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I need a device to turn on when the DC supply exceeds 12.7v

Discussion in 'Electronic Basics' started by dg, Aug 26, 2003.

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  1. dg

    dg Guest

    This has got to be so simple, yet I need some help. A buddy of mine has got
    a boat and on his boat he has a need for a device to be switched on when the
    boats DC power system meets or exceeds 12.7V. If the voltage is below 12.7,
    the device should be off. The device he needs to run has a 1.3A load at
    12V. What is the best way to accomplish this?

    Any tips are greatly appreciated!
  2. Steve

    Steve Guest

    Dear Dan
    As another poster said, the answer is probably a voltage comparator
    running a relay.

    One catch to applying a voltage comparator in this situation - you
    need to have "positive feedback" - that is you need to have a high
    value resistor from the output back to the "+" input of the voltage
    comparator, otherwise:

    when the battery voltage falls to the setpoint,
    the comparator switches off,
    turning the relay off,
    which drops the load,
    which cause the voltage to rise above the setpoint,
    which switches the comparator on,
    turning the relay on,
    which connects the load,
    which causes the voltage to fall below the setpoint
    (..and so it all repeats until the relay burns out!..)

    If you put in sufficient positive feedback (or hysterisys)
    this does not occur, and the circuit works properly.

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