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looking for a delay off circuit

Discussion in 'General Electronics' started by izzi4, Jun 18, 2005.

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  1. izzi4

    izzi4 Guest

    Hello, i'm looking for a simple delay off circuit. I have a switch which
    controls the 120vac via a relay which turns on a computer power supply. I
    am looking for something to be able to turn off the switch and then have the
    120v be turned off about 3-5 minutes later (cooling purposes) the relay
    would still have the 120 going to it, when switched off (aka no battery
    system or anything). I was planning on paralleling a second relay with the
    first, one controled by the switch and the second controlled by the timer
    circuit powered from the power supply. any suggestoins are welcome, i'm
    looking for a simple design that wouldn't be to cost prohibitive. Thanks
    for any input.
  2. John Fields

    John Fields Guest

  3. John Fields

    John Fields Guest

  4. One really simple, really cheap way to do this is to get the mechanical
    timer out of a scrapped microwave oven. This is the kind you have to
    physically turn the knob to so many minutes, and it usually goes ding at
    the end of the cycle. These should be easily obtainable from a thrift
    store. But this has the disadvantage of having to be set manually.

    I would also consider connecting power to the fans just by themselves.
    To keep the external power out of the PS, you can put a diode in series
    with the power lead from both the power supply and the external power
    supply, probably a wall wart. This is what's called a diode OR.

    Here's one simple time delay. You can make the delay much longer by
    using another transistor as an emitter follower to the first one. With
    two transistors I've had no problem getting 2 or more minutes delay.
  5. quietguy

    quietguy Guest

    Perhaps one of the turbo timer circuits would be suitable - the gizmos that
    keep the car engine running for a short time after the ignition is switched off

  6. Dingus

    Dingus Guest

    switched off

    .... pre-ignition?




  9. quietguy

    quietguy Guest

    I don't think that can happen with a diesel, but....

  10. Jasen Betts

    Jasen Betts Guest

    Perhaps one of the turbo timer circuits would be suitable - the gizmos
    I don't think a diesel engine can run without it (or something similar) happening.

    Turbo timers run the engine at idle for a few minutes to allow the
    turbocharger to cool, this allows the rotor to cool from orange-hot
    in the relatively cool idle exhaust gasses rather than cooling by
    conducttion up the shaft and cooking the bearings.

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