Connect with us

a strange problem

Discussion in 'Electronic Design' started by Myauk, Dec 30, 2008.

Scroll to continue with content
  1. Myauk

    Myauk Guest

    Hello..

    I found a strange problem while working on a typical PCBA.

    It is a kind of pneumatic control PCBA. The PCBA uses 24V power supply
    from which it draws current from 1 to 3A.

    Before its operation, normally we check the resistance measurement
    between 24V and Gnd, which should be around 2 kilo ohms.

    For a particular PCBA, the measurement showed 30 ohms, we rejected it
    for further analysis.

    After a week or two, I study the PCBA to determine what was causing
    the problem. Accidentally I supplied 24V to it and found that it was
    working normally.

    When I disconnect the powersupply and measure again, I found that it
    showed higher than 2k ohms at that time.

    I visually checked it thoroughly to see if there were any defective
    capacitor along the pcb traces but found no problem at all.

    I kept it for at least three weeks, I have tested it in operation for
    more than 5 times, and until now there is no problem in the operation
    of that particular PCBA.

    I wonder what could cause the resistance measurement to 30 ohms and
    then higher than 2 k ohms after supplying voltage to it.

    Any idea?

    Regards
     
  2. Assuming it's a double layer PCB with vias...
    do you have gnd and vcc somewhere on an edge of the pcb? maybe there
    was a hangover from the plating process giving you a low impedance
    path between the layers, maybe even a few microns. you've blown this
    away when you applied power.

    I've done similar with a home-brewed pcb where there was a remainder
    from the etching process. 12V @ 8amps did the trick - from short to
    open in 2us ;)


    Heinz
     
  3. Myauk

    Myauk Guest

    Dear Heinz,

    Thanks for sharing.

    One more thing.

    Will it be safe to use it now?

    Might it occur intermittent issue in the future?

    Regards
     
  4. TTman

    TTman Guest

    A capacitor round the wrong way??
     
  5. TTman

    TTman Guest

    Relay catcher diode round the wrong way??
     
  6. Both cap or diode round the wrong way should have been noticeable by
    drawing a higher current (maybe for a few moments) and, eventually the
    occurance of magic smoke ;)

    Heinz
     
  7. Ross Herbert

    Ross Herbert Guest

    :Hello..
    :
    :I found a strange problem while working on a typical PCBA.
    :
    :It is a kind of pneumatic control PCBA. The PCBA uses 24V power supply
    :from which it draws current from 1 to 3A.
    :
    :Before its operation, normally we check the resistance measurement
    :between 24V and Gnd, which should be around 2 kilo ohms.
    :
    :For a particular PCBA, the measurement showed 30 ohms, we rejected it
    :for further analysis.
    :
    :After a week or two, I study the PCBA to determine what was causing
    :the problem. Accidentally I supplied 24V to it and found that it was
    :working normally.
    :
    :When I disconnect the powersupply and measure again, I found that it
    :showed higher than 2k ohms at that time.
    :
    :I visually checked it thoroughly to see if there were any defective
    :capacitor along the pcb traces but found no problem at all.
    :
    :I kept it for at least three weeks, I have tested it in operation for
    :more than 5 times, and until now there is no problem in the operation
    :eek:f that particular PCBA.
    :
    :I wonder what could cause the resistance measurement to 30 ohms and
    :then higher than 2 k ohms after supplying voltage to it.
    :
    :Any idea?
    :
    :Regards


    I'll bet you used a digital VOM to measure the resistance...

    I have found a similar symptom when using my high quality digital VOM to measure
    resistance where there may be a multiplicity of components connected between the
    measuring points. The damned reading was way out from the expected value.
    Solution, I dragged out my old analog meter (AVO model 8) and now the value was
    as expected.

    Some digital VOM's don't behave as they should.
     
  8. neon

    neon

    1,325
    0
    Oct 21, 2006
    would you believe a cap
     
  9. Myauk

    Myauk Guest

    Thanks for sharing.

    I also think that there is something wrong with my digital VOM.

    On the other hand, i need a realistic and logical explanation for it
    so that I could put in my report to explain to everyone.

    There has been a lot of capacitors installed along the power rails to
    keep them away from different ripple frequencies.

    Accidental charge stored in capacitors could make the digital VOM
    readings to go wrong.

    Correct me if I am wrong..

    Regards
     
  10. whit3rd

    whit3rd Guest

    On Dec 30 2008, 1:26 am, Heinz Liebhart <h.liebhart (at) gmail.com>
    wrote:

    [concerning a PCB that had abnormal 30 ohm resistance
    across power until it was powered-up once]
    I've seen hair-thin failed-to-etch blemishes that would account
    for this; the original PCB handled some high current, so might
    have been thicker copper than most, and those always take
    annoyingly long to etch to completion.

    If it's not going to be a human-shock hazard, I'd say the
    board is fine. If there COULD be a shock hazard,
    more testing is required (HiPot type testing).
     
