Resistors Used in Electric Circuits: Types | Electrical Engineering

Depending on the material of the resistance element, resistors are classified into linear resistors and non-linear resistors. Majority of the resistors used in electronic circuits are linear resistors. Those resistors which follow ohms law and have a linear voltage current characteristic are called linear resistors. Those resistors which do not exhibit Ohm’s law are known a non-linear resistor.

Various types of resistors are mentioned as follows:

1. Linear Resistors:

i. Wire Wound Resistors:

Wire wound resistors consist of a suitable length of high resistance wire of suitable diameter wound into a heat resisting tubular or Bobbin type former. The ends of the wire are brought to suitable terminals, for connections to the external wiring either by soldering or by bolted connections. The resistance wire used may be either insulated by a suitable oxide layer to prevent short circuits between closely wound adjacent turns or bare, as in the case of the majority of power applications.

ii. Grid Resistors:

When the required current rating requires the use of resistance wire with extremely large cross section, the necessary large cross sections are produced in the form resistance grids, either cast from iron or stamped from a suitable grade of alloyed stainless steel. These are then assembled by having series and parallel connections into resistor banks of the appropriate resistance and current rating.

iii. Carbon Composition Resistor:

These are the most widely used resistors in electronic equipment. The resistance element consists of a mixture of carbon and suitable binder material which is extruded and formed into rods. After insertion of connecting leads the resistor is backed and cured to become an extremely hard structure of high mechanical and electrical stability.

The resistor is then coated and marked with the appropriate colour code. The normally available resistance range from few ohms to mega-ohms, while ratings available range from 0.25 watts to 2 watts.

iv. Carbon Film Resistor:

In these resistors, a layer of pure crystalline carbon is deposited by suitable means (such as spraying and baking) into a ceramic rod shaped body fitted with suitable metallic end-caps to which leads are attached.

The resistance of this uniform film, if required, is then brought to the required value by grinding a spiral track into it down to the ceramic base which will increase the ratio of the length of the resistor path to its width.

v. Metallic Film Resistors:

The construction of metallic film resistors is identical to that of the carbon film resistors except that, in this case, a film of high resistivity metal is applied either by spraying or by evaporation in vacuum into the ceramic rod which forms the body of the resistor. Again, the resistance value is adjusted by cutting of a suitable spiral groove into the metallic film.

vi. Light Dependent Resistors (LDR):

As the name indicates, the resistance of LDR depends on the amount of light falling on to it. It consists of a sintered ceramic disc made mainly from cadmium sulphide. Interleaved, comb like metallic electrodes are then applied to the surface facing the light, leads are attached to them and the whole assembly suitably encapsulated.

vii. Variable Resistors:

Variable resistors are those resistors whose resistance value can be changed easily.

These resistors can be classified into two categories:

(a) Slider Type Resistor:

These are in the form of wire wound on a tabular or rectangular form and tapped by way of a moving contact travelling along the length of the resistor.

(b) Rotary Type Resistors:

Here the resistor element is located along a circular path and it is tapped by a moving contact attached to a shaft capable of rotating through 270° of arc.

2. Non- Linear Resistors:

i. Negative Temperature of Coefficient (NTC) Thermistors:

These resistors are manufactured from semiconducting metallic oxides and have a large negative temperature coefficient of resistance. The oxides and other binder materials are extruded after mixing into rods or pressed into discs, depending on the particular application and fired like ceramic in a kiln at high temperature. Terminal leads are then attached to provide connections.

ii. Positive Temperature Coefficient (PTC) Thermistors:

As the name indicates, these resistors are made from semiconducting material with a high positive temperature coefficient of resistance PTC thermistors are made from a mixture of barium, titanium and strontium oxides and this ceramic material is then formed into small disc shaped pills, fired at high temperature, with connecting leads attached to metallised opposite surfaces.

iii. Voltage Dependent Resistors (VDR):

The resistance of VDR varies as an inverse function of the applied voltage. VDRs are made from silicon carbide granules and ceramic binders, using the normal techniques of shaping, forming and firing, customary in ceramic technology.

Depending on voltage and current rating the VDRs take the shape of small rods, pills prismatic slabs, discs or cylindrical blocks: Connecting leads are attached to the smaller VDRs while the larger discs and blocks are usually clamped between metallic surfaces.