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LM324A: LM324AN Failure -ve input - low impedance

Part Number: LM324A
Other Parts Discussed in Thread: LM324, LM324-N

Hi Guys,

 

I need your help. We are having some trouble with the op-amp, negative input going low impedance and I’m don’t know why this might be happening.

The circuit is shown below. The input signal can be anywhere between 0 and 24V.

I’m not sure if we have a doggy batch or I’m doing something very wrong that I can’t see.

When the device dies the inputs don’t work anymore at the connector side, but if you apply a voltage to say D7 anode, 5V the output switches okay.

I’m sort of presuming the catch diode is damaged. I personally cant see anything wrong with the circuit.

 

If you measure across the 100K when the device is damaged it measure 3K even with the ~~100k removed.

 

Any advice?

 

Kind regards

Rich

  • Hi Richard,

    why are you using an OPAmp for this typical comparator application?

    You permanently violate the common mode input voltage range at the +input of LM324 and temporarily at the -input of LM324 when inputting your 24V input signal. Datasheet says that the input voltages must stay 2V away from the positive supply voltage.

    And I don't see any current limiting resistor at the -input of LM324 protecting the OPAmp in the case of negative input voltages.

    See also section 8.3.3 of datasheet.

    Kai

  • Hi Kai,

    So your saying that the 4v bias on +ve input (R46 and R47) violate the device?

    Also, the -ve is set to a value of ~0.5V with no input signal. When 24V or so is applied then the -ve input can really only to a maxium of 5V via the 100KR resistor, the 24V is blocked by the diode.

    I agree if the input falls below <-0.7 then the internal diode could conduct.

    Is my thinking here all wrong?

    Rich

  • Hi Richard,

    So your saying that the 4v bias on +ve input (R46 and R47) violate the device?

    Yes:

    Also, the -ve is set to a value of ~0.5V with no input signal. When 24V or so is applied then the -ve input can really only to a maxium of 5V via the 100KR resistor, the 24V is blocked by the diode.

    Yes, but even +5V will violate the common mode input voltage range. See above.

    Kai

  • Hello Richard,

    There is just a diode between input connect and IN- pins. Therefore the IN- pin could be subject to ESD/ EOS. (Electrostatic discharge / Electrical over stress).  Adding a resistance in series with IN- pins could be helpful. 

    What is the supply voltage for LM324? If 5V, then the input common mode is incorrect. 

  • Hello Ron / Kai,

    So, the reason we are using the 324 is because we literally have hundreds.
    I've decided to drop the voltage on the IN+ to 2V0 or 2V5 and change the input to have a range of 0~3.0V.

    To get the voltage required with the standard components we have on hand. There is now a series resistor in-line with the IN- so any negative going signals will have less chance to damage the input.

    Hopefully I won’t see any more failures due to my incompetence!

    I will also redesign the circuit that originally exists with a correct comparator device. I guess if the Op-Amp was powered from 6V5 I wouldn't have this problem. Still not entirely sure what is actually killing the device.

    I was surprised to find that my data sheets didn't have 8.3.3 section.

    Many thanks. If there is anything else worth mentioning, I'm all ears!!
    kind regards

    Rich

  • Hi All,

    I've had second thoughts. Apart from the danger of getting a negative transient that could potentially damage the input.
    I feel that the circuit is valid as the device isn't used as an amplifier and the common mode values do not apply in this case.

    The inputs, either or both can go to 32V without damage independant of the magnitude of Vcc. This is guarateed by design.

    kind regards
    Rich

  • Hi Richard,

    have you read section 2.2 of this appnote?

    sloa277a.pdf

    By the way, which LM324 are you using? The original LM324-N from National Semiconductors or the clone from TI?

    Kai

  • Hi All,

    I'm using the TI device, pictures attached above.
    I can see there is an area of concern so as belts a braces approach I've dropped the trigger voltage to 2v5 and added a 1K series resistor on leg IN-.
    regards
    Rich

  • Hi Richard,

    you could do it this way:

    richard_lm324.TSC

    The threshold of comparator is set to 1V by the help of R1 and R2. R3 and R4 form an input voltage divider so that a 3.5V input voltage is needed to make the compatator toogle, as in your original circuit.

    R3 = 25k protects the LM324 against negative going input voltages of up to 25V and in combination with R4 positive overvoltages of up to 112V. And C2 adds some protection against ESD.

    If the LED is still glimming in off state due to the finite output voltage of LM324 (~3.8V), you can add a silicon diode in series to the LED. This can be the case when taking a high efficiency LED needing ultra low current.

    Kai

  • Hi Kai,
    Thank you for the reply. I think you mean 3V not 1V threshold.

    So with my circuit obeying the common mode voltages, the input volts can vary in range from 0V - 24V and still toggle the op-amp. Trigger now set at 2V5. I also need a 5V signal out, so that's one reason the device is powered from 5V as the output is not open collector.

    Also, the diode keeps any larger voltages at bay, but still guaranteeing that on the input side to the op-amp stays at 5V max if the cathode is biased with at least 5V.

    This has been interesting for sure.
    Regards
    Rich

     

  • Richard,

    2.5V fixed voltage input and 5V supply is fine. The other input can vary from 0V to maximum VCC. 

  • Thank you for all the replies.

    Just going back to my original problem that I don’t fully understand the mechanism how the IN- became compromised. A couple of failed devices (pin 6 IN-) measured 4KR and 20KR with reference to GND with a DMM. I am assuming partial damage occurred due to a negative going transient! Just hope it doesn't happen again with the modifications applied.

    One last note, if this pin is then driven by a lower impedance output, I wouldn’t have known about the damage as the output worked as expected.

  • Hi Richard,

    Thank you for the reply. I think you mean 3V not 1V threshold.

    No, I indeed meant a threshold of 1V, to be able to use a voltage divider at the input of circuit and to make the input of OPAmp immune against heavy undervoltages and overvoltages from -25V to 112V.

    Just going back to my original problem that I don’t fully understand the mechanism how the IN- became compromised

    Very often, heavier undervoltages and overvoltages occur in a circuit than expected. Not to forget ESD, surge and burst. Signal ground shifts during the making of cable connections or during power up and power down sequences when the involved circuits are partially or unevenly powered up or down, can be the cause of unexpected undervoltages and ovbervoltages. So it can be wise to enhance the input protection, even if there seems to be no need for it.

    By the way, your 1k current limiting resistor at the input would be too weak to fully protect the input of LM324, as the current must be limited to below 1mA. Undervoltages of more than -1V would destroy the LM324. So if you don't like my input voltage divider, then at least increase the current limiting resistance by a factor of 10 or so.

    A very time costly but useful method is to connect a robust datalooger to a signal line which is suspected to have to struggle with heavier undervoltages and overvoltages than expected. The datalooger should be battery powered to not introduce additional sources of EMI and should have a input voltage divider to be able to handle heavy undervoltages and overvoltages at its input itself.

    Kai