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

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.

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
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.

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

Franc Zabkar wrote:

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.

Agreed.

Graham

On Wed, 06 Aug 2008 06:09:45 +1000, Franc Zabkar
put finger to keyboard and composed:

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

This datasheet doesn't have the above disclaimers:
http://www.datasheetcatalog.org/data...ay/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
--
Please remove one 'i' from my address when replying by email.

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.

On Wed, 06 Aug 2008 03:05:19 GMT, Ross Herbert wrote:
: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

"Ross Herbert" wrote in message
...
On Wed, 06 Aug 2008 03:05:19 GMT, Ross Herbert
wrote:
: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


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

"Ross Herbert" wrote in message
...
On Wed, 06 Aug 2008 03:05:19 GMT, Ross Herbert
wrote:
: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



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

On Wed, 6 Aug 2008 12:44:05 +0100, "Arfa Daily"
put finger to keyboard and composed:

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

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
--
Please remove one 'i' from my address when replying by email.

"Franc Zabkar" wrote in message
...
On Wed, 6 Aug 2008 12:44:05 +0100, "Arfa Daily"
put finger to keyboard and composed:

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

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
--
Please remove one 'i' from my address when replying by email.

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

Arfa

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.

Ross Herbert wrote:

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.


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.

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

Arfa Daily wrote:

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.

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

Ancient_Hacker wrote:

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.

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

Graham