|
|
|
| |
Figure 1: A Simple if not very useful capacitor. |
|
ALL ABOUT CAPACITORS!
Any two items not directly connected together exhibit capacitance between
them, i.e., together they form a capacitor. The value of the capacitor
is proportional to the mutual area of the objects and inversely proportioned
to the distance between them. That is, the larger the object, the more
capacitance and the closer the objects are together the more the capacitance.
The capacitance value is also dependent on the permittivity of the
dielectric material (the material separating the two objects). Air
has a permittivity close to 1 (actually 1.0006), other common dielectric
materials are higher.
Simply mounting two items close to each other is not a very effective
way to make a capacitor and practical components are usually constructed
either as a laminate or a roll.
|
Figure 2: Laminated Construction |
Figure 3: Roll or Wound Type. |
|
Laminates
A laminated capacitor is comprised of two or more conductive plates
(electrodes) separated from each other by insulation (the chosen
dielectric), has alternate plates connected together and terminated
to form the two component leads as shown in Figure 2. Usually flat
rectangle form factor.
Rolls
A roll, or wound capacitor, consists of two long conductive electrodes
separated by two sets of dielectric material which are all rolled up.
A connection made to each of the electrodes is brought out to form
the terminals.
In either case, the dielectric is both longer and wider than the electrode
material to provide external insulation for the component. In both
cases the object is to achieve the largest area of the electrode, as
close as possible to its counterpart and in the least possible volume. |
Figure 4: Measuring a Charge |
|
HOW CAPACITANCE IS DEFINED
For any capacitor, there are two major parameters and five
or six minor ones. The major characteristics are the Capacitance and
the Voltage.
Capacitance is measured in Farads (after Michael Faraday) and the basic
definition of a Farad is a capacitor of a size such that one Ampere
flowing into it for one Second will change the terminal voltage by
one Volt.
This might be equivalent to a flow of one gallon per second flowing
for one second into the diaphragm unit of Figure 4 (a total flow of
1 gallon) and stretching the diaphragm to produce a change in back
pressure of one pound per square inch (psi). Energy is now stored in
the stretched diaphragm material and also in the stressed dielectric
of the capacitor. |
Figure 5: Over stressing a capacitor will puncture
the dielectric or breakdown the margin insulation. |
|
Practical Capacitance Units
The Farad turns out to be an impractically large unit and
capacitance is usually expressed in micro-Farads (uF or 1/1,000,000
par of a
Farad), nano-Farads (nF, 1 billionth of a Farad) or pico-Farads (pF,
1 millionth millionth of a Farad). Two 2" long pieces of insulated
hook up wire twisted together make a capacitor of approximately 5
pF.
The voltage rating of a capacitor is determined in its construction
by the thickness of the dielectric used. Excessive voltage applied
to any capacitor will puncture the dielectric, usually causing catastrophic
failure of the component. In the same way, excessive pressure applied
to the diaphragm unit shown in Figure 5 will cause it to rupture, with
similar results. Although some capacitors will “heal” themselves
following a low energy overvoltage breakdown – reconstituted
mica capacitors will not. Voltage breakdown can occur in the field
of the winding (puncturing the dielectric) or at the margin by tracking
around the dielectric layers. |
Figure 6: When the switch (valve) is first turned
on, the generator (pump) drives electric current (water)
through the resistor (restrictor) into the capacitor (diaphragm
unit). The voltage (pressure) across the capacitor (diaphragm)
increases until it equals that of the generator (pump) at
which time no further current (water) flows. Clearly the
pressure across the diaphragm has stressed that component
and the dielectric of the capacitor is similarly stressed.
In both cases, energy is stored and the storage medium is
the dielectric or diaphragm. |
|
A USEFUL ANALOGY
A capacitor can be conveniently and accurately modeled in the following
way.
|
Figure 7: Equivalent Circuit of a Capacitor |
|
MINOR PARAMETERS
The minor parameters are all bad news and are fairly fixed for any
given design. Also known as parasitics because of their undesirable
but tenacious presence, they include dissipation (D), equivalent
series resistance (ESR), equivalent series inductance (ESL), insulation
resistance (IR) and dielectric absorbtion. The actual circuit for
a capacitor and the water model therefore becomes:
ESR. The ESR is almost exclusively due to the fact that capacitors
are not made using superconductors. That is to say the terminal leads
and the electrodes possess electrical resistance through which the
current must flow to charge or discharge the capacitor. This accounts
for much of the ESR. In the case of the diaphragm unit, ESR is modeled
by the friction of the water flow against the walls of the inlet pipes. |
Figure 8: Equivalent circuit |
|
ESL is a little more subtle and can be most easily described
for the water model. For any length of pipe filled with water, it is
not possible to instantly stop or start flow. This is because the water
in the pipe has a mass which must be accelerated to change the flow
rate. Without an infinite pressure being available this takes time.
