a strange problem

[Myauk]

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

 

[Heinz Liebhart]

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

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

 

[Myauk]

On Tue, 30 Dec 2008 00:38:48 -0800 (PST), Myauk

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

 

[TTman]

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

A capacitor round the wrong way??

 

[TTman]

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

Relay catcher diode round the wrong way??

 

[Heinz Liebhart]

Relay catcher diode round the wrong way??

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

 

[Ross Herbert]

: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
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.

 

[neon]

would you believe a cap

 

[Myauk]

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

 

[whit3rd]

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]

.. 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 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).

 

[Ross Herbert]

:
:> 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.

 

[ehsjr]

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

 

[Ross Herbert]

: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.

 

[Ross Herbert]

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.

 

[David L. Jones]

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.

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.

 

[David L. Jones]

Thanks for sharing.

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

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.

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.

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.

 

[ehsjr]

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.

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

 

[Ross Herbert]

:
:Sorry Ross, but that's all bullshit. If you connect the
MM set to measure resistance of a circuit with a capacitor
:in parallel with a resistor and are mislead by what you see
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
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.
o you expect to get the correct value for the resistor
:displayed on the DMM? When it is not, do you blame the
MM 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.

 

[Jan]

:
:Sorry Ross, but that's all bullshit. If you connect the
MM set to measure resistance of a circuit with a capacitor
:in parallel with a resistor and are mislead by what you see
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
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.
o you expect to get the correct value for the resistor
:displayed on the DMM? When it is not, do you blame the
MM 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.

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

 

[ehsjr]

Ross Herbert wrote:

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