  11. Ross Herbert

    Ross Herbert Guest

    :
    :> I'll bet you used a digital VOM to measure the resistance...
    :>
    :> I have found a similar symptom when using my high quality digital VOM to
    measure
    :> resistance where there may be a multiplicity of components connected between
    the
    :> measuring points. The damned reading was way out from the expected value.
    :> Solution, I dragged out my old analog meter (AVO model 8) and now the value
    was
    :> as expected.
    :>
    :> Some digital VOM's don't behave as they should.
    :
    :Thanks for sharing.
    :
    :I also think that there is something wrong with my digital VOM.
    :
    :On the other hand, i need a realistic and logical explanation for it
    :so that I could put in my report to explain to everyone.
    :
    :There has been a lot of capacitors installed along the power rails to
    :keep them away from different ripple frequencies.
    :
    :Accidental charge stored in capacitors could make the digital VOM
    :readings to go wrong.
    :
    :Correct me if I am wrong..
    :
    :Regards


    That guess is as good as any. Without knowing the design details of every
    different type of digital VOM all one can conclude is that it happens for some
    unknown reason. If an analogue VOM shows the reading as expected then you can
    assume that the blame is with the digital VOM.
     
  12. ehsjr

    ehsjr Guest

    Go read John Larkin's reply and answer his question before
    going further. Forget the crap about the digital meter being
    wrong. You would be far better off learning what the meter is
    actually measuring and how it does so, than assuming it is
    somehow magically wrong because it is digital. In fact,
    unless it is broken, it doesn't read wrong - you just don't
    understand what it is showing you.

    Ed
     
  13. Ross Herbert

    Ross Herbert Guest

    :Myauk wrote:
    :> Hello..
    :>
    :> I found a strange problem while working on a typical PCBA.
    :>
    :> It is a kind of pneumatic control PCBA. The PCBA uses 24V power supply
    :> from which it draws current from 1 to 3A.
    :>
    :> Before its operation, normally we check the resistance measurement
    :> between 24V and Gnd, which should be around 2 kilo ohms.
    :>
    :> For a particular PCBA, the measurement showed 30 ohms, we rejected it
    :> for further analysis.
    :>
    :> After a week or two, I study the PCBA to determine what was causing
    :> the problem. Accidentally I supplied 24V to it and found that it was
    :> working normally.
    :>
    :> When I disconnect the powersupply and measure again, I found that it
    :> showed higher than 2k ohms at that time.
    :>
    :> I visually checked it thoroughly to see if there were any defective
    :> capacitor along the pcb traces but found no problem at all.
    :>
    :> I kept it for at least three weeks, I have tested it in operation for
    :> more than 5 times, and until now there is no problem in the operation
    :> of that particular PCBA.
    :>
    :> I wonder what could cause the resistance measurement to 30 ohms and
    :> then higher than 2 k ohms after supplying voltage to it.
    :>
    :> Any idea?
    :>
    :> Regards
    :
    :Go read John Larkin's reply and answer his question before
    :going further. Forget the crap about the digital meter being
    :wrong. You would be far better off learning what the meter is
    :actually measuring and how it does so, than assuming it is
    :somehow magically wrong because it is digital. In fact,
    :unless it is broken, it doesn't read wrong - you just don't
    :understand what it is showing you.
    :
    :Ed

    Well the evidence of what I found doesn't support your claim that it will only
    happen if the meter is broken. My meter (Gossen-Metrawatt Metrahit 25S) is fine
    and continues to be fine. The erroneous reading I got was obviously incorrect
    and I tried reversing the digital VOM polarity and eliminating every other
    possible meter fault as well without any difference to the erroneous reading.

    I would ask the OP if the same digital VOM exhibited the same result with the
    specific unit under test every time he measured? And also whether he tried the
    same measurement with an analog meter on this unit? If he did, what was the
    result?

    It is hard to argue with results proven by empirical method when all other
    logical explanations have been exhausted.
     
  14. Ross Herbert

    Ross Herbert Guest

    A bit of Googling turned up this http://zone.ni.com/devzone/cda/tut/p/id/3981

    While this article refers to PC based plug-in multimeters one would imagine that
    the PC card uses a similar technique to measure resistance as do hand held or
    bench VOM's (ie. CC).

    See the paragraph System Considerations For Resistance Measurements.

    Agilent also recognise the problem (Hint 2)
    http://www.techni-tool.com/content/...g Better Digital Multimeter Measurements.html

    Capacitance in parallel with the resistor being measured can cause erroneous
    offset measurement in a digital VOM.

    The erroneous measurement may also be dependent upon the combination of
    resistance and capacitance involved and the particular VOM being used. If it is
    auto-ranging while endeavouring to come up with a valid reading may also cause
    it to produce an erroneous reading.
     
  15. That's the problem when you try and use a multimeter ohm range to
    measure the "resistance" of an active circuit. It's not actually the
    "resistance" you are measuring, it's just what the multimeter thinks
    it is for a given test current. The figure will vary based on the type
    of multimeter you use, the probe polarity, previously charged
    capacitors, temperature, and the phase of the moon.

    Dave.
     
