[sci.electronics] Appropriate use of capacitors

jchan@rtech.UUCP (Jeff Chan) (08/04/87)

Here's a question for you old farts, audiophiles, and EEs:

What are the appropriate uses and benefits/disadvantages of all the
various types of capacitors?  For example, electrolytics are often
used in power supplies.  I suspect that polycarbonates are thermally
quite stable.

I am curious about ceramics, tantalums, electrolytics, metallized and
foil film capacitors with mylar=polyester, polystyrene, polypropylene,
polycarbonate, teflon, etc.  Please post to the net.

Jeff Chan
Summer intern at Relation Technology Inc.
(and audio hacker)

P.S. not to start any controversies, but I have heard audio differences
between electolytic and polypropylene dc blocking caps in disc players.
The film caps were pretty close, and the 'lytics did bad things to sound.
You can *easily* test this yourself.  Just put some big caps on a switch
in series through a tape loop and switch them.

bmaraldo@watdcsu.UUCP (08/05/87)

In article <1108@rtech.UUCP> jchan@rtech.UUCP (Jeff Chan) writes:
> I am curious about ceramics, tantalums, electrolytics, metallized and
> foil film capacitors with mylar=polyester, polystyrene, polypropylene,
> polycarbonate, teflon, etc.  Please post to the net.


   I am an audiophile who modifies and upgrades stereo components.  Here is
   my disertation on capacitor usage in related circuits.

   Electrolytics are available in large capacitance values, but the
   voltages generally are low.  They are also not very accurate (a typical
   accuracy might be +50% -10%).  But they are good for power circuits and
   general DC filtering.  Whenever possible, and when the circuit design
   permits, I prefer tantalums over electrolytics.  I have found that
   replacing electrolytics with tantalums in phono pre-amp sections both
   stablize the accuracy of the gain and equalization and reduces the
   level of high frequency noise that could otherwise tear at the high end.
   I also replace any mylar, polyester, ot polycarbonates with
   polypropylene which have much better accuracies and are more useful as
   bypass devices on power rails and bias currents.  Alas, I have never had
   the chance to try out a teflon cap as they are scarse around here.

   Replacing the electrolytics in passive crossovers with high quality
   polypropylene caps also cleans up the mid high to high end.


					Brett L Maraldo


-- 
               --------     Unit 36 Research     ---------
	                "Alien Technology Today"
               -------------------------------------------
	                    bmaraldo@watdcsu

max@zion.berkeley.edu (Max Hauser) (08/06/87)

In article <1108@rtech.UUCP> jchan@rtech.UUCP (Jeff Chan) writes:
>
>Here's a question for you old farts, audiophiles, and EEs:
>
>What are the appropriate uses and benefits/disadvantages of all the
>various types of capacitors?  For example, electrolytics are often
>used in power supplies.  I suspect that polycarbonates are thermally
>quite stable...

Back before I got involved with microelectronics, I had some design
jobs involving critical capacitors (all the capacitors I deal with now 
are flat, very small, and "custom"!). Rather than give opinions about
all the myriad dielectric types available in discrete capacitors, I'll
mention those with salient good features that stood out. The trade
mags like Electronic Design often show tables of this data in feature
articles, every year or two.

My comments are relevant to high-accuracy applications like signal paths
and instrumentation (I will leave power-supply filtering to someone 
-- evidently everyone -- else). Electrical properties important to these
applications include linearity, stability and low dielectric relaxation.
The last of these is especially important in pulsed applications like 
sample/hold circuits, and also the hardest to get hard facts on; you 
pretty much have to measure.  An example of the effect is for an 
abruptly discharged capacitor to exhibit some "memory" of its charging 
voltage after the discharge, due to residual polarization of the 
dielectric. In many dielectrics the magnitude can be 0.01% or more, 
which is bad news in some applications.  I regret that I don't have any 
DR figures handy.

1. Polycarbonate (Lexan (tm)) caps have some near-ideal properties,
especially their phenomenal temperature stability, which can be as
good as 0.1% over the military temperature range. They are also very
expensive (dollars apiece in quantity, instead of cents). While that
fact alone would no doubt commend them to some audiophiles ;-),
audio is not one of the applications that needs their principal
attribute of temperature stability. Instruments like precision 
oscillators exploit that property. Also, I believe TFE (Teflon (tm))
caps share similar properties with polycarbonate, but are even more
expensive, thus making them even more suitable for one-upmanship.
Only kidding, of course.

