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4hv.org :: Forums :: High Voltage
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Why polystyrene capacitors?

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woodchuck
Tue May 12 2015, 05:29PM Print
woodchuck Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
Just about all film capacitors from China that have a voltage rating in excess of 10kv use a polystyrene dielectric. Why? I see that polystyrene has very good dielectric strength, but not higher than teflon or polypropylene. So I start to suspect it has something to do with mechanical/manufacturing properties. Is it less expensive to make high quality PS film vs PP? I'd appreciate any enlightenment on this subject. :)
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Sulaiman
Tue May 12 2015, 07:31PM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I have some large polystyrene capacitors, and from looking at them,
I suspect that manufacturing cost is very low as the seal is just melting the ends!

Decades ago, polystyrene was the ultimate dielectric
close tolerance (5% or 2.5% common), low loss at r.f., temperature stable (at least predictable)
so there was a lot of manufacturing research, but, from faded memory,
polystyrene film was expensive and PET/Mylar became the first choice.
(and melts easily)
..... maybe high quality polystyrene film is now cheap in China?
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woodchuck
Tue May 12 2015, 08:13PM
woodchuck Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
Thank you for that insight Sulaiman. It truly could be as simple as the self-sealing ends... Vacuum potting each end of a HV cap has got to be slow and expensive. Meanwhile, HV PS caps stand out by their thick, clear, almost optically perfect ends.
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Ash Small
Tue May 12 2015, 08:22PM
Ash Small Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
There was, apperently, a company in Germany that manufactured capacitor grade polystyrene, but it shut down a number of years ago. Maybe the Chinese bought up the equipment, or something?....I know the Chinese do that sort of thing, I once had to re-furbish a carbon fibre oven, etc. that they'd found in a scrapyard in Israel that was originally built for a factory in Birmingham. They then copied it and built more plants in China.

I thought the only real advantage of PP or whatever over PS was the increased working temperature?
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woodchuck
Wed May 13 2015, 02:06AM
woodchuck Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
I understand that PP has about the lowest dielectric absorption and dissipation factor of any capacitor film. Electrons find it a little more 'slippery'. This recommends it for high power high frequency applications.
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Sulaiman
Wed May 13 2015, 05:49AM
Sulaiman Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
From memory;
.the company 'Suflex' was synonymous with polystyrene when I was (much) younger
.the low dielectric adsorbtion made 'suflex' the best choice for sample-and-hold capacitors
.the (usually) negative temperature coefficient closely matches the positive coefficient of air core inductors,
giving reasonably low temperature drift of frequency in resonant circuits.


P.S. just remembered;
the band usually indicates the end that is connected to the outermost layer of foil
so would normally be at the 'earthy' end of a circuit to minimise capacitive coupling to other components.
... self-shielding components !
.. polycarbonate capacitors for near zero temperature drift ... needed to justify their use due to relative cost.

P.P.S. at work we still get PCBs with polystyrene capacitors on them,
I have seen fractured leads but not yet come across a faulty one.
They melt if a hot soldering iron gets too near,
and some of my ex-tv service colleagues will not wash PCBs with 'suflex' capacitors
(a bad batch or season by manufacturer ?)
I do and so far no problems.
Some solvents make polystyrene capacitors look dusty/grey or even melted... new ones fitted.
For hobby use I like the characteristics of polystyrene capacitors and use them occasionaly
but for a given capacitance and voltage they are so BIG
and the termination of film/foil polypropylene capacitors
can carry many times more current than the puny terminations of PS caps.

I too was fascinated by the apparent homogeneity of the sealing of larger polystyrene capacitors ... component porn.
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Mads Barnkob
Wed May 13 2015, 10:28AM
Mads Barnkob Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
http://en.wikipedia.org/wiki/Film_capacitor wrote ...
Polystyrene (PS) film capacitors[edit]
Polystyrene film capacitors, sometimes known as "Styroflex Capacitors", were well known for many years as inexpensive film capacitors for general purpose applications, in which high capacitance stability, low dissipation factor and low leakage currents were needed. But because the film thickness could be not made thinner than 10 µm, and the maximum temperature ratings reached only 85 °C, the PS film capacitors have mostly been replaced by polyester film capacitors as of 2012. However, some manufacturers may still offer PS film capacitors in their production program, backed by large amounts of polystyrene film stocked in their warehouse.
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woodchuck
Wed May 13 2015, 01:59PM
woodchuck Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
Makes you wonder whether the current spate of HV caps from China is just a flash in the pan or something one can depend on? Either way, I've been taking full advantage of the situation. :)
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GrantX
Thu May 14 2015, 02:51PM
GrantX Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
I've used two of those polystyrene capacitors from a Chinese ebay store over the past few years. One of them was rated at 1 nF and 80 kVDC, it was built into a 'Terry filter' on the output of an 8.2 kV transformer and has survived quite a bit of spark gap Tesla coil action. The other was 20 nF 50 kVDC, and only made it about 5 minutes before shorting out internally while connected across the output of a 14 kV transformer.

They aren't very expensive and seem to perform nicely if kept far below their voltage rating. The 50 kV cap was emitting a distinctive hiss at 14 kV, so perhaps the film was slightly damaged during shipping and the resulting corona discharge ate a hole through it. The 80 kV cap has witnessed some rather large peak currents and voltages during its service and has showed no signs of degradation.
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woodchuck
Fri May 15 2015, 08:19PM
woodchuck Registered Member #39190 Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
GrantX: Thanks for the report on these caps. I'm particularly gratified to hear that the 1 nF 80 kV cap worked so well. Going through a few calculations with the second cap, it looks like a 20 nF capacitance has an impedance of about 160 kOhm @ 50 Hz. Straight across the output of a 14 kV transformer, that would be about 1230 VA! IOW, it may not have been dielectric breakdown that was the problem as much as it was overheating.
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