HamCall (October 1991)/HamCall (Whitehall Publishing)(1991).bin/amateur/hamnew/caps.txt

Text File | 1991-10-03 | 15KB | 253 lines

AN INTRODUCTION TO CAPACITORS

by David Johnson, Los Banos, CA.

(This article originally appeared in the Canadian LF/VLF newsletter
_Northern Observer_ [issue #23, mid-December 1990].  It is posted here
by permission of the author.)

Inexperienced techs, and engineers fresh out of school, frequently
assume that once you know the value of a capacitor you know almost
everything there is to know about it that's important.  After all,
this rule of thumb works with resistors most of the time.  But there
are very important differences between the various types of
capacitors-- which is the reason that so many different types are
marketed.  I will discuss the important properties of the more common
types of capacitors that you are likely to encounter.  This discussion
will not cover specialty capacitors.

ALUMINUM ELECTROLYTICS pack a lot of capacitance into a relatively
small volume, and are cheap.  They are used primarily for power-supply
filtering, low-frequency AC coupling, and DC blocking.  In general
they are not suitable for other applications because they have very
sloppy tolerances, exhibit marked temperature drift, have low Q,
require DC biasing of known polarity (except special nonpolarized
units), and have high leakage currents, especially if the capacitor
has not been used for a long period of time.  If biased with the wrong
polarity of DC voltage, they usually explode within several seconds or
minutes, creating a substantial hazard.  Aluminum electrolytic
capacitors gradually deteriorate with age, though this problem is not
a serious one with modern units.  Never use old or surplus
electrolytics-- buy new; they're cheap.  Aluminum electrolytics give
best life and performance when used at close to their rated voltage.
Don't use a 100 volt unit in a 10 volt circuit.  A 16 volt unit would
be much better.  Aluminum electrolytics are widely available in
ratings from 1 to 10,000 microfarads (uF), with voltage ratings up to
several hundred volts in the low to medium capacitance ranges.

SOLID TANTALUMS.  These are solid semiconductor electrolytics with
properties that are generally better than aluminum units.  They are
widely used for power-supply decoupling and low-frequency AC coupling.
They are much more stable with time and temperature than are aluminum
electrolytics, and have higher Q as well.  It's OK to use them at
voltages much lower than their rating, but they _are_ polarized and
should never be allowed to become reverse-biased.  A reverse-biased
solid tantalum may work for seconds to several days, but it _will_
fail, usually with a horrible stench and possibly fire.  Solid
tantalums are sometimes used in non-critical timing and frequency
filtering circuits.  Solid tantalum capacitors are widely available in
ratings from 0.47 to 47 microfarads at up to 35 volts in the lower
capacitance values.

POLYESTER (Mylar (tm)) FILM CAPS.  These popular capacitors are widely
available in ratings from .001 to 10 uF, at voltages up to 630 volts
in the medium and low capacitance values.  They receive use in a wide
variety of applications, particularly in audio-frequency circuits.

Their temperature curve is fairly flat at room temperature, providing
good stability when used in equipment that will not be used outdoors.
Aging is generally less than 1% over the life of the product.  Q
factor is several hundred at audio frequencies, but plummets at higher
frequencies, making them generally unsuited for RF work.  Dielectric
absorption and DC leakage preclude their use in high-performance
sample-and-hold circuits.

POLYCARBONATE FILM CAPS are rather similar to polyester caps, but most
of their electrical properties are better.  The temperature curve is
relatively flat over a much wider range and is generally within the
range of +25 to -50 ppm per degree C at room temperature.  Q is about
1000 in the audio range, dropping to about 100 at 1 MHz.  Dielectric
absorption and leakage are low enough to allow use in medium-
performance sample-and-hold circuits.  Polycarbs are bulkier than
polyesters for a given capacitance and voltage rating, and they cost
more.  They are available in tighter tolerances (1%) than polyesters,
and they generally age less than 0.2% over the life of the product.
Polycarbs are manufactured in the same capacitances and voltage ranges
as polyesters, but are not as widely available as polyester since they
are more costly.  Probably the most common uses for polycarbs are
high-Q audio filters, tight-tolerance RC timers, and precision
instrumentation circuits.

