Comparator circuit misbehaviour

[Murugesh_89]

Hi,
the comparator circuit i am using is given below..



When the non inverting point is at 3.3V and inverting point is at 8.5V i was expecting a 0V at the output where as i got it as 5.3V.

What is the problem with this simple comparator circuit using LM358?

Thanks,
Murugesh

 

[Audioguru]

The opamp has such a high gain that when the circuit is built on a breadboard then it oscillates. When oscillating, its output is switching high and low at a high frequency. Your multimeter is measuring the average DC output voltage which is near half the supply voltage.

Usually, a comparator circuit has hysteresis (a little positive feedback) so that the output can switch quickly then latch.

 

[Murugesh_89]

what is the little positive feedback? what should i need to do to get proper switching?

 

[Audioguru]

First of all you must use an oscilloscope to see if the circuit is oscillating.

The datasheet for the LM339 quad comparator and the LM393 dual comparator ICs talk about adding a little hysteresis (positive feedback)so that the output can switch quickly then latch to prevent oscillation.

Hysteresis can be added to your circuit by adding a 1k resistor in series with the input pin 3 and adding a 100k resistor from the output pin 1 to the input pin 3.