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LM293: LM293

Prabu K1
Prodigy 40 points
Part Number: LM293

Hi, we are facing some output issue with the comparator LM293. Attached the circuit where LM293 is used in our application. There are 100 nos. of boards which are in field, In some boards we are receiving the feedback that the output of the comparator is always low though the non-inverting input is high. Below the measured voltages. Sometimes the output is oscillating between Low & High though the Non-Inverting Input is at 2.5V & Inverting Input is at 0V. Could you pls help to sort out what could be the reason for this issue.

  • 0
    TI__Guru 53536 points
    Hello Prabu,

    Your voltage labels are covering the schematic connections and pin numbers near the inputs.

    Can you please repost the schematic w/o the labels?

    What is the supply voltage on the logic gates (U11D)?

    Have you looked at the power supply with a scope to see if it is drooping due to the relay current? What is the frequency of the oscillation?

    Is there a large bypass capacitor (>100uF) on the supply line?

    My immediate guess is that the relay current is causing the supply to droop and the resulting "noise" on the supply line is causing problems with the comparator (PSRR problem). The oscillation may be high enough in frequency that the relay armature cannot respond - so it looks as if the relay is not operating - but current is still pulsing through the coil.
  • 0 in reply to Paul Grohe
    Prodigy 40 points

    Hi,

    Pls find attached some analysis feedback from our side.

    Problem with LM293D Comparator.pdf

  • 0 in reply to Prabu K1
    TI__Guru 53536 points
    Hello Prabu,

    The impedance at the non-inverting node (pin 6) is pretty low (~30k), so a standard 10 Megohm scope or DMM probe should not add enough error to be noticed. What model scope probe are you using? Double-check the R206, R78 and R213 resistor values by measuring them to make sure they are not off by a decade or two.

    The "observations" seem to point to soldering problems - perhaps with the LM293 leads and/or the resistors around it. Are these commercially assembled boards or hand soldered prototypes?

    Are you probing *at* the LM293 pins, or at the test points? Measurements should be done by probing the "shoulder" of the IC pin to reveal any intermittent pad connections.

    Does pushing on or flexing the board cause any changes? Is there any cracking of the components or IC package?

    By "randomly", how long does ti take the "problematic" circuit to finally respond? 100ms? 1 second? 10 seconds? It really sounds like you have a floating input.

    Electrically, I do not see any reason for this behavior. The circuit is too simple. Can you send a picture of the layout? And of a good and bad board?
  • 0 in reply to Paul Grohe
    Prodigy 40 points

    Pls find the below reply to the queries.

    What model scope probe are you using?

    We are using Keysight probe model number  N2873A

    Attenuation - 10:1,Typical Probe BW - 500MHz

     

    Are these commercially assembled boards or hand soldered prototypes?

    These boards are not hand soldered proto. These are manufactured at our supplier end as mass production.

    Are you probing *at* the LM293 pins, or at the test points?

    Yes, we are probing at LM293 pins.


    Does pushing on or flexing the board cause any changes? Is there any cracking of the components or IC package?

    We sent the boards to our supplier end, to check for further analysis for component/IC damages.



    By "randomly", how long does ti take the "problematic" circuit to finally respond? 100ms? 1 second? 10 seconds? It really sounds like you have a floating input.

    We are not able to define the time limit. Sometimes it changes in few seconds, some times it takes around 10 minutes.

    Attached the layout for reference.