There are a bewildering array of capacitor
characteristics and specifications associated with the humble capacitor
and reading the information printed onto the body of a capacitor can
sometimes be difficult especially when colours or numeric codes are
used. Each family or type of capacitor uses its own unique set of
capacitor characteristics and identification system with some systems
being easy to understand, and others that use misleading letters,
colours or symbols.
The best way to figure out which Capacitor Characteristics
the label means is to first figure out what type of family the
capacitor belongs to whether it is ceramic, film, plastic or
electrolytic and from that it may be easier to identify the particular
capacitor characteristics.
Even though two capacitors may have exactly the same capacitance
value, they may have different voltage ratings. If a smaller rated
voltage capacitor is substituted in place of a higher rated voltage
capacitor, the increased voltage may damage the smaller capacitor.
Also we remember from the last tutorial that with a polarised
electrolytic capacitor, the positive lead must go to the positive
connection and the negative lead to the negative connection otherwise it
may again become damaged. So it is always better to substitute an old
or damaged capacitor with the same type as the specified one. An example
of capacitor markings is given below.
Capacitor Characteristics
The capacitor, as with any other electronic component, comes defined by a series of characteristics. These
Capacitor Characteristics
can always be found in the data sheets that the capacitor manufacturer
provides to us so here are just a few of the more important ones.
1. Nominal Capacitance, ( C )
The nominal value of the
Capacitance,
C
of a capacitor is the most important of all capacitor characteristics.
This value measured in pico-Farads (pF), nano-Farads (nF) or
micro-Farads (µF) and is marked onto the body of the capacitor as
numbers, letters or coloured bands. The capacitance of a capacitor can
change value with the circuit frequency (Hz) y with the ambient
temperature. Smaller ceramic capacitors can have a nominal value as low
as one pico-Farad, ( 1pF ) while larger electrolytic’s can have a
nominal capacitance value of up to one Farad, ( 1F ).
All capacitors have a tolerance rating that can range from -20% to as
high as +80% for aluminium electrolytic’s affecting its actual or real
value. The choice of capacitance is determined by the circuit
configuration but the value read on the side of a capacitor may not
necessarily be its actual value.
2. Working Voltage, ( WV )
The
Working Voltage is another important capacitor
characteristic that defines the maximum continuous voltage either DC or
AC that can be applied to the capacitor without failure during its
working life. Generally, the working voltage printed onto the side of a
capacitors body refers to its DC working voltage, ( WV-DC ). DC and AC
voltage values are usually not the same for a capacitor as the AC
voltage value refers to the r.m.s. value and NOT the maximum or peak
value which is 1.414 times greater. Also, the specified DC working
voltage is valid within a certain temperature range, normally – 30°C to +
70°C.
Any DC voltage in excess of its working voltage or an excessive AC
ripple current may cause failure. It follows therefore, that a capacitor
will have a longer working life if operated in a cool environment and
within its rated voltage. Common working DC voltages are 10V, 16V, 25V,
35V, 50V, 63V, 100V, 160V, 250V, 400V and 1000V and are printed onto the
body of the capacitor.
3. Tolerance, ( ±% )
As with resistors, capacitors also have a
Tolerance
rating expressed as a plus-or-minus value either in picofarad’s (±pF)
for low value capacitors generally less than 100pF or as a percentage
(±%) for higher value capacitors generally higher than 100pF. The
tolerance value is the extent to which the actual capacitance is allowed
to vary from its nominal value and can range anywhere from -20% to
+80%. Thus a 100µF capacitor with a ±20% tolerance could legitimately
vary from 80µF to 120µF and still remain within tolerance.
Capacitors are rated according to how near to their actual values
they are compared to the rated nominal capacitance with coloured bands
or letters used to indicated their actual tolerance. The most common
tolerance variation for capacitors is 5% or 10% but some plastic
capacitors are rated as low as ±1%.
4. Leakage Current
The dielectric used inside the capacitor to separate the conductive
plates is not a perfect insulator resulting in a very small current
flowing or "leaking" through the dielectric due to the influence of the
powerful electric fields built up by the charge on the plates when
applied to a constant supply voltage.
This small DC current flow in the region of nano-amps (
nA) is called the capacitors
Leakage Current.
Leakage current is a result of electrons physically making their way
through the dielectric medium, around its edges or across its leads and
which will over time fully discharging the capacitor if the supply
voltage is removed.
When the leakage is very low such as in film or foil type capacitors it is generally referred to as “insulation resistance” ( R
p )
and can be expressed as a high value resistance in parallel with the
capacitor as shown. When the leakage current is high as in
electrolytic’s it is referred to as a “leakage current” as electrons
flow directly through the electrolyte.
Capacitor leakage current is an important parameter in amplifier
coupling circuits or in power supply circuits, with the best choices for
coupling and/or storage applications being Teflon and the other plastic
capacitor types (polypropylene, polystyrene, etc) because the lower the
dielectric constant, the higher the insulation resistance.
Electrolytic-type capacitors (tantalum and aluminium) on the other
hand may have very high capacitances, but they also have very high
leakage currents (typically of the order of about 5-20 μA per µF) due to
their poor isolation resistance, and are therefore not suited for
storage or coupling applications. Also, the flow of leakage current for
aluminium electrolytic’s increases with temperature.
