Reading capacitance on SM capacitor

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I'm looking at a laptop (Toshiba Sat 3000, to be specific) motherboard
that having some SMD suspecting  leaky capacitors; Some basic questions
if someone can bring me up-to-date to the world of surface mount device,
that would be nice:

* There are four of them in parallel; ohm reading both way, in circuit,
about 50 Ohm. They are around the power charging area, and the circuit
having a blown fuse. Would this sound like a strong posibilty for leaky
cap ? (I can take them out one by one for sure...)

* The capacitors are the good size ones in the SMD world (6mmx4mmx3mm);
There is + sign printed on one end, and the printed note say "220" on
first line and "g14" the next line. Would that be 220 pF ? Seem little
for that size...

Thanks for any advice.


Re: Reading capacitance on SM capacitor

I bet to differ. I'd say all "can" capacitors are uF.

Testing in-circuit won't do u any good, as you are seeing other
components at the same time.

Unsolder them, when u ohm, u should read a short first then within a
few seconds go up to infinity.

Curious, why are u blaming the caps?

Re: Reading capacitance on SM capacitor

bobb wrote:
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As I read on-line so far, the figure refer to pico Fara with the last
digit indicate the number of zero to follow. So I'm confused as if they
are 22 pF for that size of a surface mounter device.

I know I should remove the cap to measure for sure, but I need the
replacement just in case, and therefore need to read the value correctly

The circuit is having a blown fuse, so I'm looking around to see what's
wrong; and this low Ohm reading across the caps causes some concern...

Re: Reading capacitance on SM capacitor

nospam_needed wrote:

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220 pf is the best bet.  If they are showing 50 ohm both directions then
either they're completely dead or you're measuring resistance through some
other part of the circuit--for a capacitor you should be seeing a low
initial resistance that increases over time as the capacitor charges.

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(was jclarke at eye bee em dot net)

Re: Reading capacitance on SM capacitor

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Those are 220 uF, probably tantalum. The ordinary failure mode is a short
circuit in which case you would read much less than 50 Ohms. Some other part
of the circuit is responsible for the 50 Ohm reading.

Re: Reading capacitance on SM capacitor

Frank346 wrote:
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I tend to think that's 220uF as well, but all the references I've read
so far on-line indicate that the number refer to pico Fara, with the
last digit indicate the number of zero's to follow... That would make
this 22 pF, which is against any other indicators (polarity sign is not
popular for small cap, physical size of the cap...) Unless I'm still
missing other way to read capacitor from a specific manufaturer.

I know measuring resistance in circuit is not conclusive; but before
taking the cap out, I need the replacement ready just in case, therefore
I need to read the capacitance value ...

Any other hint ? Thanks for the opinion so far.

Re: Reading capacitance on SM capacitor

When you see a 3-digit number on a part (resistor or capacitor), IF the
number is the "value" of the part at all (and often it's not), the usual
format is that the first two digits are the two significant digit and
the 3rd is the power of tens exponent.  For example, to take a resistor:

470 = 47 ohms
471 = 470 ohms
472 = 4,700 ohms
473 = 47,000 ohms


Both resistors and caps come with only a few "first two significant
digits", most commonly:

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, 91

[In "high tolerance" parts, there are additional standard values, and on
special order, any value can be custom made, but the above are the most
common "low tolerance" sizes]

Frank346 wrote:

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Re: Reading capacitance on SM capacitor

First, the largest problems with leaking capacitors were with
electrolytics, these are usually large capacitors and are usually not
surface mounted.

Testing a capacitor with a voltmeter (ohm-meter) the way that you did
won't tell you much and can destroy the item (the laptop).  What you
measured was mostly the resistance not of the capacitor, but of the REST
of the circuit, and in measuring it you applied voltage to it, possibly
in the wrong polarity.  That can destroy the circuit.  If you are going
to test a capacitor, you need some specialized equipment and/or you need
to test it "out of circuit".

 From your size descriptions, I'd guess that they are tantalum rather
than electrolytic, thus could not leak (no electrolyte).  But they could
still be bad, of course.  "220", if it's the capacitance at all, most
likely means 22 x 10**0 or 22;  220pf is a tiny, very small capacitor,
do you perhaps mean 22 MF (there is only a difference there of
1,000,000:1)?  It's hard to be sure, but my guess would be 22MF, IF the
"220" is the capacitance at all.  Sometimes they put part numbers on the
parts and not the actual values.  It can be very hard to tell.  It COULD
be 22MF at 14 volts, but then again, those could be part numbers and the
values could be anything.

nospam_needed wrote:
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