We can define the resistance as that component that puts up certain difficulty against the step of the electric current. Namely it offers resistance to allow to cross for the electric current in the most varied values according to the type of component, so that there can complir such diverse functions like the polarization of load, tension limiters, etc.

The resistances, there are the elements that more the electronic circuits abound. When we uncover any box that contains semiconductors we will see them with profusion, distinguished immediately by hoops of living colors that wrap them and that, they indicate the value of his resistance óhmica, in accordance with his code.

Classes of Resistances:

We will establish a classification of the resistances in accordance with the way of being contruidas, and also of acuerdocon the materials with it carries out this construction.

a) Agglomerated resistances.
b) Resistances of layer or movie.
c) Resistances Bobinadas.

The agglomerated resistances consist of a homogeneous mass of graphite mixed with an agglutinative element, hard pressed in form cylindrical and encapsulated in a muff of insulating material like the plastic. The value óhmico of a coal resistance, that is to say, his biggest or minor facility to allow to spend the electric current depends on the proportions of the graphite and agglutinating agent used in his fabricaión.

In the resistances of layer or movie, the element resistivo is the thinnest coal layer on an insulating body, of form also cylindrical. The central body is, in some cases, a minuscule crystal pipe with the terminals of connection connected to every end. A variant of this type of resistances there are the called resistances of metallic movie, in which the coal layer has been replaced with a metallic alloy of high constant resistiva (nickel, chromium or gold - platinum) or a metallic oxide as the oxide of tin.

In the resistances bobinadas a conductive thread is used that poseauna resistance específicaespecialmente high. The conductive thread is wound on a body, generally a ceramics pipe. As for the ends of the thread, they are fixed generally with clamps that to his you see can serve like connections for the assembly and, even, if the clamps are desplazables partial values of resistance can be obtained. In many ocaciones they are placed also inside a ceramic prism of square section and are sealed by a special silicone so that they are properly protected.

Value óhmico and tolerance of the resistances

What more we are interested in of the resistances is, from, his value óhmico, that is to say, the opposition that offers the step of the electric current. This value does not have any relation with the size, but the constituent materials of the resistance. As for the value óhmico it is necessary to bear in mind that this one remains affected by the heat, the heat takes place whenever the current eléctricapasa across a resistance, and this increase of the temperature modifies the value of the resistances. For this motive, in some measurement devices it is necessary to wait until the resistances should have warmed up before doing the measurement so that it stops the change of resistance that these elements provoke. It be born in mind that, after certain time, the balance state is established between the produced heat and the radiated heat, with what the temperature does not keep on increasing. Anyhow, the value assigned to a resistance is always approximate, and hence it should always be counted with a tolerance, so that the nominal value can change inside certain limits.

Why do they serve the resistances

In the electronic circuits, both the tensions and the currents it is necessary to control them to obtain the wished effects. We cannot, for example, order indiscriminately current to the base of a transistor; on the contrary, these bases always need polarization tensions so that they could work inside the correct límitres, which means that the issuer must support the base tension of a transistor to a constant tension with regard.

Indication of the value of the resistances

It is interesting to us really to know the value of each of the resistances that are part of a circuit, since if one day some resistance has to change that we can replace it with other one of the suitable value.
The value of the resistances is recorded on them and can be indicated by means of numbers, for rings of color or for points of color, engraving all this, as we say, on the exterior surface of the component and in accordance with a code that we have to know. The use of identical rings with color is the most current system used in electronics, and it is the one that we are going to study on this page.