Resistors

RESISTANCE
The resistor is an electrical device whose primary function is to introduce resistance to the flow of electric current. The magnitude of opposition to the flow of current is called the resistance of the resistor. A larger resistance value indicates a greater opposition to current flow. The resistance is measured in ohms. An ohm is the resistance that arises when a current of one ampere is passed through a resistor subjected to one volt across its terminals. The various uses of resistors include setting biases, controlling gain, fixing time constants, matching and loading circuits, voltage division, and heat generation.
Where
ρ is the resistivity of the resistor material (W· cm),
l is the length of the resistor along direction of current flow (cm), and
A is the cross-sectional area perpendicular to current flow (cm2)
Resistor Power Rating:-
When current passes through a resistor, electrical energy is lost in the resistor in the form of heat and the greater this current flow the hotter the resistor gets. A resistor can be used at any combination of voltage (within reason) and current as long as its power rating is not exceeded and the power rating of a resistor indicates how much power the resistor can convert into heat without damage to itself.
Fixed resistors: -
Carbon composite resistors: Carbon resistors are the most common type of composition resistors. Their resistive element is manufactured from a mixture of finely ground carbon dust (graphite) and a non-conducting ceramic (clay) powder. The ratio of carbon to ceramic determines the overall resistive value of the mixture and the higher this ratio the lower the resistance. The mixture is moulded into a cylindrical shape and metal wires are attached to each end to provide the electrical connection before being coated with an outer insulating material.
Carbon composite resistors are prefixed with a "CR" notation (eg CR10kΩ) and are available in E6 (±20% tolerance (accuracy)), E12 (±10% tolerance) and E24 (±5% & ±2% tolerance) packages with power ratings of 0.125 (1/4) W to 2W.
Film Resistors: -The generic term "Film resistor" consists of Metal Film, Carbon Film and Metal Oxide Film resistors, generally made by depositing pure metals, such as nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate. The resistance value of the resistor is controlled by increasing the desired thickness of the film and then by laser cutting a spiral helix groove type pattern into the film. This method of manufacture allows for closer tolerance resistors (1% or less) as compared to the simpler carbon composition types. The tolerance of a resistor is the difference between the preffered value (i.e., 100 ohms) and its actual manufactured value i.e., 103.6 ohms, and is expressed as a percentage, for example 5%, 10% etc, and in our example the actual tolerance is 3.6%. Film type resistors also achiever a much higher maximum ohmic value compared to other types and values in excess of 10MΩ (10 Million Ω´s) are available.
Wire wound Resistors: Wire wound resistor, is made from winding a metal alloy wire (Nichrome) or similar wire onto an insulating ceramic former. These types of resistors are generally only available in low ohmic high precision values (from 0.01 to 100kΩ) due to the gauge of the wire and number of turns possible on the former. They are also able to handle much higher currents than other resistor of the same physical size and power ratings in excess of 300 Watts are available. These high power resistors are molded or pressed into an aluminum heat sink body with fins attached to increase their surface area and promote heat loss. These types of resistors are called "Chassis Mounted Resistors". They are designed to be physically mounted to heat sinks or metal plates to further dissipate the generated heat increasing their current carrying capabilities.





The heater elements of an electric fire use wire wound resistor principles converting current into heat with each element dissipating 1000 Because the wire is wound into a coil, it acts like an inductor as well as exhibiting resistance and effects the way the resistor behaves in a.c. circuits at high frequencies and above. The length of the actual resistance path in the resistor and the leads contributes inductance in series with the "apparent" d.c. resistance resulting in an overall impedance path Z. Impedance (Z) is the combined effect of resistance (R) and inductance (X), measured in ohms and for a series a.c. circuit is given as, Z2 = R2 + X2.
When used in a.c. circuits this inductance value changes with frequency (inductive reactance, XL = 2πƒL) and therefore, the overall value of the resistor changes. Inductive reactance increases with frequency but is zero at d.c. (zero frequency). Then, wire wound resistors must not be designed into a.c. or amplifier type circuits where the frequency across the resistor changes. However, special non-inductive wirewound resistors are available.




Metal oxide resistor: -
A resistor is a component of an electric circuit that produces heat while offering opposition, or resistance, to the flow of electricity. A resistor can introduce resistance into an electric circuit. Resistors are primarily used for protection, operation or current control. The resistor materials are generally classified into metallic resistor materials, metal oxide resistor materials, and non-metallic resistor materials. Of these materials, the metal oxide resistor materials have heat resistances and energy breakdowns for absorbing a high electrical energy, which are higher than those of other materials.

Variable Resistors
There are two general ways in which variable resistors are used. One is the variable resistor which value is easily changed, like the volume adjustment of Radio. The other is semi-fixed resistor that is not meant to be adjusted by anyone but a technician. It is used to adjust the operating condition of the circuit by the technician. Semi-fixed resistors are used to compensate for the inaccuracies of the resistors, and to fine-tune a circuit. The rotation angle of the variable resistor is usually about 300 degrees. Some variable resistors must be turned many times to use the whole range of resistance they offer. This allows for very precise adjustments of their value. These are called "Potentiometers" or "Trimmer Potentiometers."

\Special Type of Resistor

(1)Thermistor (Thermally sensitive resistor)
Nonlinear resistors - a. NTC, b. PTC

The resistance value of the thermistor changes according to temperature.
NTC (Negative Temperature Coefficient Thermistor): With this type, the resistance value decreases continuously as the temperature rises.
PTC (Positive Temperature Coefficient Thermistor): With this type, the resistance value increases suddenly when the temperature rises above a specific point.
CTR (Critical Temperature Resister Thermistor) : With this type, the resistance value decreases suddenly when the temperature rises above a specific point.
The NTC type is used for the temperature control.The relation between the temperature and the resistance value of the NTC type can be calculated using the following formula.
(2) Resistor Array
This resistor is called a Single-In-Line (SIL) resistor network. It is made with many resistors of the same value, all in one package. One side of each resistor is connected with one side of all the other resistors inside. One example of its use would be to control the current in a circuit powering many light emitting diodes (LEDs)
.
(3) LDR: As the name implies, light dependent resistor (LDR), is a device whose the resistance changes in response on the amount of light falling in it. An LDR’s resistance value in the presence of strong light is just a few ohms , but in absence of light , the value can be tens of megaohms. It is an important to note that they not linear in their response. The basic material from which the LDR can be prepared is calcium sulphide or lead sulphide. Since LDR are fairly slow in their response to changing in light ,circuit which required a high speed response , such as optical communication system , use photodiode or phototransistor .
An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases.
Darkness: maximum resistance, about 1M . Very bright light: minimum resistance, about 100 .