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    Transformer Phase shift used in isolated zero crossing detectors

    In this and other forums, the question arises whether the same low power transformer used in the power supply, can be used to obtain an isolated sample for a zero crossing detector.

    I remember many years ago that there was a small error, but could not remember the magnitude.

    Thus on a lazy Sunday with nothing better to do, I rigged a small setup and measured it. I decided to share it, for future reference.
    See attached photo.

    The light blue trace is the actual 120V, 60 Hz line viewed with a 100X diff probe. This voltage is applied to the transformer's primary.
    The magenta trace is the voltage coming out from the transformer's secondary.

    It can be seen that the secondary voltage leads about 100 microseconds, which on a 60Hz powerline means around a 2.2 degree error.

    For whatever it is worth, the photo shows the transformer I used. It is a very small transformer and therefore the L/R ratio is small. I suspect that larger transformers would have even less phase shift.
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    Re: Transformer Phase shift used in isolated zero crossing detectors

    Hi,

    Ideal transformers don't have phase shift between V_in and V_out.

    Real transformers have. It is caused by the stray (series) inductance, the ohmic wire resistance, the load current ....and I assume even the coupling factor.

    Some years ago I had the problem, I tried to use a small power transformers as zero cross detector.
    I found out that especially the "short circuit proof" transformers are bad.
    And even worse: the phase depended (a little) on temperature.

    Btw: Your transformer plate says "signal transformer" and not "power transformer". Maybe it is a transformer optimized for low phase shift, maybe especially made for zero cross detection.


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    Re: Transformer Phase shift used in isolated zero crossing detectors

    A voltage measuring transformer with specified accuracy class will show lower phase error, but be much more bulky. See https://en.wikipedia.org/wiki/Instrument_transformer and the transformer specification quoted therein.

    2.2 degree sounds fair and can be probably compensated down to 0.1 or 0.2 degree.

    As analyzed by schmitt trigger, primary winding resistance and magnetizing inductance are the major error sources, thus their temperature dependency must considered.



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    Re: Transformer Phase shift used in isolated zero crossing detectors

    Indeed, for applications which are not critical, this slight error could be calibrated via software to a much smaller error.

    I made another experiment, although I did not take waveforms:
    This is a an "universal" 115/230V device depending how one connects the primary, in parallel or series.

    In the second experiment, I connected the primary windings in series. The phase shift appeared to reduce slightly to 70 microseconds, but I must confess that measuring such shallow slope is difficult.
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    Re: Transformer Phase shift used in isolated zero crossing detectors

    it is in fact the capacitance ( coupled with leakage L & R ) that causes the delay ....

    - - - Updated - - -

    loading the Tx should reduce the 100uS seen



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