In the diaphragm unit the water in the inlet pipes must be started
and stopped each time the pressure is changed and the inertia of the
water has to be overcome. This is an exact analogy to electrical inductance
which is present in every piece of conduction material used in every
circuit.
IR. If the diaphragm of the diaphragm unit was absolutely water proof
no water could flow through the unit. Likewise in the capacitor, if
the dielectric was a perfect insulator then no current whatsoever would
flow through it and insulation resistance would be infinite. In practice
this ideal state of affairs is never attained and a small leakage current
(converted to insulation resistance using Ohms Law) always exists.
Leakage Current. This is indicated schematically by a parallel resistance
(restrictor) of a very high value (very low flow rate) which is also
the insulation resistance. |
Dissipation and Dielectric Absorbtion. These are not shown schematically
for either the capacitor or the diaphragm unit in Figure 7 but are
explained as follows.
Any elastic medium, such as a spring or a rubber band can be deformed.
To cause the deformation it is required that some work be done, e.g.
stretching the rubber band. When the band is released most of the work
put into stretching it is returned but some is lost in the mechanics
of the deformation and subsequent reformation. That is, some of the
energy is dissipated in the elastic medium itself and causes heating
of that medium. This is certainly so for the diaphragm and also fot
the capacitor dielectric. Different materials have different internal
frictions (some rubber balls are bouncier than others) and the same
is true of dielectric materials.
Dielectric absorbtion is included for both interest and safely reasons.
If a charged capacitor is rapidly discharged (by briefly touching the
terminals together for example) a fraction of the initial voltage will
reappear at the terminals within a few minutes as the dielectric stresses
relax. This is true for all capacitor types but is primarily of importance
for high voltage parts since “shocking” voltages can accumulate
from this phenomenon.
Beyond this, there are several other parameters common to all capacitors.
The most important of these are the temperature coefficient (change
of capacitance with temperature), the voltage coefficient (change of
capacitance with applied voltage) and age dependence or percentage
change of capacitance per decade of time, that is after 1 week, 10
weeks, 100 weeks, and so on. |
Figure 9: Filtering application showing input and
output. |
|
HOW ARE CAPACITORS USED?
Capacitors find their way into practically every electronic circuit
that exists or that can be designed. Specific circuit functions cover
an enormous variety of uses dependent upon the ingenuity of each
designer, but the following general classes cover the majority of
these.
1. Decoupling, also referring to as smoothing, filtering, resovoir
and energy storage applications. Typically seen at rectified AC inputs,
DC supply outputs and situations requiring occasional large current
pulses.
The operation should be clear from the figures – fluctuations
in voltage (pressure) at the input produce small current (water) flows
through the resistor (restrictor) which are largely absorbed by the
capacitor (diaphragm) which in turn exhibits much reduced voltage changes
when compared to the input. In this way the undesirable fluctuations
at the input, usually refred to as noise, are decoupled or filtered
from the output resulting in the smoothing of the waveform by the reservoir
capacitor (diaphragm unit) |
Figure 10: Capacitor charging from source at a
low rate via a resistor |
Figure 11: Discharge at high rate into much smaller
resistance |
|
Special Case – Rapid Discharge
Clearly, in the long term, the current which can be drawn
from the capacitor in Figure 10 will be limited by the resistor connecting
it to the source. In the short term, however huge currents can be
drawn until the energy stored in the capacitor is exhausted. This
the general principle of rapid discharge energy storage circuits.
For example, in a typical EBW or EFI detonator firing circuit, a capacitor
is charged at the rate of a few milli-Amperes over about 1/10 second
and is the discharged with a peak current of 6,000 Amperes in 50 billionths
of a second to initiate the explosion. |
Figure 12: Application in which the 2 volt AC signal
is COUPLED to the output while the DC voltage is BLOCKED
by the capacitor. |
|
2. Coupling Applications. Also referred to as DC blocking, this application
is really the reverse of the previous one. Situations frequently occur
in electronics where the desired information is a small AC waveform
which exists in a circuit where a large DC level is also present. The
AC signal may be a radio signal, an audio signal or the output of some
other transducer. A capacitor can readily separate the AC from the
DC as shown in Figure 12.
3. Tuned Circuits. Also referred to as Tank Circuits, L-C circuits,
resonant circuits, and so on.
These applications are better addressed by capacitor technologies other
than reconstituted mica and would involve considerable digression into
inductors to characterize. |
Figure 13: Basic RC circuit |
|
4. Timing Circuits. The final broad class of circuits
in which capacitors are used are timing circuits. From the definition
of capacitance given
earlier, it is clear that if the current flow into, or out of, a capacitor
is know the voltage across the capacitor with respect to time can be
deduced. I.e. the voltage is related to time. It is a fact that the
product of the resistance in Ohms multiplied by thje capacitance in
Farads for the simple RC circuit shown in Figure 13 is actually seconds
and is known as the time constant of the circuit.
