Question about zero crossing comparator

xiaxingxing

Question about zero crossing comparator [Copy link]

As shown in the figure below, I want to use LM393 to make a zero-crossing comparator:

Figure a, a 10k resistor is connected in series at the positive and negative ends of the comparator, and the output is as expected;
Figure b, there is no series resistor at the positive and negative ends of the comparator, why is the output like this? I really can't understand it. Please guide me, thank you!

maychang

I don't know what happened to the reply just now, it disappeared.

When there is no series resistor at the input, the instantaneous value of the input signal exceeds the allowable value of the chip input signal range. The LM393 allows the input terminal to be at most 0.3V lower than the negative terminal of the power supply, but in your circuit it reaches 1V lower than the negative terminal of the power supply.