Power Off Delay Circuits for AUX fan

Discussion in 'The Projects Forum' started by reinchua, Jul 7, 2010.

  1. reinchua

    Thread Starter New Member

    Jul 7, 2010
    4
    0
    Hi,

    Can any one help me to design a circuit for my AUX fan?. The AUX fan must remain on for a certain of minutes like 5-10 when the power from 12Vdc is cut off. but it will reset the timer every time the supply returns.

    I would really appreciate any kind of information provided here

    Regards
    Rein
     
  2. Dx3

    Member

    Jun 19, 2010
    87
    7
    You should look at 555 timer circuits. The idea is to set up a one shot timer that activates when the +12 volts stops. The timer is connected to the continuous 12 volt supply so it can time out after the rest of the circuit has turned off.
     
  3. reinchua

    Thread Starter New Member

    Jul 7, 2010
    4
    0
    Hi Dx3 what kind of circuit shiuld i look for? is this monostable circuit? i have a pocket books about IC555 timers and none of the circuits that i'm looking for. I have a 12v timer relay but i dont know how to configure it. any idea how to do? I really appreciate for the help.

    thank's
    Rein
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Why does this AUX fan need to stay on? Temperature too high?

    Instead of using a timer, why not compare the temperature of whatever the AUX fan is cooling to the outside air temperature, and then when the difference is less than, say, 20°C, turn everything off.

    If you just run the fan for a set period of time, it might not run long enough, or run for an excessive amount of time, depending on what the temperature differential was to begin with.
     
  5. reinchua

    Thread Starter New Member

    Jul 7, 2010
    4
    0
    The Circuit is intend to use in car aircon condenser cooling system. the old models of the car does not have any aux fan in condenser unlike for the new models they have 1 some models have 2 aux fan, we are making some experiment for this kind of circuit for the old models using additional aux fan.
     
  6. Dx3

    Member

    Jun 19, 2010
    87
    7
    The time delay 555 circuit is called a monostable multivibrator. Use a 12 volt supply that turns off to make the reset pulse (with a capacitor). Then the first resistor and capacitor tell the 555 how long to stay on. It's output will stay high until the timing capacitor is full enough to switch off. You will need a relay or transistor on the output to get enough current to run a fan motor.
     
  7. reinchua

    Thread Starter New Member

    Jul 7, 2010
    4
    0
    Thank's DX3 for the info. you give me an idea, i'll try this configuration. thank's
     
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