2. Polystyrene caps have temperature stability approaching that of
polycarbonate, but are manufactured to a coarser initial tolerance
and are MUCH less expensive (order of few cents in quantity 100).
They are useful for applications where stability may be important but
initial value is not (a trimmed function generator, for example). 
They are also clean in other respects. A drawback is physical size;
they are typically more bulbous than, say, Mylar (tm) caps of the same
value and voltage, though the Mylar units are much less stable.
Mylar caps are more common by far than polystyrene and denser, as I've
said, but no cheaper, and their second-order dielectric properties
as I recall are largely inferior to polystyrene.

3. Ceramic caps, especially the monolithic versions developed for
digital-IC decoupling service, are relatively dense in the large values
(0.1 uF and larger), and their high-frequency response is superior to
those with plastic dielectrics (polywhatever), which typically contain
a spooled sandwich structure with larger series inductance. The
ceramic caps also have poorly controlled value, usually rated GMV
(guaranteed-minimum) or -20, +80% and often, in my experience, show
enough leakage conductance to be a problem in high-impedance
instrumentation circuits, and their temperature stability is among
the worst. They are therefore most useful in wideband and power-supply
decoupling applications.

The foregoing dielectric types are about all you need for low-value
(1 uF and smaller) applications. A rub comes if you need something
larger, where these types all get bulky, in signal-path applications.

A large divide exists between electrolytic and non-electrolytic types,
so much so that it is unfortunate they are both called capacitors;
this causes lots of confusion among the inexperienced, who don't
realize the fundamental difference. Electrolytics work of course by
forming a (temporary) insulating layer between a metal surface and
an electrolytic gel or liquid, in response to an applied DC voltage.
The ONLY reason they are used is to gain the extremely high capacitance
density possible from the very thin dimension of the electrochemically-
formed insulating layer. They are inherently polar devices; must have
a steady DC bias (and are, simply from that, nonlinear); and show, in
most models, limited lifetime. Because of these three drawbacks they
must be used with great care and awareness, and they are very 
successful in certain applications, like the low-frequency filter
capacitors in DC power supplies.

*************  Anti-electrolytic flame follows  ****************

Unfortunately, designers have been sticking electrolytics in
everywhere they needed a large-value capacitor. The result?
Audio distortion, for one thing (as already noted here and in
rec.audio); but also, in-circuit failures where the electrolytics
experience no or inadequate DC bias (they need a "forming" voltage
equal to a fraction of the "working" rating) or simply from their
limited lifetime. 

I have been repairing audio equipment since about 1970 and I cannot
count the times I have seen "right channel is out, or nearly out,
and (maybe) comes back with loud music." (If the right channel is
completely out, it may well be due to an electrolytic interstage
coupling capacitor that failed; if the volume is way down, quite
possibly it's an emitter- or source- or opamp-resistor bypass 
capacitor, also electrolytic.) Often these capacitors fail because
they are rated for 25 volts and are operated with a two-volt DC bias.

Anyway, it's worth avoiding electrolytics of all kinds in analog
signal paths and relegating them to power supplies where they are
fat and happy facing DC and some 60-hertz ripple. It's just hard to
do this if you need 100 or 1000 microfarads in your signal path.
(One place you generally *don't* need capacitors, at least fundamentally,
is in series with the speaker at an audio output stage. With proper
design it's not hard for modern amp circuits to maintain stable 
zero-DC output voltages if the power supply is roughly symmetric.)

If you can figure out a way to pack, say, 100 uF at 50 volts into a
cubic centimeter or so, using cheap, dry, stable, voltage-independent
materials, the world will beat a path to your door. Unfortunately, with
readily manufactured insulators, physics is against you.

