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Sunday, 17 March 2019

Resistors - Specifications and Properties

A resistor is the most basic electronic component invariably found almost every electronic circuit. It is a passive element. The most important function of resistor is to resist the flow of current. The property of a resistor is known as resistance.


A resistor can be used as load, as potential divider and also as a biasing element in different circuits. It also acts as filter and timer in combination with capacitor. Resistors are basically available in two types viz., fixed resistors and variable resistors. Now a days resistors are available in a large variety of types and sizes, each type has specific advantages, disadvantages and applications.

Such variety types of resistors are shown in figure.




SPECIFICATIONS OF RESISTORS:


All resistors will have three main specifications that are to be considered. They are

1. Resistance value
2. Tolerance
3. Power rating

1. Resistance Value : It gives the value of resistor R in ohms. It's value is either printed or colour coded over the body depending upon the type of resistor. In general resistors from 1Ω to many MΩ are available.

2. Tolerance : It gives the variation of resistance value from the indicated value. It is generally expressed in percentage. It's typical values are ranging from ±1% to ±20%. Resistors with low tolerance values are preferred.

3. Power Rating : The power rating of a resistor is given by the maximum wattage. The resistor can dissipate without excessive heat. It is expressed in watts. The resistors with power ratings ranging from 0.1 watts to hundreds of watts are available. The power rating depends on the size of the resistor. Since it is current which produces heat, power rating also gives some indication of the maximum current a resistor can safely carry. However there are some other specifications that are to be considered while selecting a resistor. They are



4. Temperature Coefficient of Resistance : It gives the variation of resistance with a change in temperature. It is usually measured with reference to resistance at 25°C.

5. Voltage Coefficient : It is measured as the change of resistance of a resistor with a change in the applied voltage.

6. High Frequency Performance : Even though resistors are insensitive to frequency but at higher frequencies some factors like distributed capacitances in carbon resistors and inductive reactances in wire wound resistors are become dominant causing a change in resistance. Generally the resistors with lower resistance values will have better high frequency performance.

7. Noise Figure : When a d.c current is passed through a resistor R the voltage drop across the resistor is not only Ids. R, but it is superimposed by fluctuations called noise. Noise in resistors is of two types (1) Thermal agitation noise and (2) Current noise. Wire wound resistors exhibit little current noise when compared with carbon composition resistor.

8. Stability: A resistor under test is said to be more stable, if it is used for a long period under atmospheric condition, and its value measured at room temperature is nearer to it's initial measured value. carbon film, metal film and wire wound resistors are more stable than carbon composition resistors.

9. Size, Shape and Leads : Resistors are available in different sizes (small, big) and shapes (rod, disc, washer) with different types (axial, radial, lug) of leads or terminals. The selection of particular type of resistor depends on the requirement.

PROPERTIES OF RESISTANCE:


A resistor is a passive electronic component that offers a specific amount of electrical resistance to the flow of current when connected in a circuit. The property of a resistor is "resistance", it is defined as the "opposition to the flow of current". This opposition comes from the electrons present in the atom of material. In materials like copper, the electrons are more free to move compared to the electrons in a material like plastic. In other word copper has less resistance than plastic. Resistance is denoted by the letter 'R'.

The unit of resistance is the 'Ohm', its symbol is '01. It is defined as the resistance between the two ends of a conductor.

According to Ohm's law the resistance of a conductor is said to be one Ohm; when a constant current of one ampere flows through it for an applied potential difference of one volt.
In other words the resistance 'R' can be defined as the ratio of voltage applied to the current flowing through the conductor.

Mathematically it can be given as R = V / I

Where R = Resistance of the conductor in Ohms
V = Voltage applied in Volts.
I = Current in amperes.

The typical values of resistance can be varied from 0.1 ohm to several hundreds of ohms.


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