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Discussion in 'Electronic Repair' started by Arfa Daily, Aug 5, 2008.

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  1. Arfa Daily

    Arfa Daily Guest

    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. 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
     
  2. Franc Zabkar

    Franc Zabkar Guest

    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.electronics/msg/ed8b3fd1f1dd56d6
    http://groups.google.com/group/aus.electronics/msg/b82fd5492cacd442
    http://groups.google.com/group/aus.electronics/msg/6702c887350f4adb

    If your meter uses a different testing methodology, then the above
    observations may not be relevant.
    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
     
  3. Eeyore

    Eeyore Guest

    Blue tends to suggest polypropylene caps IME (good for pulses) which figures in
    such an application although you don't state where they are in the circuit.

    How about some more info on what they do ?

    Graham
     
  4. Eeyore

    Eeyore Guest

    Agreed.

    Graham
     
  5. Franc Zabkar

    Franc Zabkar Guest

    This datasheet doesn't have the above disclaimers:
    http://www.datasheetcatalog.org/datasheet/vishay/mkp3384.pdf

    However, page 6 has a voltage versus frequency graph which shows that
    the maximum RMS AC voltage of 300V starts dropping from about 300Hz.
    At just above 10kHz the max voltage rating is only 10V, at 1kHz it is
    about 100V.

    - Franc Zabkar
     
  6. Ross Herbert

    Ross Herbert Guest

    :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
    :eek: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
    :eek: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
    :eek: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.
     
  7. Ross Herbert

    Ross Herbert Guest

    :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.

    The X2 you are using is probably this one
    http://www.vishay.com/docs/28118/mkp3384.pdf

    Since X2 caps are generally physically larger for a given value than a standard
    MKP or other film type the space available appears to allow a component
    measuring at least 5 x 11 x 17.5. You may be able to find ahigher voltage
    standard pulse cap of similar size.

    Sticking with the Vishay (BC) brand, the 2222 378 64153 in this range would be
    ideal http://www.vishay.com/docs/28134/mkp378.pdf
     
  8. Arfa Daily

    Arfa Daily Guest


    Thanks all for your views and good input. Plenty of food for thought and
    further research. I have looked at Vishay's list of re-sellers and will have
    a look at their websites to see if I can find any that keep the 378 series
    in the value I need. The 'usual suspects' like RS and Farnell are there, but
    these caps don't seem to be in their catalogues here in the UK, although
    others - the X2s that I have tried for instance - are. I will have a look at
    Arrow etc when I have time.

    TA

    Arfa
     
  9. Arfa Daily

    Arfa Daily Guest



    OK. Here's some additional info (I did post a reply to what we've come up
    with so far, but it doesn't seem to have appeared - strange ...)

    Looking again at one of the original caps, the trade name is "PILKOR" which
    seems to be a Korean component manufacturing arm of Vishay. So far so good.
    The numbers "378" do appear, so this would suggest that it is indeed one of
    Vishay's 378 series MKP film caps. Now, Farnell keep some of the 378 range,
    but not this one. They do, however, keep that value in the "385" series. As
    far as I can see, the main difference here seems to be that the caps are
    rated at 2000v dc rather than 630v dc, and are physically taller, whilst
    still having the same footprint and lead pitch as the 378s. Taller would not
    be a problem. Higher voltage may be an advantage ...??

    Any comments or opinions on this range being more suited, appreciated.

    The caps are located on the primary side of a resonant mode smps. It is a
    sort of half bridge circuit with two FETs. The drain of the upper one is
    connected straight to the 400v rail, and the upper connection of the
    transformer primary, is returned to the same point via one of these caps.
    The upper transformer connection is also connected to the second capacitor,
    the remaining end of which is returned to deck via a very low value R, which
    is used to sense primary current. The lower FET has its drain connected to
    the upper one's source, and it is this 'mid point' which hooks to the lower
    connection of the primary winding. The source of the lower FET is returned
    to the same deck as the low value R detailed above. So I guess that the caps
    are for coupling and resonating the primary. Looks like they would be
    working hard ...

    Arfa
     
  10. Franc Zabkar

    Franc Zabkar Guest

    That explains a lot. For example, you measured an ESR of only 38 ohms
    instead of 80, which is what you would expect if the two caps were
    connected in parallel. In fact your circuit does effectively parallel
    the two caps via the low ESRs of the current sense resistor and 400V
    bulk capacitor.

    If one cap is OC, then you will see an ESR of 80 ohms, which is the
    ESR of the other good cap. If both are good, then you will measure 40
    ohms.

    - Franc Zabkar
     
  11. Arfa Daily

    Arfa Daily Guest

    Ha! Well spotted Franc ! Good answer, and obvious now I look at it again ...

    Arfa
     
  12. Metalized film has many drawbacks, you really have to read the fine
    print on the spec sheets. The metalization is soo thin it can't take
    much peak current.

    So instead of looking for exact replacements, which will just repeat
    the cycles of failure,
    I'd do the exact opposite-- look for something sturdier.


    The caps will be slightly larger but if you can squeeze them in they
    circuit might have a chance of lasting quite a bit longer.
     
  13. Eeyore

    Eeyore Guest

    I agree with your comment about Evox-Rifa.

    Epcos (formerly Siemens) also do several ranges of polypropylene caps with various
    pitches which may help. The data on them is also very extensive, indicating max
    pulse handling etc.

    Graham
     
  14. Eeyore

    Eeyore Guest

    X2s AIUI are not designed for *repetitive* pulse work. Polyprops are. You also
    have a choice of film/foil or film/foil/film construction (at least from Epcos)
    which affects peak and average current handling.

    Graham
     
  15. Eeyore

    Eeyore Guest

    That's why Epcos make film/foil/film ! Not just metallised film.

    Graham
     
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