How to test capacitors in-circuit?

[phaeton]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Thanks.

-ph

 

[Phil Allison]

"phaeton"

How do you accurately test a capacitor in the circuit?

** Generally, that is impossible.

Aside from
bulging packages, how do you know a capacitor has failed?

** Only electros bulge.

Do they
usually just short and start passing DC?

** Film and tantalums caps sometimes do.

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

** Electros can be checked - in circuit - with an ESR meter.

But this does not find leaky ones.

Particular kinds of circuit misbehaviour indicate a faulty cap at a certain
spot.





........ Phil

 

[Michael Black]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Are you trying to find something specific, or is this a general
question?

One old trick is when you suspect a capacitor, jumper it with
another capacitor of the same value. If things improve, then
it may be the capacitor. But of course, this implies that the
original capacitor is open or has lost capacitance, rather than
shorted.

Michael

 

[phaeton]

Are you trying to find something specific, or is this a general
question?

It is a general question.

(and not a homework question!)

-phaeton

 

[Richard Seriani]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Thanks.

-ph

For electrolytics in most circuits, an ESR meter works pretty well. In some
circuits, the capacitor will still have to be removed for testing.

Richard

 

[[email protected]]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Thanks.

-ph

You have a look at either side of it with a 'scope and estimate what
you should be seeing e.g. if it is a large coupling capacitor in an
audio path then both sides should be the same size.

If it is some sort of first order filter then work out the capacitor's
Xc and compare that to the "load" and if e.g. they are the same then
half the signal should be dropped across a working cap.

If it is part of a UHF oscillator (that won't) then do as the other
poster says i.e. bridge it (hold in place with end of plastic trim
tool).

A worse case is where you have a large output transistor with four
caps soldered to strip-line pcb layout at UHF and the output power is
10Watts instead of 20. This is straight off the production line and
has never worked before. Then you have no option but to remove all
four and measure them with a bridge.

Robin

 

[Rich Grise]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

If you're getting paid by the hour, it's probably cheaper to just
swap out the cap.

I once worked repairing video game PCB's, and when we'd get a really
knotty problem that took too much time to diagnose with the scope,
we'd "shotgun" it - just replace every chip on the board, because
it was cheaper to take an hour to replace 20 or so chips at $0.30
apiece than to spend four hours troubleshooting at $30.00/hr.

And I've been in the business since 1968, and have never seen a
capacitor tested in-circuit, let alone done it.

Good Luck!
Rich

 

[Jamie]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Thanks.

-ph

I have a nice B&K tester that works very good with
in circuit stuff. It's an LCR meter and it's been
around for a bit but it's what I use for in circuit
tests.
http://www.cs-sales.net/bk875.html

That model isn't have I have but it's
close.

 

[Jamie]

How do you accurately test a capacitor in the circuit? Aside from
bulging packages, how do you know a capacitor has failed? Do they
usually just short and start passing DC?

Sure you can desolder it, test it out of the circuit, and resolder it
back in. But by time you've done all that you could have just skipped
testing the suspect and soldered in a known good one.

Thanks.

-ph

I posted a URL of what I use which works very well, I can tell you
a basic theory of how it works.
The in circuit uses the resistive load in the circuit to offset the
reading from the test cycle on the capacitor.
It's not perfect but it's very close for doing debug work.
what I have found is this. In many cases where a cap test
resulting in much higher value than should be, may indicate a
leaky cap. Of course, one that test much lower is most likely
a weak cap.

As far as doing repairs, I haven't yet pulled a cap from a board
using this LCR meter that showed a below normal reading to be false.

I have seen where at times, when it shows far to much Uf on a cap
in circuit that it may be a false reading. But this hasn't happen to
much.

Don't go for a cheap cap meter. Get one that is designed for
in circuit tests.