. eng


    Feb 1, 2015
    the dc motors can include CPU FAN OR SMPS FAN while we can take off the 10Ω resistence and applied a voltage 12v battery.But my big problem is R and I in the base of transistor.what do you think for it?
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    You have to decide whether you want to switch the fan ON and OFF depending on whether the temperature is above or below a certain threshold, or whether you want to vary its speed according to the temperature.

    In other words, you need to draw a graph with temperature on the horizontal axis, and fan speed (or fan voltage) on the vertical axis, and define how you want temperature to control fan speed.

    The solution to your problem will probably involve at least one op-amp or comparator. Once you have drawn a graph, I will suggest a design.

    Edit: If you use an ON/OFF control method, the control loop should include hysteresis. If you don't know what that is, look it up on Wikipedia or with Google.
    Last edited: Feb 2, 2015
    eng and Bluejets like this.
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