Multimeter reads zero when testing capacitance

[Toby Corkindale]

Hi,
I have a what is probably a stupid newbie question.. again..
I finally had a need to measure the capacitance of some capacitors,
and for the first time ever tried using said function on my
multimeter. However no matter what I try, it always reads zero, on
every scale.
I tried with some possibly-bad capacitors, and then with a couple of
known-good ones.

I've double-checked that I'm using the right connection points on the meter.

Is there anything else silly that I could have missed here?
Is this function supposed to work with electrolytic caps, or some
other sort that I'm not using?

Thanks,
Toby

[Darren Freeman]

On Wed, 2014-01-08 at 00:50 +1100, Toby Corkindale wrote:
> multimeter. However no matter what I try, it always reads zero, on
> every scale.

> Is there anything else silly that I could have missed here?

Not that I can think of - it should just work. (Assuming the capacitor
is not in a circuit - it has to be essentially rolling around on the
bench.)

Try measuring resistance with the probes opened and shorted, just to
make sure there is no break in them and no short.

It's possible that all of the caps that you are measuring are either
over or under the range that the meter can handle. You should be able to
measure a 100 nF ceramic capacitor just fine.

Depending on what other functions are shared with that input, perhaps
you've blown a fuse? Does anything else work on that input?

Have fun,
Darren

[Toby Corkindale]

On 8 January 2014 01:49, Darren Freeman <dar...@freemaninstruments.com> wrote:
> On Wed, 2014-01-08 at 00:50 +1100, Toby Corkindale wrote:
>> multimeter. However no matter what I try, it always reads zero, on
>> every scale.
>
>> Is there anything else silly that I could have missed here?
>
> Not that I can think of - it should just work. (Assuming the capacitor
> is not in a circuit - it has to be essentially rolling around on the
> bench.)

Yep, they're all unattached. (Either desoldered from a board, or have
never been soldered to a board)

> Try measuring resistance with the probes opened and shorted, just to
> make sure there is no break in them and no short.

All good there.

> It's possible that all of the caps that you are measuring are either
> over or under the range that the meter can handle. You should be able to
> measure a 100 nF ceramic capacitor just fine.

The ones I'm trying should be easily within the multimeter's range -
between .1 and 47 µF.

> Depending on what other functions are shared with that input, perhaps
> you've blown a fuse? Does anything else work on that input?

Resistance, current (DC) and voltage (DC) were all working fine. (I
think the AC functions are good too.. I just hadn't cause to use them
last night)
The Cx mode uses a unique arrangement of inputs -- the probes are
attached to the mA plug and the combined V/R/etc plug. (ie. The common
plug isn't used.)


Weird.. Maybe my multimeter has just had a busted capacitance test for
the whole time? I've never used it before so it's possible.

Thanks for the advice,
Toby


--
Turning and turning in the widening gyre
The falcon cannot hear the falconer
Things fall apart; the center cannot hold
Mere anarchy is loosed upon the world

[Robert Powers]

The Cx mode uses a unique arrangement of inputs -- the probes are
attached to the mA plug and the combined V/R/etc plug. (ie. The common
plug isn't used.)

That is unique in my experience. What is the model of this DMM? 

Cheers,

Bob

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[Toby Corkindale]

On 8 January 2014 12:09, Robert Powers <rdpo...@gmail.com> wrote:
>> The Cx mode uses a unique arrangement of inputs -- the probes are
>> attached to the mA plug and the combined V/R/etc plug. (ie. The common
>> plug isn't used.)
>
> That is unique in my experience. What is the model of this DMM?

Tenma 72-7725
A manual seems to be online at:
http://www.mcmelectronics.com/content/ProductData/Manuals/72-7720.pdf

[samotage]

DMM's can be kinda fail testing capacitors, what is really important is the internal resistance, the ESR which increases in fail situations.

Check out this from the venerable Dave Jones at @eevblog

http://www.youtube.com/watch?v=72sJ5Infuu0 

Sam,

@samotage

[John Bosua]

I have an ESR meter if you want to borrow it.
John Bosua

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[Toby Corkindale]

On 8 January 2014 13:03, samotage <sam....@esskware.com.au> wrote:
>> DMM's can be kinda fail testing capacitors, what is really important is
>> the internal resistance, the ESR which increases in fail situations.

So I've heard. But without an easy way to test it, I thought I'd test
for capacitance first and see if that identified anything.
Although the more I read up, the more it sounds like ESR testing is
required to be sure.


Given the requirement to remove caps from the board to test them, I'm
thinking it's going to be easier to just start replacing all caps in
decreasing order of likelihood-of-being-failed then. Hmm.

