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ZCD saturates! Please advise

Discussion in 'General Electronics Chat' started by Faiyaz, Oct 26, 2003.

  1. Faiyaz

    Faiyaz New Member

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    I tried building a ZCD (zero crossing detector) making use of the 741 opamp.
    The supply provided by me was +15V and GND to pin 7 instead of negative voltage.( I wanted to make use of single supply).
    The problem occuring is that the o/p of opamp saturates and does not change its state when the input sine wave crosses zero.
    On replacing the GND at pin 7 by a -15V supply the ckt is working perfectly.
    I would like to operate the ZCD using a battery source ( single supply source)
    Please suggest any modifications necessary.
    The ckt has been attached.
     

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  2. kinjalgp

    kinjalgp Active Member

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    741 will not work with single supply in above given configuration. You need to connect inverting terminal to a positive reference which is in between 15V and GND i.e. connect inverting terminal to a potential divider of 10k resistors connected between +15 and GND. Thus inverting terminal will be at +7.5V reference and that will be your change-over point.

    Insted a simple way is to use LM324 op-amp which can operate on single supply without use of any resistors as mentioned above.
     
  3. mechie

    mechie New Member

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    Zero Crossing Detector

    That would look like the attached then ...
    The second circuit allows for adjustment of the switching point.
    Both circuits require a DC offset which will appear at the input so you may wish to add a series capacitor at the input to block it.

    I guess they are both 7v5 crossing detectors :roll:
     

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  4. dave

    Dave New Member

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  5. Optikon

    Optikon New Member

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    These op amp circuits shown above still saturate to a couple volts below 15V and a few mV above 0V(741 is not R-2-R)

    but I wonder...
    Are you using the analog output voltage to drive something else?
    If you really need your zero detect signal to end up in a digital part, use a comparator for this. Running the opamps in saturation is not a good practice. You can use the same divider scheme shown by the others but add some hysteresis and the output can be pulled up (O/C type) right to the digital supply. Only use the opamp if you intend to wrap more circuits around the thing that will keep the amp in linear range.

    Since comparators are designed for saturated operation, you'll get a much faster and more acurate zero-detect response out of it. Op amps can take a "long" time to come out of saturation.

    Just my opinion!
     

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