LM293A: Comparator implementation with Hysteresis

Ishan Panchal

Part Number: LM293A
Other Parts Discussed in Thread: LM293,

Hi There,

Please see attached image of LM293 used as comparator to monitor +24VDC supply. I am bit doubtful about the hysteresis provided as the reference is on inverting input. Can you please verify and provide your inputs to same.

Also diode D28 seems to be placed wrongly in circuit as at TP12 we are getting around 2.4V. It was placed to protect MCU pin from possible over voltages as opam is biased on 5V.

I want to use hysteresis on this existing circuit.

Thanks..!!!!

over 1 year ago

0 kai klaas69 over 1 year ago

Guru 140200 points

Hi Ishan,

D28 makes no sense at all. Replace it with a 4k7 resistor or similar.

The LM293A has an open collector output and by this provides a perfect level shifting. Just connect the pull-up resistor to a suited potential:

ishan_lm293a.TSC

Kai

0 Ishan Panchal over 1 year ago in reply to kai klaas69

Intellectual 640 points

Hi Kai,

Okay, I will do that.

If I want to provide hysteresis, can I just tweak 1Mohm feedback resistor. I need a hysteresis for Vil=2.3 and Vih=2.6V on non-inverting input.

Please let me know.

Thanks..!!!

0 kai klaas69 over 1 year ago in reply to Ishan Panchal

Guru 140200 points

Kai

0 Chi Nguyen1 over 1 year ago in reply to kai klaas69

TI__Genius 11701 points

Thank you kai for your support as usual.

Hi Ishan,

Thanks for reaching out. Yes, you can tweak the input and feedback resistors to achieve the hysteresis as you need, as Kai have shown. See Non-inverting comparator with hysteresis circuit (Rev. A) for the formulas. R179 and R184 will now have a thevenin equivalent resistance that will need to be incorporated in the equations.

0 Ishan Panchal over 1 year ago in reply to Chi Nguyen1

Intellectual 640 points

Hi Kai and Chi,

Thank you for you kind inputs, I am in process to implement the circuit and practically check same.

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LM293A: Comparator implementation with Hysteresis