On Tue, 5 Aug 2008 17:51:39 +0100, "Arfa Daily" wrote:
: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
ver 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
nes 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
ut 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. 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
:
X2 is not recommended for the particular application.
I have always tried to use Evox-Rifa (in the 60's they were simply Rifa) because
they were very reliable. Of course, with all the mergers/takeovers since those
days I don't know how their reliability has changed, if at all.
Have a look at the range of PHE426 pulse caps here
http://www.evoxrifa.com/n_america/specs_pulsecaps.htm
This series does not have 15nF/630V in 15mm lead spacing but if you want more
than a few you may be able to order the 10mm spacing with lead length of 30mm
instead of the standard 6mm.