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zero crossing detector

Discussion in 'Electrical Engineering' started by Warren Thai, Aug 13, 2007.

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  1. Warren Thai

    Warren Thai Guest

    anyone know any circuits for a fast (fast as in response lag time of
    preferably nanoseconds or very very little microseconds) and preferably
    simple zero crossing detector. The input to the circuit would be a 50Hz sine
    wave at approx 5Vpeak and should produce a square wave.
  2. Palindrome

    Palindrome Guest

    Take the period of a 50Hz sine wave.
    Now convert your "nanoseconds" into the change in angle that it
    represents for a signal with that period.

    Now take that change in phase angle and calculate the maximum change in
    voltage that can occur during that change in phase.

    Now tell me how you are going to keep noise on that signal well below
    that change in voltage..

    How then to do it is fairly simple. Connect your signal through a
    resistor to base of a transistor connected as common emitter, with a
    diode between base and emitter to protect against reverse voltages. Take
    the output pulse train from the collector, with a resistor going from
    collector to your desired dc supply.

    That will give you your pulse train of zero crossings. If the lag,
    before the mains voltage rises enough to switch on the transistor, is
    too great, include another resistor to make the base a summing point and
    add an offset voltage to decrease the lag to "zero".

    You can achieve zero lag time with that circuit - although noise and
    thermal effects *will* cause errors - including causing the zero
    crossing point to wander. So, if you want to improve that, you will need
    extra complexity. If you can hold the temperature of the transistor
    reasonably constant, it won't wander much.

    Simple enough?
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