Sharing is caring!

Solid tantalum capacitors have extremely low series resistance, making them a preferred component for power decoupling on cards.

Tantalum capacitors, however, have a defect: they exhibit a slight non-linearity, which is why these capacitors are not recommended for signal transmission (creation of even harmonics) except when they are associated with other non electrolytic capacitors for form a composite capacitor (Safety Certified Capacitors).

Solid tantalum capacitors also have another defect: tantalum may catch fire if the current is exceeded or in case of failure. That is why they are little used in applications where it presents a danger for the user (automobile for example).

Modelization

An electrolytic capacitor is modeled more or less realistically.

As a first approximation, the main characteristics of the component are described:

  • the value of the capacity;
  • series resistance;
  • series inductance;
  • parallel resistance;

By making a more elaborate modeling, one accesses finer characteristics:

  • load hysteresis;
  • the battery effect;
  • others (influence of temperature , aging of materials, etc.).

The load hysteresis is an effect that, below a threshold voltage (low), the jelly does not let current flow (for example, a large electrolytic capacitor of 1 farad subjected to a voltage of 5 microvolts n will not accumulate a charge of 5 microcoulombs). As a result, the weak alternating signals come out with a distortion that is comparable to that of a pure class B amplifier , although much less.

The battery effect, less negligible, is due to the existence of a parasitic electrolysis reaction and electrosynthesis which take place in the presence of an alternating signal or a DC voltage. This charge and discharge of battery is not to be confused with a charge and discharge of capacitor, because its time constant is much greater. To observe it, you can charge an electrolytic capacitor, let it charge for a few minutes (which causes the phenomenon) and then unload it abruptly by short-circuiting it for a short time. By means of a voltmeter , one will then observe at the limits the reappearance of a tension: it is the load of battery.

Another effect of this technology is that the insulating layer does not necessarily have the same thickness, even for the same model. The thickness depends on several factors: the temperature, the microscopic micro-asperities of the metal, the vibrations, the humidity during manufacture, the age of the capacitor, the use to which it has been subjected, etc. This is why the electrolytic capacity is always presented with great tolerance (typically -20% to +100% for big), which makes them bad candidates to make accurate filters or time bases.

Insulating capacitors

They are manufactured according to the classic definition of the capacitor: a metal conductor split with an insulator. As always, the insulation, chosen according to the use one wants to make, will determine the nature of the capacitor (Safety Certified Capacitors).

Air

This category includes variable / adjustable capacitors and some low value capacitors realized using the printed circuit itself. Its insulation characteristics are quite weak and sensitive to ambient humidity.Example:

  • Adjustable air condenser (used in radio receivers for the choice of stations).

They are constituted of movable armatures one compared to the other; the facing surfaces determine the value of the capacitor.

Advantages:

  • the advantages of an extremely low inductance and a particularly high series resistance, this is why capacitors with ceramic insulation are widely used:
  • in high frequency applications (up to hundreds of gigahertz)
  • in high voltage applications (valve circuits ( tubes ) for example)
  • for surface components, as they lend themselves to miniaturization.

Disadvantages:

  • to be mechanically fragile
  • to have a field of lightning not particularly high. They require a certain distance between the plates and therefore lend themselves poorly to large capacities (which does not matter in the high frequencies).