POLYPROPYLENE FILM CAPS.  Somewhat similar to polycarb, but reserved
for even ore critical applications.  Aging is generally less than 0.1%
over the life of the product, dielectric absorption and DC leakage
specs are even better, and Q is several thousand in the audio-
frequency range.  The temperature coefficient is fairly steep-- about
-220 parts per million per deg. C.-- but the curve is flat, making it
possible to use other components to compensate for temperature drift
over a wide range if that is necessary.  Polypropylene film caps are
used to some extent in lower frequency RF work, where positive tempco
inductors can be used to balance the tempco of the capacitor to
produce a very stable tuned circuit.  Polypropylene caps are generally
available in the range of 470 pF to 1 uF, up to 630 volts, are harder
to find than polyesters and polycarbs, and are more expensive and
bulkier than polycarbs.  Digi-Key sells 2% tolerance units and most
manufacturers have 1% tolerance units available.  Most usage of
polypropylene caps is in high-performance tuned circuits, timers, and
sample-and-hold circuits; however, they are starting to see use in
switching power-supplies because their high Q allows them to handle
large amounts of power without overheating or reducing efficiency.

POLYSTYRENE FILM CAPS.  Extremely low dielectric absorption and DC
leakage make this a popular capacitor for high-performance S/H
circuits and integrating A/D converters.  Its superb stability (ageing
typically less than .05% over life) and predictably low temperature
coefficient (-150 to -190 ppm per deg. C) over a very wide temperature
range make this a common capacitor in high-performance
instrumentation.  Its extremely high Q (over 10000 in the audio range,
and several hundred at 1 MHz) make it popular for lower-frequency RF
tuned circuits.  Readily available in values from 100 pF to .01 uF--
Higher values are difficult to find, and they are very bulky.
Polystyrene caps are very easily damaged by heat from soldering, and

by solvents.  Because of this, polystyrene capacitors are losing
popularity (primarily to polypropylenes and to NPO ceramic
capacitors).

EXOTIC PLASTIC-FILM CAPS include Teflon (tm), polysulfone, parylene
and other dielectrics.  These are generally available only on special
order from the factory, and they aren't used except when nothing else
will do.  The exotics generally exhibit some combination of high
stability and/or operation at high temperatures.

CERAMIC CAPACITORS.  These are even ore diverse a group than the
plastic film capacitors, and are even less interchangeable.  When you
buy surplus or used ceramic caps, you usually don't know what kind you
are getting.  If you don't know what kind they are, don't buy 'em.

NPO (COG) CERAMICS.  These are very stable with time and temperature,
and have high Q at RF.  Temperature coefficient is les than +-50
ppm/deg.C and ageing is extremely low.  Widely available in values
form 5 pF to 1000 pF (and less widely available 1 to 10000 pF) in two
popular styles:  disc and multilayer.  The multilayers are more
compact in the larger capacitance values.  A tubular style used to be
popular but is rarely seen anymore.  When handling and soldering
ceramic capacitors of any type, use care--  the leads on some
manufacturers' units are poorly bonded to the capacitor plates, and
detach easily.  On some, the attachment solder melts at a temperature
lower than that of ordinary solder, so if in doubt, use pliers as a
heat-sink on the leads.  If the wire _does_ detach, the protective
coating will often hold it in place, leaving an open or intermittent
capacitor that looks physically perfect.  NPO capacitors are widely
used in RF circuitry.  They are gradually displacing micas in most
applications because they have higher Q and better stability than most
micas.  In addition, the multilayer NPO's are much smaller than micas.

X7R CERAMICS.  These are designed for general-purpose use where
stability is not critical.  A rather wobbly temperature curve keeps
capacitance variation due to temperature change within +-5% of room
temperature value from -40 to +90C.  Capacitance can decrease up to
several percent with aging.  Initial tolerance is generally 10 or 20%-
- tighter tolerances are not available because aging effects would
make tighter tolerances meaningless.  Three styles are popular-- disc,
multilayer, and military CK05 box style.  Popular values range from
 .001 to 0.1 uF; lower and higher values are available.  Polyester film
caps are gradually displacing X7R ceramics in many applications;
however, where space is critical, multilayer X7R's can still beat
polyester film caps for size.

