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Power on delay circuit needed

Discussion in 'General Electronics' started by Thomas, Jul 24, 2005.

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  1. Thomas

    Thomas Guest

    I need a simple 2-10second delay circuit. When power is supplied, I need it
    delayed. Will a 555 delay in that manner? Needs to be simple and no
    relays. Whatcha got?

    Thanks in advance.
  2. Rich Webb

    Rich Webb Guest

    Variable between 2-10 or fixed, roughly in that range?
    power = ?

    3 VDC, 300 mA for a flashlight bulb?
    12 VDC, 200 mA to run a PC fan?
    120 VAC, 1 A for a reading light?
    750 VDC, 800 HP marine motor?
  3. Thomas

    Thomas Guest

    12VDC @ 100mA should do it. Thanks.
  4. default

    default Guest

    Will sinking current work? The 555 is always an option.

    A simpler approach is just a NPN transistor emitter to ground,
    relatively large electrolytic cap on base to ground, relatively large
    resistor charging the cap from +12.

    To reset the circuit quickly, a reverse biased diode to the cap and
    +12 will discharge the cap faster.

    Works much better with 2N7000. That's an N channel mosfet with a pin
    out similar to other TO92 NPN transistors. It requires a ~4 volts on
    the gate to turn on so you get a longer delay for the same RC network,
    the reset diode works better, it has lower leakage.

    Very handy part that 2N7000 . . . and cheap.
  5. Thomas

    Thomas Guest

    With the 555, in what config would allow me to pause for x-seconds then go
    high until V+ is lost?

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