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Problems in relay

  1. Feb 26, 2012 #1
    Hello everyone, I've made a thermistor controlled comparator circuit which has got a relay connected to its output, when the temperature of the thermistor rises, the relay switches and the fan on its output is turned on the coll the thermistor down.

    Now, my circuit is working perfectly, but I'm getting a problem with my relay, which is that in normal condition when the coil of the relay is not being charged from the thermistor output, my relay is switching extremely fast and again and again, it switches from NC to NO then again from NO to NC at about twice or thrice per second. But when the coil is charged, it then switches permanently to start the fan connected to it.

    I dont know what I must do about the problem I'm facing in the normal condition.

  2. jcsd
  3. Feb 26, 2012 #2
    First, unless the current through the thermistor is what is heating the thermistor, a fan will not cool it. In fact due to the power dissipated by the fan, it will heat the air slightly. The reason blowing air over skin feels cool is that it aids evaporation of perspiration which does cool the skin.

    To avoid the condition you describe in which the relay is switching rapidly from one state to the other, hysteresis is introduced by feeding back a small amount of the output voltage to the the non-inverting (+) input of the opamp. I would start with about 1% and adjust it up or down from there.
  4. Feb 28, 2012 #3


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    Staff: Mentor

    Can you show the circuit here?
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