Z5U CERAMICS.  These are used almost exclusively for power-supply
bypassing.  Their capacitance changes markedly with temperature, and
there is an appreciable capacitance loss with aging.  Both disc and
multilayer styles are popular, and values from .01 to 2 uF are readily
available.  The multilayer type can be smaller than an aluminum
electrolytic of the same capacitance.  Z5U's have lower impedance at
high frequencies than do electrolytics, and do not require a DC bias
voltage since they aren't polarized.  Because of these reasons, they
are more popular than aluminum electrolytics in values below 1 uF.


TEMPERATURE-COMPENSATING CERAMICS are generally similar to NPO
ceramics, but are designed to have a specified negative temperature
coefficient.  They are hard to find, and are used primarily for
temperature compensation of other components such as inductors or
quartz crystals at RF.

REDUCED TITANATE (BARRIER TYPE) CERAMICS.  Not used much anymore.
This group comprises a wide variety of temperature coefficients and
capacitance values.  They require DC polarization, have low voltage
ratings (as low as 3 volts), have extremely high leakage.  They met a
need for small capacitors in the early days of the transistor, before
modern multilayer ceramics were developed.

OTHER CERAMICS.  There are other ceramic types available, the most
popular of which are probably formulations designed to get more
capacitance per unit volume for power-supply bypassing applications.
These formulations are generally less stable than Z5U and should be
used only for power-supply bypassing.  Values up to about 10 uF are
available.

MICA CAPACITORS have a low temperature coefficient, good stability,
and high Q at radio frequencies.  However, the performance of NPO
ceramics is in general slightly superior, and the NPO's are usually
cheaper and smaller as well.  Mica capacitors are readily available in
values from about 3 pF to several thousand pF.  They are still a bit
easier to obtain in tight tolerances (to 1%) and high voltage ratings
than are NPO's.

PAPER CAPACITORS used to be _the_ general-purpose capacitor, but have
been almost entirely superseded by polyester and ceramic capacitors.
They are still used to some extend in applications involving AC line
power, for instance, motor-starting capacitors.  The paper is normally
impregnated with an oil or some other material that is the actual
dielectric.

FILM VARIABLE CAPACITORS.  These are the little plastic variable
capacitors in modern portable radios.  The dielectric is a plastic
film-- polyethylene, I think, but possibly something else.  The
temperature coefficient is probably about -200 ppm/deg C, which
compensates a tuning inductor having a tempco of +200 ppm.  Installing
a proper knob on one of these (or even installing it, period) can take
some ingenuity or access to rare hardware.  Sometimes good hardware
can be cannibalized from a junked receiver.

AIR VARIABLE CAPACITORS.  More stable than the film variables, and
available in a much wider range of voltages, capacitances and styles.
Unfortunately, there are few manufacturers still in business, and
prices of new units are quite high.  Experimenters usually obtain air
variable caps through surplus houses or by cannibalizing old
equipment.  The high voltages encountered in most transmitters
preclude the use of film variable caps.  Many air variable caps come
with a 1/4 inch diameter shaft, making it easy to install a knob or
other hardware.  Maximum capacitance available is about .002 uF.


VACUUM VARIABLE CAPACITORS withstand very high voltages, and are used
in the power and antenna circuits of radio transmitters.  They are
very expensive, generally over $200 if purchased new.  Experimenters
usually obtain them from surplus houses or by cannibalizing old
equipment.

TRIMMER CAPACITORS.  These are designed for set-and-forget, not to be
frequently adjusted like the other variable capacitors discussed.
Popular types are rotary )air, film or ceramic dielectric), and mica
compression types.  Air types are most stable, and generally most
expensive.  Film and ceramic types often have rather high temperature
coefficients.  Mica compression type has low temp. coefficient, but is
prone to come out of adjustment by itself.  The mica compression type
is available in maximum values up to 2000 pF, ceramic and film types
up to about 150 pF, and air type to about 60 pF.

DOUBLE LAYER "MEMORY BACKUP" CAPACITORS.  These devices have
electrical properties intermediate between an electrolytic capacitor
and a rechargeable battery.  Nominal capacitance values range from
about .02 to several Farads.  They must be used at about their rated
voltages (which is usually 5.5 volts).  They are used to provide power
backup to CMOS memory for up to several hours in the event of a power
outage.  I am not aware of any other use for these "capacitors."

--

posted with author's permission by   Frank Reid  W9MKV
                                     [e-mail]
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