Max Hauser, UC Berkeley EECS Department, IC Design Group

   UUCP: ...{!decvax}!ucbvax!eros!max
   Internet (old style): max%eros@berkeley
   Internet (domain style): max@eros.berkeley.edu

(PS: for those who asked, I will post answers to the old-fart quiz soon.)

kunert@nicmad.UUCP (Dick Kunert) (08/07/87)

In article <1108@rtech.UUCP>, jchan@rtech.UUCP (Jeff Chan) writes:
> 
> Here's a question for you old farts, audiophiles, and EEs:
> 
> What are the appropriate uses and benefits/disadvantages of all the
> various types of capacitors?  For example, electrolytics are often
> used in power supplies.  I suspect that polycarbonates are thermally
> quite stable.
> 
> I am curious about ceramics, tantalums, electrolytics, metallized and
> foil film capacitors with mylar=polyester, polystyrene, polypropylene,
> polycarbonate, teflon, etc.  Please post to the net.

In February and March, 1980, "Audio" magazine published a two part
article on picking capacitors. See if your local library can get it
for you. 

Without going into too much detail, for audio applications you
want the lowest possible dielectric absorption and dissipation factor.
Polystyrene, teflon and polypropylene caps are among the best, polycarbonate
nearly as good, and polyester (mylar) caps not as good but still much
better than electrolytics or ceramics. The article went into a fair amount
of detail on why you should never, never put any kind of electrolytic or
ceramic cap in your signal path.

Basically, dielectric absorption is a "memory" effect- when the cap is
discharged it doesn't *stay* discharged, but builds up to some percentage
of its previous voltage. This causes a sort of low level "grundge" in the
signal. A high dissipation factor will cause phase and amplitude to vary
non-linearly with frequency.

Most mid-fi audio gear has aluminum electrolytics liberally sprinkled
throughout the signal path. Some of the more enlightened manufacturers
bypass the electrolytics with small film caps; this is sort of a
compromise. High end manufacturers often use polypropylenes , etc. but
this is directly reflected in the price (look up pricing for a 10 uF,
35 volt polypropylene cap sometime). The film caps are also quite LARGE.

> P.S. not to start any controversies, but I have heard audio differences
> between electolytic and polypropylene dc blocking caps in disc players.
> The film caps were pretty close, and the 'lytics did bad things to sound.
> You can *easily* test this yourself.  Just put some big caps on a switch
> in series through a tape loop and switch them.

Electrolytics do sound bad, but to be fair you really need to supply 
sufficient DC bias so the signal doesn't reverse-voltage the caps.
-- 
                      ihnp4-\                  "I'm looking for a lifestyle
Dick Kunert      seismo!uwvax!nicmad!kunert     that doesn't require my
                     decvax-/                   presence..." 

qwerty@drutx.ATT.COM (Brian Jones) (08/07/87)

>    Electrolytics are available in large capacitance values, but the
>    voltages generally are low.  They are also not very accurate (a typical
>    accuracy might be +50% -10%).  But they are good for power circuits and
>    general DC filtering.  Whenever possible, and when the circuit design
>    permits, I prefer tantalums over electrolytics.
		       ^^^^^^^^^ ^^^^ ^^^^^^^^^^^^^
> 
> 					Brett L Maraldo
> 	                    bmaraldo@watdcsu

Electrolytic capacitors have a dielectric formed by electrolytic action.

They provide the most farads per dollar, in the least volume of any type of
capacitor.  There main disadvantage is their inability to withstand reverse
voltages.  They are also poor performers at high frequency, high temperature,
and subject to internal heating in the presence of larger AC voltages.

Common types:
	Dry Aluminum
	Solid Tantalum
-- 

Brian Jones  aka  {ihnp4,allegra}!{drutx}!qwerty  @  AT&T-IS, Denver

wtm@neoucom.UUCP (Bill Mayhew) (08/08/87)

In the capacitor wars, I've got to agree that electolyic capacitors
are terrible.  Perhaps, it would be a good idea to start a Jihad
against them.  As mentioned, electrolytic units are appropriate for
brute force bulk regulation of power supplies.  In power supplies,
the continuous application of DC keeps the liquid dielctric
material formed and acting as an insulator.

One point to keep in mind is that capacitors integate current.
When a large brute force filter capacitor is present in a supply,
it will draw a very large current while it attempts to nearly
instantaneously reach operating voltage.  This property can result
in blown out fuses and/or rectifier diodes if percautions are not
used.  A number of methods are possible to slow down the initial
charging.  Often, a current limiting resistor is placed between the
rectifier and the filter capacitor.  A modest timing circiut can be
used to close a relay contact across the resistor.

