Sunday, 26 January 2014

Types of Resistors-Fixed resistors

Types of Resistors-Fixed resistors
A.Carbon Composition Resistors
These resistors are cylindrical rods which are a mixture of carbon granules and powdered ceramic. The resistor value depends on the composition of the ceramic material. A higher quantity of ceramic content will result in more resistance. Since the rod is coated with an insulated material, there are chances of damage due to excessive heat caused by soldering.

High current and voltage can also damage the resistor. These factors bring irreversible changes in the resistance power of these resistors. This type of resistor is rarely used nowadays due to their high cost and are only preferred in power supply and welding circuits.
B.Carbon film resistors
This resistor is formed by depositing a carbon film layer on an insulating substrate. Helical cuts are then made through the carbon film to trace a long and helical resistive path. The resistance can be varied by using different resistivity carbon material and modifying the shape of the resistor. The helical resistive path make these resistors highly inductive and of little use for RF applications.

C.Metal Film resistor:  
These resistors are made from small rods of ceramic coated with metal (such as a nickel alloy) or metal oxide (such as tin oxide). The value of resistance is controlled mainly by the thickness of the coating layer (the thicker the layer, the lower is the value of resistance). A fine spiral groove can be cut along the rod using a laser to split the carbon or metal coating effectively into a long and spiral strip, which forms the resistor. 

D.Wire wound resistor
Wire wound resistors vary in size and physical appearance. Their resistive elements are commonly lengths of wire, usually an alloy such as Nickel/Chromium or Manganin wrapped around a small ceramic or glass fiber rod and coated in an insulating flameproof cement film. They are normally available in low values of resistance but are capable of dissipating large amounts of power.
These resistors can get very hot during use. For this reason, these resistors are housed in a finned metal case that can be bolted to a metal chassis to dissipate the heat generated. Protection from fire is important and fireproof cases or coatings are vital. Lead-out wires are normally welded rather than soldered to the resistor. Enamel resistors are used in scenarios where high power is involved and are encapsulated in heat proof bases.
Since wire wound resistors are primarily coils, they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed directions can minimize inductance. Other techniques employ bifilar winding to reduce cross-section area of the coil. For the most demanding circuits, resistors with Ayrton-Perry windings are used.

E.Thin film and thick film resistors:
The principal difference between thin film and thick film resistors is how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). Thin film resistors are made by sputtering (a method of vacuum deposition) the resistive material onto an insulating substrate whereas thick film are made using screen and stencil printing processes.
Ceramic conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and bismuth iridate (Bi2Ir2O7) are the materials commonly used for making thin film resistors. Thick film resistors are usually made by mixing ceramics with powdered glass. Thick films have tolerances ranging from 1 to 2% and a temperature coefficient between ±200 or ±250 ppm/K.
Thin film resistors are usually more expensive than thick film resistors. Thin film resistors are preferred for microwave passive and active power componentssuch as microwave power resistors, microwave power terminations, microwave resistive power dividers, microwave power attenuators.

F.Surface mount resistor (SMT)
This type of resistor helps to achieve very low power dissipation along with very high component density. Most modern circuits use tiny SMT resistors. These are made by depositing a film of resistive material such as tin oxide on a tiny ceramic chip. The edges of the resistor are then accurately ground or cut with a laser to give precise resistance across the device. Tolerances may be as low as 0.02%. Contacts at each end are provided, which are soldered directly onto the conductive print on the circuit board, usually by automatic assembly methods. These are mostly used where space is an important factor.

G.Network resistors:
These resistors are the combination of resistances which may be giving identical value at all pins, with one pin acting as a common terminal. These resistors are available in both single in line package and dual in line package and may be surface mount or through hole. These are used in applications such as pull up/pull down, DAC etc

No comments:

Post a Comment

Readers' Choice