problem with zero crossing detector using 741.

Discussion in 'The Projects Forum' started by onlyvinod56, Apr 16, 2013.

  1. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008

    Iam designing a zero crossing detector (not exactly).

    I need a positive pulse during the positive half cycle of an ac signal.
    I am using 741 but the results are not as as expected. see the schematic.
  2. SgtWookie


    Jul 17, 2007
    You don't have a -Vcc supply; only ground. You must have a negative supply for your design. Also, you probably have smoked the noninverting (+) input, because you have most likely caused that input to go more than 0.3v below ground. You must never cause the inputs of an opamp to exceed Vcc or Vee by more than 0.3v. There ARE some exceptions (certain opamps & comparators are designed to allow "over the top" input, but these are the exceptions and are few and far between.)

    The 741 opamp is ancient, horribly slow, and can't "see" within about 1.5v of Vcc or Vee. You should use a different component.
  3. bertus


    Apr 5, 2008
  4. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008
    thanQ sgt & bertus.

    Yes the 741 is dead.
    I dont have a -ve supply on my board and i dont want to setup it.
    Can i use LM339.?
  5. onlyvinod56

    Thread Starter Senior Member

    Oct 14, 2008
    I tested with my workbench dual power supply.
    Its working.
    The output is swinging from +12 to -12.
    I connected a diode and a 10k b/w the output and ground.
    finally i got the positive pulses.

    Now i dont have a 12 0 12 transformer right now.
    does the 339 works suit here?
  6. vk6zgo

    Active Member

    Jul 21, 2012
    Do you really need a zero crossing detector?

    In other words,does the pulse have to occur exactly at the zero crossing,or just sychronous with the +ve half cycle?

    If the latter,you could 1/2 wave rectify the AC (no filtering),then use a Schmitt trigger to produce a Square wave pulse,which you could then fed through a differentiator & use the +ve going spike to trigger a monostable with its RC time constant variable,so you could adjust the resulting pulse duration.

    Or,you could connect a DC supply in series with the transformer secondary,giving you a DC offset AC signal,which you could feed into one input of a Comparator,the other input being an adjustable DC source.

    You would then be able to adjust the trigger point of the Comparator.