Capacitor
a capacitor
is a component made up of 2 parallel conductors, called plates separated over
the entire extent of their surface by a thin insulator, expressed by its
dielectric strength εr
(epsilon) or relative permittivity.
The capacitor is an electronic component which has the particularity of being able to store energy when it is subjected to a voltage. This component is essential in the field of electricity, it is almost as common as resistance.
Capacitor Symbol
Charge and
energy stored in a capacitor
| Capacitor Symbol |
The charge
that can be stored in a capacitor of capacitance C in farad is given by the
formula:
Q = C x U
The constants of this equation are :
C: capacitor capacity in Farads (F)
Q: capacitor charge in Coulombs (C)
U: voltage across the capacitor in Volts (V)
Definition
of farad: capacitance of a capacitor containing 1 coulomb under 1 volt.
The energy
stored in joules can be calculated with:
Electrical model of real capacitors
the manufacturing method and the inevitable presence of the connection wires lead to the appearance of an inductive component Ls.
The resistance Rs represents the resistance of the connections, the resistance Rp the equivalent value due to the losses in the dielectric and C the value of the ideal admitted capacitance.
capacitors in parallel
When two identical capacitors of dielectric e and surface S are connected in parallel, Note that the surface S doubled.
the total capacity is calculated by adding the capacity of each of the capacitors.
capacitors in series
Connecting two identical capacitors in series is equivalent to doubling the thickness of the dielectric e, the capacity of the equivalent circuit is half the capacity of a capacitor. The insulation voltage of the equivalent capacitor is almost doubled.
the total capacity is calculated using the following form:
ceramic capacitor
Ceramic capacitors are inexpensive components that are widely used in devices and in all areas.
There is a
wide variety, both in characteristics and in manufacturing methods.
Multilayer ceramic capacitors are beginning to compete with electrolytic capacitors for capacities up to several tens of µF. The dielectric is a ceramic,
that is to
say a synthetic material obtained by compression at high temperature of a
powder of more or less complex composition (magnesium, aluminum silicates, etc.
to which titanium, calcium are added...). The composition of the ceramic
determines the characteristics of the dielectric, in particular the
permittivity which can vary widely, and the temperature stability.
Characteristics
According
to the type of ceramic (Type 1)
- dielectric: metal oxides and titanates.
- Capacity and (capacity / unit volume): low.
- Insulation resistance (MW):> 20,000
- Coef. temperature (ppm / ° C): -1500 to +100, NP0 / C0G
- Temperature range (° C): -55 to +125
- Tolerance: 5 to 10%
- Permittivity (er): <200
advantages
an extremely low inductance and a high
series resistance, this is why ceramic insulating capacitors are used a lot:
- in high frequency applications (up to
hundreds of gigahertz).
- for surface components, because they lend
themselves well to miniaturization.
the inconvenients :
- mechanically brittle.
- not particularly high flash field. They
require a certain distance between the plates and are therefore unsuitable for
large capacities (which does not matter in high frequencies).
- They have a slight charge hysteresis and
generate a tiny bit of noise when the dV / dt (current therefore) is high
(large signal amplitude or particularly high frequency). This noise being white
noise has little effect on high frequency circuits, these being most often
tuned on a narrow band.
classes of ceramics according to their
temperature resistance:
- C0G or NP0
have great stability and are used for high frequency applications..
- X7R, less
stable: about 10% variation between –10 ° C and +60 ° C. These ceramics are
reserved for applications that do not restrict high stability.
- Y4T and
Z5U have temperature drifts of the order of 50% in the ranges mentioned above,
and are therefore reserved for decoupling functions.
Use
- mechanically brittle.
- not particularly high flash field. They require a certain distance between the plates and are therefore unsuitable for large capacities (which does not matter in high frequencies).
- They have a slight charge hysteresis and generate a tiny bit of noise when the dV / dt (current therefore) is high (large signal amplitude or particularly high frequency). This noise being white noise has little effect on high frequency circuits, these being most often tuned on a narrow band.
classes of ceramics according to their temperature resistance:
- C0G or NP0 have great stability and are used for high frequency applications..
- X7R, less stable: about 10% variation between –10 ° C and +60 ° C. These ceramics are reserved for applications that do not restrict high stability.
- Y4T and Z5U have temperature drifts of the order of 50% in the ranges mentioned above, and are therefore reserved for decoupling functions.
The use of
ceramic capacitors is so wide that it is easier to cite the cases where other
types are preferred:
- in oscillators where high capacitance stability is required, we prefer mica, polystyrene or polycarbonate capacitors
- in filtering and decoupling circuits where a very large capacity is required, electrolytic capacitors (aluminum and tantalum), although polarized, reign supreme.
- in low frequency circuits because their capacity is generally too low
On the other hand, for all the coupling (between stages) and decoupling (grounding of an unnecessary signal) functions, ceramic capacitors are widely used.
Comments
Post a Comment