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Old 20th September 2006, 12:42 AM   #1
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Default Are Polystyrene caps suitable for digital circuit decoupling/bypassing?

I have long used polystyrene caps in audio (analog) circuits , with good results. I was wondering if there might be problem using them for decoupling in digital circuits to stunt noise. I read somewhere that their construction makes them inductive , which concerns me in this application. Any cap gurus out there? Thanks
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Old 20th September 2006, 01:06 AM   #2
lineup is offline lineup  Sweden
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make a search for
polystyren
in DIGITAL Forum

I am sure this has been discussed in several topics
by those experts that build DACS, Clocks or other digital circuits

you should of course also search for
polypropylene
and
tantal
I know i have read detailed topics on this before!
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Old 20th September 2006, 02:33 AM   #3
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I tried the search thing , but the discussion about these caps is never clear as to wether or not they are suitable.
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Old 20th September 2006, 06:51 AM   #4
jcx is offline jcx  United States
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The best info is probably on Cap manufacturer's sites

many polystyrene are "rolled foil" construction with leads attached to just one end of long strips, the clear ended polystyrenes

the rolled foil type is not good for digital/ high frequency applications due to the resistance and inductanace of the long foil strips

"extended foil" construction has each foil extending beyond the insulation, one foil on each end of the roll, then the one edge of each foil strip can be shorted along the whole length of the strip making a much better but expensive cap for high frequency applications

most caps for digital applications are "stacked" construction, flat versions of the extended foil concept - look for "stacked metal(lized) film" or "smf"

ploystyrene melts at too low a temp for wide use, mylar and pps are available as smt smf caps that can survive soldering to the board without long leads
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Old 20th September 2006, 11:49 AM   #5
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JCX
Thank-you for that very good explanation. I just didn't want to shoot myself in the foot by using the incorrect component.
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Old 23rd September 2006, 08:44 AM   #6
gerhard is offline gerhard  Germany
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Quote:
Originally posted by homemade
I tried the search thing , but the discussion about these caps is never clear as to wether or not they are suitable.
There is a long discussion about decoupling somewhere, that has a lot of substance.
Google for "bypass stuff about, anyway"

Gerhard
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Old 23rd September 2006, 12:05 PM   #7
max29 is offline max29  Netherlands
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have worked a lot with polystyrols but pay attention that most are unrealiable. Only 2 or 3 fabrics on the market which you can solder in. Many are made in a very disorderly, chaotic and rough way.To use them in an audioapplication first cut one open and look closely how the foil and leads are made.
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Old 23rd September 2006, 01:16 PM   #8
gerhard is offline gerhard  Germany
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Quote:
Originally posted by max29
have worked a lot with polystyrols but pay attention that most are unrealiable. Only 2 or 3 fabrics on the market which you can solder in. Many are made in a very disorderly, chaotic and rough way.To use them in an audioapplication first cut one open and look closely how the foil and leads are made.
Polystyrol deteriorates above 60C and melts above 90C, so PS
capacitors are hard to solder no matter how they are built.
There are no PS SMDs, and with the new wave of leadfree products
the solder temperatures have risen again.

BASF was the only maker of polystyrol foil for capacitors and they have
given up production many years ago. So anything you can buy now
is really old stock.

Gerhard
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Old 29th September 2006, 03:37 PM   #9
max29 is offline max29  Netherlands
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Have some from Sell , Trw and Itt who i really like and are stable.
But when temperature is rising they are gone as well as a silver mica. You see them becoming darker and darker in secondairie (power)supply pcb even in an old little tuner.

Really a pity because is a very audiominded material.Never understood why the leads are thin as a hair. But when i opened one saw how fragile they are. Think the lead cannot handle higher temperature and current to start with so tells it all.

If i came near a 50v cap no polystyrols.
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