  16. There is almost certainly nothing wrong your VOM.
    Go and measure your circuit with different brand VOM's and you'll
    almost certainly get 3 different values, then try the probes backwards
    and that'll really screw you up.
    Then measure just a resistor on its own with the same meters and they
    will all read the same value.
    What you are trying to measure is non-linear and not purely resistive,
    that's your problem. This is not something a resistance range of a
    multimeter is designed to measure accurately or at all.

    Dave.
     
  17. ehsjr

    ehsjr Guest

    Sorry Ross, but that's all bullshit. If you connect the
    DMM set to measure resistance of a circuit with a capacitor
    in parallel with a resistor and are mislead by what you see
    on the meter, that is *not* the meter's fault. It's yours.
    You seem to think that the meter should show you the value
    of the resistor, and that, if it doesn't, it's wrong. It's
    not wrong, you are, for not understanding what you are
    measuring and how it can affect the reading.

    Suppose there is a resistor in the circuit and you connect
    your DMM, set to measure resistance, across it, not knowing
    that the circuit is applying a voltage across the resistor.
    Do you expect to get the correct value for the resistor
    displayed on the DMM? When it is not, do you blame the
    DMM for being wrong?

    Ed
     
  18. Ross Herbert

    Ross Herbert Guest

    :
    :Sorry Ross, but that's all bullshit. If you connect the
    :DMM set to measure resistance of a circuit with a capacitor
    :in parallel with a resistor and are mislead by what you see
    :eek:n the meter, that is *not* the meter's fault. It's yours.
    :You seem to think that the meter should show you the value
    :eek:f the resistor, and that, if it doesn't, it's wrong. It's
    :not wrong, you are, for not understanding what you are
    :measuring and how it can affect the reading.
    :
    :Suppose there is a resistor in the circuit and you connect
    :your DMM, set to measure resistance, across it, not knowing
    :that the circuit is applying a voltage across the resistor.
    :Do you expect to get the correct value for the resistor
    :displayed on the DMM? When it is not, do you blame the
    :DMM for being wrong?
    :
    :Ed

    What you are suggesting is that a resistance reading taken with a digital meter
    can not necessarily be relied upon. And where an unexpected reading does occur
    the user should completely analyse the system undermeasurement in order to
    determine why the expected resistance measurement is not being returned. That is
    just ludicrous.

    A technician using a digital meter to measure a specific resistance combination
    should have no reason to suspect that there should be a significant difference
    in the result comapred to using an analog meter. The fact that there may be some
    parallel capacitance will have no effect on the analog meter reading (once the
    capacitance is charged) and the analog meter reading will be accurate.

    A technician has every right to expect that a digital meter will also present an
    accurate reading without having to analyse whether or not any particular value
    of capacitance might be present to upset the reading. The fact is that digital
    multimeters, being sampling devices, can be upset by a certain combination of
    resistance and capacitance, but is it wrong to say that where an unexpected
    result occurs, the fault lies with the user because he has failed to analyse
    what might be upsetting the meter? Of course not. The digital meter is just a
    measuring tool the same as the analog meter and the technician should not have
    to be conversant with the specifics of the design of the two items in order to
    determine whether a reading is correct or not.

    The user has every right to expect the same result (within reason) for the same
    measurement whether using an analog or digital meter, "particularly for
    resistance measurements". If the digital meter produces an unexpected result and
    the analog meter doesn't then where does the fault lie? The meter producing the
    unexpected result is obviously "wrong", despite any excuses, legitimate or not,
    which might be made to explain its erroneous measurement.
     
  19. Jan

    Jan Guest


    If you don't know how to use a DVM then you're not a technician.
    You can't just poke your probes in a circuit and expect to get a meaningfull
    reading.

    Jan
     
  20. ehsjr

    ehsjr Guest

    Ross, I have no wish to argue with you - it's not personal.
    But I must correct your statement about what I am suggesting.
    It is not *I* who is suggesting that the reading taken with
    the DMM cannot be relied on, it is *YOU*.

    I have said you have to know what you're doing. If you don't,
    and you think the meter reading means something that it
    doesn't mean, then you are misleading yourself.

    If you want to go ahead and measure things with your DMM and/or
    analog meters without knowing what you are measuring, no one can
    stop you. Have a wonderful time. It is glaringly obvious that
    ignoring _what_ is being measured can deceive you. That is *not*
    the meter's fault - it is yours. All you said below about what
    a technician or a user has a "right" to expect is just bullshit.
    An expectation does not change what the digital meter shows
    by the most significant digit, least significant digit, or
    any digit in between, and it does not move the needle on the
    analog meter movement, at all. It is up to the technician or
    user to understand what the meter is shoing him/her.

    If you want to have a technical discussion, fine. Tell us what
    the ohmmeter reading should be when you connect it to a resistor
    in a circuit that has a voltage applied to it by the rest of the
    circuit. Lets stipulate: said resistor is a bleeder resistor
    across a cap that is charged to 450 volts, and the resistor is
    open. Aren't you glad, after your digital meter gives a screwy
    reading (and hopefully protects itself), that you connected that
    nice analog meter to measure the resistance? I don't think it's
    *fun* to watch the needle wrap itself around the pin. That is
    one scenario of what can happen if you stubbornly refuse to learn
    what you are connecting your meter to before doing so.

    Ed
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-