Capacitor Characteristics


The characteristics of a capacitors define its temperature, voltage rating
and capacitance range as well as its use in a particular application


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
to understand 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.



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.



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, (WVDC).

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.
 


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%.



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.
leakage current





When the leakage is very low such as in film or foil type capacitors it is
generally referred to as “insulation resistance” ( Rp ) 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.



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 -30oC to +125oC
with nominal voltage ratings given for a Working Temperature of no more than
+70oC especially for the plastic capacitor types.

Generally for electrolytic capacitors and especially aluminium electrolytic
capacitor, at high temperatures (over +85oC 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 -10oC, as the electrolyte jelly freezes.



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/oC), 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/oC or “N200”, which is -200 ppm/oC 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/oC through to -5000
ppm/oC). 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.



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.
polarization




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”.



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.
equivalent resistance




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 0.01μF)
generally do not pose much danger to humans. However, when their
capacitance’s start to exceed 0.1μF, 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.
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