Zero-crossing detector

Posted on Jun 29, 2012

This zero-crossing detector uses a dual LM393 comparator, and easily controls hysteresis by the reference levels which are set on the comparator inputs. The circuit illustrated is powered by +10-V power supplies. The input signal can be an ac signal level up to + 8 V. The output will be a positive going pulse of about 4*4 V at the zero-crossover point. These parameters are compatible with TTL logic levels. The input signal is simultaneously applied to the non-inverting input of comparator A and the inverting input of comparator B. The inverting input of comparator A has a +10 mV reference with respect to ground, while the non-inverting input of comparator has a -10 mV reference with respect to ground.

Zero-crossing detector
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As the input signal swings positive (greater than +10 mV), the output of comparator "A" will be low while comparator "B" will have a high output. When the input signal swings negative (less than -10 mV), the reverse is true. The result of the combined outputs will be low in either case. On the other hand, when the input signal is between the threshold points (±10 mV around zero crossover), the output of both comparators will be high. If more hysteresis is needed, the ± 10 mV window may be made wider by increasing the reference voltages.

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