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Original question:

LM358: is it OK to have input voltage present while power source V+ = V- (gnd) ie device not powered?

LM358: LM358 comparator transition issue

Ramana Murthy
Expert 1765 points
Part Number: LM358

Dear Sir,

we are using LM358 as a comperator.we are facing transition Issue from positive +15V DC to Negative output -15V DC on Pin number 1.
while touching with multimeter probe on pin no:1.state is changing properly. what would be cause for this.please provide some solution.

  • 0
    TI__Guru*** 149766 points

    Hello Murthy,

    It seems that physical pressure on pin 1 is making connection where no probe has an open connection somewhere. Check or better yet, re-touch the solder joints in this area. 

    Does this effect happen on just one board or on all the boards?

  • 0 in reply to Ron Michallick
    Expert 1765 points

    Dear Sir,

    There is no connection issues. I am using comparator to switch on mosfets by comparing Voltage inputs from different source.similar like Hot Swap.comparator choose the hihgest value power source between available sorces.But Some times op Amp goes in middle stage like 0 to 1.8 V at pin 1. it try to on both Mosfets.But Current is flowing through one mosfet only.i want to make sure that Opamp Voltage transition is going well from +ve to -ve output on Pin 1.please changes what i required to make it properly switch between to stages.I have tried in two different boards reacting same.Your support will be helpful

    Thank you

    Regards

    Murthy

  • 0 in reply to Ramana Murthy
    TI__Guru*** 149766 points

    Murthy,

    This comparator circuit has no hysteresis, so the output may have issue if both inputs (plus their noise) are the same voltage. When the output appears to be in the middle, perhaps it is switching between low and high (and average voltage is read).   

    Try to measure waveforms on all the pins of the op amp when the issue occurs. 

  • 0
    Guru 140200 points

    Hi Murthy,

    switcher are known to produce huge amounts of ripple and noise. You will need to low pass filter the input and outputs of switcher. Pi filters using a choke work the best.

    Also, mount a cap in parallel to R12 to decrease the effect of ripple and noise at the input side of switcher (battery). Mount a hysteresis resistor of 100k...1M from pin 3 to pin 1 of LM358. The small signal diodes can be 1N4148. There's no need to take an 1A rectifier diode.

    The wiring of Q1 looks strange. By the way, you don't need D1, R3, C2, C3 and Q1 at all. Just add a transistor inverter to the "BAT_SIG" output.

    Kai

  • 0 in reply to kai klaas69
    TI__Guru*** 149766 points

    Murthy,

    Q1 does look strange but will likely work as a saturated bipolar transistor can conduct collector to emitter current in both directions. How well this works depends on the beta of the transistor with the collector and emitter swapped. A simulation only supported 7mA (not the 15mA needed). So increase R6 (to 6.8k) or decrease R3 (to 2k).  That and well chosen input capacitors and hysteresis should fix the issues.