The Ubiquitous Capacitor – Real World Values
In any common consumer electronics item such as a television, video
recorder, AM/FM radio, washing machine, etc. capacitors can be found.
The range of values and voltages is large. Voltage ratings vary from
10 VDC to as much as 30,000 VDC (3000 to 1) while capacitance values
will extend from a few pF to tens of thousands of uF a range of perhaps
a billion to one. Reconstituted mica capacitor coverage is indicated
in the following figures. |
 Figure
14 |
|
|
Figure 15 |
|
|
Figure 16
|
|
|
Figure 17: Typical styles for variable and silvered
mica capacitors |
|
The gaps in the coverage of Figure 15 are, of course, easily filled
since all types can be operated safely below their rated voltage and
therefore can extend leftward to the Y axis. Conflicts in overlap areas
are usually unambiguously resolved by consideration or secondary parameters
giving each technology its own niche in the marketplace.
Application Overview
Variable Type
Radio set tuning controls (moving vane assemblies seen in AM radios)
and trimmer type used for final adjustment of frequencies etc.
Silvered Mica and Natural Mica
High accuracy, extremely stable, excellent parasitics. Usage limited
to low level tuned circuits and similar. Relatively expensive. |
Figure 18: Typical aluminum (left) and tantalum electrolytic
styles |
|
Electrolytics
Usually polarized, i.e. must be connected correct way in circuit. Electrolytics
offer the only realistic way to achieve many micro-Farads in a small
volume. Limited to about 600VDC. Poor parasitics. Aluminum types
are low cost, tantalum medium and high cost. Used in electronic photographic
flash packs, power supply filters, timing circuits and so forth,
where large tolerances are acceptable.
Figure 19: Oil Filled Paper |
|
Paper & Oil, paper and Other
Economical choice for high capacity (tens of micro-Farads) high voltage
(1 kV to 100 kV). Frequently used in power distribution. Good performance
generally with the penalty if liquid fill and hermeticity problems.
Medium to low cost. Excellent energy density. Reasonable parasitics. |
Figure 21: Low voltage ceramic disc capacitor.
Up to about 1kV and 1 micro-Farad. |
|
Figure 20: Metalized Polypropylene tubular film capacitor |
|
Film Types
Perhaps the most common “good capacitor”. Secondary attributes
vary considerable between types and usually determine selection. All
types have low leakage and dissipation and are reasonably stable both
over temperature and voltage. Usage ranges from “brute force” low
cost types in florescent lamps to precision, ultra-stable types for
filter applications. High voltage limited to 600 or 1000V although
some are available to 6kV. Not usually impregnated. Medium cost.
Ceramic Types
Highest numerical count for all. In lower voltages (up to
3kV or so) are usually lowest cost by considerable margin. Secondary
characteristics
generally poor with high leakage, poor temperature, |
Figure 24: Surface mount type. Extremely small
size and very low cost. Used in computers and similar. |
|
Figure 23: Epoxy coated, radial lead monolithic
ceramic style. Good stability, up to 100VDC and 1 micro-Farad. |
|
Figure 22: High voltage "Door Knob" type. 15
to 20 kV used in transmitter applications. |
|
coefficient and so on. High frequency performance however is good
and a major use in decoupling power supply lines. Often installed one
per integrated circuit, these components are small (good energy density)
and very low cost (less than ($.03) each for surface mount type) Higher
voltage types, especially in 1nF or more, necessitate multi-layer construction.
Most characteristics stay the same but the cost increases significantly
usually approaching that of alternates including reconstituted mica.
Excellent energy density, poor leakage and temperature characteristics. |
Figure 25: Form Factor considerations. |
|
Reconstituted Mica
As indicated in Figure 15, these components cover a range
of 2kV to 60kV using standard techniques and can be made in capacitance
values
from 10pF or so to several micro-Farads.
The most economical, i.e. cost competitive applications are those
that can be met with a single section capacitor. Tampering with the
length and
width dimensions can also lead directly to multi-section, expensive
solutions as shown in Figure 25. |
Figure 26: Rather than the four capacitors in the
circuit diagram, a module containing all components is tailored
to fit the space and includes mounting means. |
|
Where does Reynolds Fit In?
RELIABILITY
- Superb environmental performance, shock, vibration & thermal
- Short
and long term stability
- Long life, greater than 100 years operation
or storage
- Excellent ultimate
breakdown margin – usually 3 times operating
voltage.
- Withstand repeated rapid discharge with no degradation.
PERFORMANCE
- Excellent temperature capability –55 to +125
C with no derating – better
than all competing technologies.