Actually, there's a new question now -- how do I know if the original
caps were specified as low leakage, or low esr, or both or neither? If
I'm replacing caps, is it safe to always use a particular sort, or
could low leakage/low esr caps cause problems in circuits that aren't
expecting them?

Thanks,
Toby

[Toby Corkindale]

On 8 January 2014 13:55, John Bosua <bosu...@gmail.com> wrote:
> I have an ESR meter if you want to borrow it.
> John Bosua

Oh, cool. Thanks. It's not urgent, since I'll probably just replace a
dozen caps anyway, but I would like to eventually find out if any/all
caps were faulty.
I might try building an ESR meter with an arduino myself. I found this online:
http://forum.arduino.cc/index.php/topic,80357.0.html

Thanks,
Toby

[Clifford Heath]

On 08/01/2014, at 2:45 PM, Toby Corkindale <to...@dryft.net> wrote:
> On 8 January 2014 13:03, samotage <sam....@esskware.com.au> wrote:
>>> DMM's can be kinda fail testing capacitors, what is really important is
>>> the internal resistance, the ESR which increases in fail situations.

> Given the requirement to remove caps from the board to test them

Most ESR meters use a low-voltage AC test which works just fine
while the capacitor is in-circuit. Any test voltage under 500mV won't
turn on semiconductor junctions so the meter only sees parallel
resistances, which typically are far above valid ESR levels.

> I'm
> thinking it's going to be easier to just start replacing all caps in
> decreasing order of likelihood-of-being-failed then. Hmm.

If any electrolytics have high ESR, others are likely on the way and will
develop it soon, even if not yet - so replacing them all is a good stratagem.

> Actually, there's a new question now -- how do I know if the original
> caps were specified as low leakage, or low esr, or both or neither?

Check the specs, if any, and the circuit if not. An SMPS will typically have
low-ESR caps.

> If
> I'm replacing caps, is it safe to always use a particular sort, or
> could low leakage/low esr caps cause problems in circuits that aren't
> expecting them?

Unlikely, but possible. Increasing the temperature rating doesn't hurt either.

Clifford Heath.

[Luke Weston]

With regards to the original problem of seeing whether the capacitance test works on that particular multimeter, if we've checked/ruled out the following then I've got absolutely no idea.

- Check that the multimeter works on other functions, including voltage, resistance and the current ranges.

- Are the fuses intact? If the mA-scale current shunt is separately fused this may both knock out that current range and the capacitance test, and if these aren't multimeter features you commonly use then you might not notice it.

- Replace the battery, just to be cautious. Low battery can cause funny multimeter behaviour even if it appears "still alive".

- Connect the capacitor to the meter with the right polarity, if applicable for that capacitor type.

- Just to be fussy, work through the step-by-step documented procedure for a capacitance measurement as given in the manual

- Check that the probes/leads you're using are intact.

- Test using a capacitor that is not too large or too small, say just something like 0,1uF or 1uF for a test.

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[Toby Corkindale]

On 8 January 2014 17:21, Luke Weston <reindeer...@gmail.com> wrote:
> With regards to the original problem of seeing whether the capacitance test
> works on that particular multimeter, if we've checked/ruled out the
> following then I've got absolutely no idea.
>
> - Check that the multimeter works on other functions, including voltage,
> resistance and the current ranges.
>
> - Are the fuses intact? If the mA-scale current shunt is separately fused
> this may both knock out that current range and the capacitance test, and if
> these aren't multimeter features you commonly use then you might not notice
> it.

So, the common/mA fuse was fine, and all the other functions I could
were working fine.

Then I dismantled the multimeter further, and found a second,
non-user-serviceable, fuse -- which is blown.
Annoyingly, unlike the main fuse, it's soldered directly to the back
of the circuitboard. What a stupid design. And why would you do that,
when the other one is easily user-replaceable?
But once my iron heats up, I'll replace it and see if the Cx
measurement starts working again.

*grumble*

Thanks for all the help and suggestions so far!

[Toby Corkindale]

Quick update to say that, after a couple of attempts I managed to get
a good, similarly-rated fuse soldered on neatly, and now the Cx
measurement works again.
F***ing stupid design choice though. Why have one fuse that's easily
user-replaceable, and another one soldered onto the back of the
circuitboard?!

[Toby Corkindale]

.. and then after soldering half a dozen components onto a board, my
soldering iron died and won't come back on.
It's not been a good few days for equipment around this house :/

[Lauren Shearer]

Why make that design choice?

Because, unlike most other professions electronic engineers believe in fostering inquisitiveness, analysis, and technical skill in others with the unlikely hope that they, too, may one day become employed in the field and elevated to to godlike nirvana that that entails.

Or it cost too much to put in two.