5. Working Temperature, ( T )
Changes in temperature around the capacitor affect the value of the
capacitance because of changes in the dielectric properties. If the air
or surrounding temperature becomes to hot or to cold the capacitance
value of the capacitor may change so much as to affect the correct
operation of the circuit. The normal working range for most capacitors
is -30°C to +125°C with nominal voltage ratings given for a
Working Temperature of no more than +70°C especially for the plastic capacitor types.
Generally for electrolytic capacitors and especially aluminium
electrolytic capacitor, at high temperatures (over +85°C the liquids
within the electrolyte can be lost to evaporation, and the body of the
capacitor (especially the small sizes) may become deformed due to the
internal pressure and leak outright. Also, electrolytic capacitors can
not be used at low temperatures, below about -10°C, as the electrolyte
jelly freezes.
6. Temperature Coefficient, ( TC )
The
Temperature Coefficient of a capacitor is the
maximum change in its capacitance over a specified temperature range.
The temperature coefficient of a capacitor is generally expressed
linearly as parts per million per degree centigrade (PPM/°C), or as a
percent change over a particular range of temperatures. Some capacitors
are non linear (Class 2 capacitors) and increase their value as the
temperature rises giving them a temperature coefficient that is
expressed as a positive “P”.
Some capacitors decrease their value as the temperature rises giving
them a temperature coefficient that is expressed as a negative “N”. For
example “P100″ is +100 ppm/°C or “N200″, which is -200 ppm/°C etc.
However, some capacitors do not change their value and remain constant
over a certain temperature range, such capacitors have a zero
temperature coefficient or “NPO”. These types of capacitors such as Mica
or Polyester are generally referred to as Class 1 capacitors.
Most capacitors, especially electrolytic’s lose their capacitance
when they get hot but temperature compensating capacitors are available
in the range of at least P1000 through to N5000 (+1000 ppm/C through to
-5000 ppm/C). It is also possible to connect a capacitor with a positive
temperature coefficient in series or parallel with a capacitor having a
negative temperature coefficient the net result being that the two
opposite effects will cancel each other out over a certain range of
temperatures. Another useful application of temperature coefficient
capacitors is to use them to cancel out the effect of temperature on
other components within a circuit, such as inductors or resistors etc.
7. Polarization
Capacitor
Polarization generally refers to the
electrolytic type capacitors but mainly the Aluminium Electrolytic’s,
with regards to their electrical connection. The majority of
electrolytic capacitors are polarized types, that is the voltage
connected to the capacitor terminals must have the correct polarity,
i.e.
positive to
positive and
negative to
negative.
Incorrect polarization can cause the oxide layer inside the capacitor
to break down resulting in very large currents flowing through the
device resulting in destruction as we have mentioned earlier.
The majority of electrolytic capacitors have their negative,
-ve
terminal clearly marked with either a black stripe, band, arrows or
chevrons down one side of their body as shown, to prevent any incorrect
connection to the DC supply.
Some larger electrolytic’s have their metal can or body connected to
the negative terminal but high voltage types have their metal can
insulated with the electrodes being brought out to separate spade or
screw terminals for safety.
Also, when using aluminium electrolytic’s in power supply smoothing
circuits care should be taken to prevent the sum of the peak DC voltage
and AC ripple voltage from becoming a “reverse voltage”.
8. Equivalent Series Resistance, ( ESR )
The
Equivalent Series Resistance or
ESR, of a
capacitor is the AC impedance of the capacitor when used at high
frequencies and includes the resistance of the dielectric material, the
DC resistance of the terminal leads, the DC resistance of the
connections to the dielectric and the capacitor plate resistance all
measured at a particular frequency and temperature.
ESR Model
In some ways, ESR is the opposite of the insulation resistance which
is presented as a pure resistance (no capacitive or inductive reactance)
in parallel with the capacitor. An ideal capacitor would have only
capacitance but ESR is presented as a pure resistance (less than 0.1Ω)
in series with the capacitor (hence the name Equivalent Series
Resistance), and which is frequency dependent making it a “DYNAMIC”
quantity.
As ESR defines the energy losses of the “equivalent” series
resistance of a capacitor it must therefore determine the capacitor’s
overall
I2R heating losses especially when used in power and switching circuits.
Capacitors with a relatively high ESR have less ability to pass
current to and from its plates to the external circuit because of their
longer charging and discharging
RC time
constant. The ESR of electrolytic capacitors increases over time as
their electrolyte dries out. Capacitors with very low ESR ratings are
available and are best suited when using the capacitor as a filter.
As a final note, capacitors with small capacitance’s (less than 0.01
uF) generally do not pose much danger to humans. However, when their
capacitance’s start to exceed 0.1 uF, touching the capacitor leads can
be a shocking experience.
Capacitors have the ability to store an electrical charge in the form
of a voltage across themselves even when there is no circuit current
flowing, giving them a sort of memory with large electrolytic type
reservoir capacitors found in television sets, photo flashes and
capacitor banks potentially storing a lethal charge.
As a general rule of thumb, never touch the leads of large value
capacitors once the power supply is removed. If you are unsure about
their condition or the safe handling of these large capacitors, seek
help or expert advice before handling them.
We have listed here only a few of the many capacitor characteristics
available to both identify and define its operating conditions and in
the next tutorial in our section about
Capacitors, we look at how capacitors store electrical charge on their plates and use it to calculate its capacitance value.
ref:http://www.electronics-tutorials.ws/
p: 
Posts