One other thing is electrolyitic units is that there is finite
leakage current.  Sometimes, the leakage current is considerable.
I once saw a guy burn up a plate supply transformer in a tube
preamp by using a humungous filter cap bank.  It turned out that
the filter bank drew more leakage current than the preamp itself
drew to operate.

I try to avoid electrolytic capacitors in audio circuits.  I'm
amazed at how many times I've seen electrolytics used to directly
couple the audio path in Compact Disk players.  They'll almost
certainly fail after a few years because there isn't a DC bias
(usually) of sufficient magnitude to maintain the integrity of the
dielectric film.  Perhaps this is Japan getting even for WW II by
building in planned failure into their products?  Note that smaller
"working voltage" values are useful if you insist in using an
electrolytic cap in an audio circuit.  A 5 volt rated capacitor is
more likely to stay working than a 25 volt rated electrolyic
capacitor in a circuit that has a 3 volt DC bias.  It's difficult
to believe that with all the fancy electronics, optics and
mechanics in CD players that engineers can't concoct a stable
direct coupled audio output circuit.

For general non critical applications, I use stacked mylar (tm)
capacitors.  These are OK for most audio applications such as for
bypassing the Miller-effect feedback resistor in the emitter of a
transistor circuit.  Values are commonly available in the nF to 1
to 2 uF range.  Mylar has a quite modest dielectric absorption,
and is value stable at consumer temperature ranges.  A product
example is Panasonic V-series.  They are also pretty cost
effective.  On drawback is that tolerances are about +/- 5 to 20%.

For critical needs such as sample & hold capacitors and highly
stable audio oscillators, I use polystyrene capacitors.
Polystyrene is very temperature stable and has very low dielectric
absorption.  Mallory offers two lines of polystyrene units.  One
line has a leakage R of 100-1000 megohms.  The other series is up
over 1000 megohms.  The disadvantage of polystyrene is that it
results in physically large capacitors.  It is also modestly
expensive.  Values range from pF to nF typically.  Tolerances are
available as low as +/- 1%.

Polystyrene degrades in dielectric performance at RF frequencies.
For RF, mica capacitors are quite good. Silver/mica capacitors are
quite expensive relative to ceramic types.  Mica offers very good
temperature stabilty and quite good dielectric performance.  Mica
caps are moderate in size.  C vales are .5 to 1000 pF
approximately.  Tolerances are available to +/- 1% or better.  A
typical vendor for mica units is Cornell-Dublier Electronics (CDE).

Polycarbonate capacitors are better than polystyrene in performance
of the dielectric.  Polycarbonate units are also available from
several sources with very tight tolerence values.  Unfortunately,
polycarbs often cost several DOLLARS each (ouch!).  They're mostly
used for applications where high untrimmed accuracy and wide, wide
temperature range stability is needed, for example space based
radios.

Most electronics industrical suppliers such as Newark or Pioneer
should be able to burry you in a ton of capacitor data sheets
(well, almost) if you phone-up and request some.  Much of the
discussion present in magazines such as "Audio Amateur" seems more
opinion than fact-- especially in re capacitors.  I'll admit some
opinion that I'm a relatively demanding listener; I really can't
acknowledge any real noticable sonic differences in mylar,
polystyrene and similar type capacitors when used appropriately in
audio coupling circuits.  Electrolyic units do certainly add
noticable coloration to sound, but then electrolytic capacitors are
readily measurably inferior in performace on very inexpensive test
gear (such as a $14.95 pocket multimeter from Radio Shark).

I get a laugh when I see electrolytic capacitors used in coupling
applications in CD players.  In an apparent move of appeasement,
they often have a red plastic sleeve (as opposed to the more common
blue or orange) that is emblazoned "AUDIO GRADE".  I wonder just
who the manufacturers are trying to impress, since the same CD
players bear labels on the outside of the cabinet saying something
to the effect of "dont open-- danger of hideous government
punishment.." or some similar rot.  Seemingly, "AUDIO GRADE" ==
"inferior to industrial grade".

  --Bill