LM393 comparator issue

[libbresse]

Simulating a circuit where LM393 comparator is in, everything works just fine until i get a negative input, then the comparator turns on ?

Should I put a led on the input to get rid of the negative signals ??

http://www.pixbox.se/pic_show_id26529066.html

[bharbour]

Originally Posted by libbresse Simulating a circuit where LM393 comparator is in, everything works just fine until i get a negative input, then the comparator turns on ? Should I put a led on the input to get rid of the negative signals ?? http://www.pixbox.se/pic_show_id26529066.html

You can't bring the inputs below the lower power supply voltage and get predictable results. Since the lower power supply voltage is ground, any negative input will be bad. A diode between the input and ground with the cathode on the input will help. Don't use an LED, the forward voltage drop is too high, a standard silicon diode or maybe a schottky diode would be much better.

Also, simulations are useful for looking at circuit operation in normal situations. The models do not model a part exactly and when you start doing things like reverse biasing inputs, the simulation and reality are likely to differ.

Good Luck,
BobH