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TL082: Schmitt trigger not cutting off at set threshold

Part Number: TL082
Other Parts Discussed in Thread: OPA2196, , OPA2180, LM393, LM358, TL072


I am experimenting some functionalities based on schmitt trigger made using TL082. Ive put non inverting voltage as 2.04V and voltage at inverting input is variable. I can see the output toggles when input is 1.92V and not 2.04V. In my experiment it should atleast cut off at 2.0V - 2.02 range if not 2.04V. Can someone tell what can be the possible issues in it? Also if this IC is not suitable for Schmitt trigger application can you guys suggest some IC. I have found OPA2196 and OPA2180 but these two are out of stock on TI. 

I am sharing the circuit below. 

The resistor R17 is passed through some current and schmitt trigger will measure voltage accross R17 which is also its input voltage. The Opamp is working at 20V unipolar supply and the upper threshold voltage is set at 2.04V(measured on non inverting input of opamp) as per multimeter.  

  • Hi Swapniil,

    you violate the common mode input voltage range of TL082. See section 6.6 of datasheet. Keep in mind that the TL082 is no-input-rail-to-rail OPAmp.

    Why not using a dedicated comparator? Something like the LM393? Or some of these?;asc


  • I have designed same circuit 2 months ago on a separate PCB and it is still working flawlessly. This time I have attached a controller on the new pcb and since then its causing problem. There is no shorting of any pins too. 

    My main doubt is even if this opamp is not a good comparator, why is it working on one PCB and not Other. 

    Also can I use LM358 for this purpose? as I have a deadline of this project by next 2 days.


  • Hi Swapniil,

    OPAmps can show big manufacturing tolerances. I think it was sheer luck that the one circuit with the TL082 worked and the other does not. You may also want to read section 9.3 of datasheet of TL072. The TL072 and the TL082 are very similar, if not identical.

    Your circuit may work with the LM358. But in any case "C8" should be removed. It's counterproductive having a cap at the +input of comparator when using hysteresis. Remember that the feedback from the output of comparator to the +input (hysteresis) shall help to traverse the threshold region of comparator faster and to a greater extent. But "C8" will slow down this traversing again and by this destroys the hysteresis effect. It is as if you would brake and accelerate in a car at the same time. Think about what it means to the +input of comparator when the output of comparator toggles from low to high or high to low.


  • This information is very helpful. I have added capacitor just in case threshold input varies due to noise so cutoff will occur at any voltage and not exactly what i have set. But surely I will keep in mind your feedback and try again by removing it. Thanks a lot Kai. 

  • Hi Swapniil,

    noise filtering is always a good idea. But I would do it this way:

    The 47k resistor is splitted into a 2k2 and 44k8 resistor (44k8 + 2k2 = 47k) and the 2k2 resistor in combination with C1 forms a low pass filter with a corner frequency of 70Hz. You can modify the component values to your needs, of course, to provide any other corner frequency.