On Tue, 5 Aug 2008 17:51:39 +0100, "Arfa Daily"
put finger to keyboard and composed:
A particular board that I'm currently involved in repairing, makes use of a
couple of 15nf 630v dc caps in the primary side of a smps transformer. They
go open or low value, and are easily found initially by running an ESR meter
over them. Good ones go about 38 ohms, bad ones over 80 ohms or beyond the
range of the meter. Once you've got them out, a value check will confirm
that they are faulty. The originals are those 'boxy' blue metalized film
ones with 15mm pitch on the leads. Some are fitted with the shiny brown
epoxy dipped metalized film types, which interestingly, don't seem to fail.
My problem is that I am having difficulty finding any suitable replacements.
I can find blue box metalised film caps with a 630v dc rating, but 15nf
seems to be a rare value. Also, they are smaller than the originals, and
have only a 10mm lead pitch, and legs that are too short to be able to open
out to 15mm pitch. I can't find any 'dipped' ones at all in that value /
voltage rating.
What I have found is an X2 rated metalized film cap in that value which
quotes an ac rating of 300v, and a dc rating of 630v. Its from BC, and it's
their 2222 338-4 Series. It is the right physical size, and seems to work ok
in the application. Thing is, when you run an ESR meter over these caps,
they go about 80 or 90 ohms. Now I know that you can't trust the reading of
an ESR meter that's designed to evaluate large value electrolytics, on a
small value film capacitor, and the fact that I was using it at all, was
just a matter of convenience in that it quickly gave a useful indication of
the state of the caps, without having to dig them out of the board.
Some time ago I investigated the accuracy of Bob Parker's ESR meter
for low valued caps, including X2 film types. Perhaps the following
posts may provoke some thoughts:
http://groups.google.com/group/aus.e...8b3fd1f1dd56d6
http://groups.google.com/group/aus.e...2fd5492cacd442
http://groups.google.com/group/aus.e...02c887350f4adb
If your meter uses a different testing methodology, then the above
observations may not be relevant.
So, am I
seeing this much different figure on the ESR meter, because of the way the
meter works, and the fact that these caps are designed to be low inductance,
whereas the originals are just bog standardly constructed ? Can anyone see
any problem in using an X2 capacitor in a pulse application like this, given
that they supposedly have a superior pulse characteristic, and claim to be
low loss in pulse applications ?
Opinions for discussion, appreciated.
Arfa
I'd say don't risk it. According to this datasheet, these caps "are
not intended for continuous pulse applications", nor "for series
impedance applications":
http://www.vishay.com/docs/28119/mkp3382.pdf
APPLICATION NOTES
• For X2 electromagnetic interference suppression in across the line
applications (50/60 Hz) with a maximum mains voltage of 275 V (AC).
• These capacitors are not intended for continuous pulse applications.
For these situations, capacitors of the AC and pulse programs must be
used.
• These capacitors are not intended for series impedance application.
For these situations in case safety approvals are requested, please
refer to our special capacitors of 1772 series with internal series
connection.
- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.