- Low and predictable temperature
coefficient
- Extremely low (unmeasurable) voltage coefficient
- Good dissipation
factor
- Very low leakage
- Good ESR & ESL for DC applications
USER FRIENDLY
- Readily package into next higher assembly
- Cost competitive with major
competing technology (ceramic)
- Flexible Form Factor readily customized.
Any situation requiring
extreme ruggedness (environmentally or electrically), good secondary
features, wide temperature
range or any combination of these is our business. |
MAKE or BUY made Easy
The Santa Maria Division of Reynolds possesses excellent facilities,
procedures and skills to produce a wide selection of molded capacitor
assemblies. Whenever an application requires more than one capacitor
there is a potential opportunity for big savings by considering a
molded assembly. Benefits usually include significant space, weight
and cost savings and are readily available to customers by working
with the plant to develop a module containing all of the required
capacitors as outlined in Figure 26.
In addition to all internal interconnections being made, the customer
receives a fully tested and burned-in assembly, complete with integral
mounting means and terminations, for easy installation into the next
higher assembly. The make/buy decision usually favors this approach. |
Past, Present & Future Applications
The majority of current and past applications for Reynolds
capacitors has been for output filter capacitors (decoupling applications)
used
in TWT and CRT supplies with values ranging from 500 nF at 3 kV
to 1 nF and 25 kV. Applications have also emerged in Radar modulators
and related high frequency AC situations and characterization effort
to
support such AC applications continues.
New Applications
In common with other Reynolds Industries products, mica
capacitors excel in extremely arduous environments wherever space
and weight are
at a premium. These attributes present a narrow market but, in those
markets, there is none better. With continued emphasis on technology
advances and product improvement, new opportunities continue to appear.
Potential uses to be aware of for Reynolds Capacitors include:
- Compact
Laser projects – Airborne and Vehicular
- X-Ray equipment – Baggage
and Freight inspection, Medical
- Electrostatic Generators – Flue
Scrubbers & Air Cleaners
- Ion Generators – Laboratory atmosphere
control
- Marx Generators – High voltage research equipment
- RADAR Modulators – High
Pulse Rate missile RADARs
- Pulse Mode LASERs – High Power Pump
Components
|
How to be YOUR OWN Expert
To determine first, whether a new application can be met with a reconstituted
mcia capacitor and then detail the capacitor configuration, simply
fill out the following Questionnaire.
Determine if Technology is suitable:
1. Capacitance (more than 5 pF, less than 10 uF)
2. DC Voltage Rating (more than 2kV, less than 60kV)
3. AC Voltage Rating (if more than 2 kV consult factory)
Single Capacitor Requirement:
4. Single or Multi-section
5. Length_____”; Wigth _____”; Thickness _____"
6. Axial or Radial Leads
(NOTE: Radial Leads require 1/2” with for each kV)
7. Wrapper Finish – Mylar, Glass, None (Raw)
(skip to question 12 if not molded type)
Multi-Value or Other Molded Units:
8. Value and Voltage of Capacitors for Multi-Value type
9. General Description (include size, volume, weight as known)
10. Terminations (Pee Wee, Turret, Flying Lead etc.)
11. Mounting Means (Studs, Inserts, Lugs etc.)
12. Required Quantity
Call (805) 928-5866 for Assistance |
Glossary of Terms
Capacitance n. the ratio of charge to potential, coulombs/voltage
Coulmb n. the charge transferred by one ampere flowing for one second
Dielectric n. a nonconductor of electricity, insulator
Dissipation factor n. the ratio of the energy dissipated
to the energy stored in a dielectric per hertz, also equal to the tangent
of the
loss angle
Electrode n. a solid electric conductor through which an electric current
enters or leaves in a medium
Energy density n. figure of merit usually expressed in Joules per cubic
inch for capacitors
Farad n. capacitor with a charge of one coulomb on each plate and a
potential difference of one volt between plates.
Field, capacitor n. body of a capacitor where electrodes
are present as distinct from the margin where there are no electrodes
Margin, capacitor n. dielectric area at the edges of a capacitor with
no electrode present as distance from the field of the capacitor
Natural mica n. group of mineral silicates which can be split into
sheets. Chemical formula K2O.3Al2O3.6SiO2.2H2O. Electrical grade used
for capacitors is mined in India
Ohm n. unit or resistance in which a current of one ampere flows when
one volt is connected across it
Permittivity n. the ratio of electric flux density produced by an electric
field in a medium to that produced in a vacuum by the same field
Reconstituted mica n. flaked natural mica reconstituted
by paper making process into flexible sheets from .0005” to .002"
in thickness
Resonant circuit n. an electric circuit with inductance and capacitance
selected to produce specific natural frequency
Time constant n. in physics is the time for an exponential process
to attain 63% of its final value after starting |
|
|
|
