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Capacitors With Variable Capacity.

When the plates are brought together, the capacity increases rapidly, as does the voltage gradient (ie, the electrostatic field). For example, the field in a capacitor subjected to only 5 volts and whose plates are 5 micrometers apart is 1 million volts per meter! Insulation therefore plays a vital role. The perfect insulation would have unlimited resistance and total transparency in the field, would have no flash point (field gradient where an arc appears), would have no inductance (which limits the reaction to high frequencies: a perfect capacitor let the light through, for example), etc. We must therefore choose an insulation according to the desired objective, that is to say the use that (y capacitors).

Electrolytic capacitors

Electrolytic capacitors are used:

  • when you need a large storage capacity
  • when you do not need to have a perfect capacitor
  • high resistance in series
  • poor response to high frequencies
  • great tolerance

Manufacturing

Unlike any other capacitor, when we produce them, we do not put insulation between the two conductors. Moreover, a new electrolyte conducts the direct current ! In reality, one of the conductors is metallic, the other is a conductive gel: the metallic conductor is simply inserted into the jelly. When applying a voltage for the first time, a chemical reaction (called electrolysis hence the name) takes place, which creates an insulating interface on the surface of the metal. Of course, as soon as it is formed, this layer prevents the current from passing and consequently its own formation. This results in a particularly thin insulating layer (a few molecules thick) hence the very large capacity of electrolytic.

Hence also their limited maximum voltage, which nevertheless makes it useful for low voltage power supplies (less than 200 volts ). However, the jelly is not as good conductor as a metal: an electrolytic therefore has a significant series resistance which creates a “zero” in the sense of the transfer functions ( low pass filter ) with the capacitance. Furthermore , passing in the jelly deforms the orbitals of the electrons of the valence layers that bind the jelly, creating a small mechanical vibration in the jelly, hence:

  • an inertial effect ( inductance ) important;
  • poor response to high frequencies.

Let’s just say that these capacitors were simply not designed to be used for decoupling or signal filtering purposes.

Tantalum capacitors

There are 2 technologies of tantalum capacitors:

Solid Electrolyte Tantalum Capacitors : These are capacitors where the first electrode is tantalum, and the second is the manganese dioxide MnO2 . The contact with the manganese dioxide is ensured by a layer of metallization based on silver. This technology brings the following benefits:

  • reduced series resistance (ESR);
  • low series inductances;
  • weak resonances;
  • no degradation in time, storage or use;
  • low cost.

Capacitors tantalum liquid electrolyte (WET Tantalum): These are capacitors wherein the first electrode is tantalum, and the second conductive gel.

  • more series resistance (ESR) than “solid” models;
  • low series inductances;
  • weak resonances;
  • high self-healing capacity, hence high reliability;
  • higher cost.

Indeed, the liquid electrolyte is indeed capable of oxidizing the tantalum in case of a fault in the oxide layer, this regeneration makes them very reliable capacitors, they are frequently chosen for applications where reliability is a criterion determinant; example: use in a satellite. On the other hand, this possibility means that a higher leakage current is possible, to be taken into account in the design (y capacitors).

Capacitors with liquid electrolyte are more expensive, because of the materials used: silver or solid tantalum for the case (because of the acidic electrolyte), but also more complex manufacturing processes (waterproof assembly), they are in fact reserved for “high-end” applications.