Resistors

Resistors

The resistor is a one of a most common and reliable electronic component. It is used as a load or a part of a load, in most electronic circuits. Its major purposes are to control current and divide voltage. Some types of resistors can operate at temperature as high as 3000C & they are manufactured in a wide range of resistance value. Resistance of less than 1 ohm to over 100 Meg ohms is available.

Resistors can be classified into two basic categories

· Fixed
· Variable


Fixed Resistors

Fixed resisters are commonly used in electronic circuits. The resistance of them are fixed, but varying in a small range called tolerance mentioned their with its colour code.







Circuit symbol




Resistor Colour Code

As the fixed resisters are very small, we use special standard colour code to represent its resistance value. It is called the resister colour code.





Here is a physical diagram of a 4-colour band fixed resister. The following diagram shows what are the things represented by each colour band.






There is another method of expressing resistor values. This method is normally used when writing the values with digits. When writing values with digits, the decimal points may be erased or accidentally miss. Therefore special characters are used as the decimal point. They are as follows.
Letter Multiplier
R 1
K 1000
M 1000,000


Example:
560R means 560
2K7 means 2.7 k
39K means 39 k
1M0 means 1.0 M

The method has another letters to express the tolerance. They are as follows.


Letter Tolerance
F ±1%
G ±2%
J ±5%
K ±10%
M ±20%


Types of resistors
Resistors are grouped according to the type of material or process used to make the resistive element. The major types and their characteristics are listed bellow.


Carbon composition
The resistance element is made of finally powdered carbon and held together with an inner binding material. The resistance of the element is determined by the ratio of carbon to binding material. The physical size determines the power rating not the resistance. Carbon composition is used for both fixed and variable resistors.


Wire wound Resistor



These are used for all classification of resistors variable, fixed, adjustable and tapped. In fact nearly all taps and adjustable resistors are wire wound and the class as power resistors. The wire for wire-wounded resistor all made of alloy such as nickel-chromium or copper - nickel. The wire is wound on an insulator and the ends are connected to solder coated copper ends.
Fixed wire wound resistors come in many sizes and shapes. Some are looked just like carbon composition resistors. Wire wounded resistors are made with power rating in excess of 1000W.
Wire wound resistors have low temperature coefficient, good stability, and can be made to close tolerance.

There are other kind of fixed resistors. Surface mount, printed resistors and resistor banks are some of them.


Variable Resistor
Variable resisters can be again categorized as potentiometers, rheostat and presets.



Potentiometers






Circuit Symbol


There can be categorized in two ways as,
· Rotary arm
· Slide arm

Or


· Linear (LIN)
· Logarithmic (LOG)

They have 3 terminals. Use to vary voltage. Most potentiometers are linear. That is, one degree of sharp rotation results in the same change of resistance regardless of the shaft location. Logarithmic potentiometers have non-liner tapers. This means that the rate of change of resistance varies as the shaft is rotated. Also called tapped potentiometers. Verities of tapers are available. Tapped potentiometers are some times used as volume and tone control in stereo amplifier.


Rheostats
Circuit Symbol

Rheostats have only two terminals, turning the shaft change the resistance between these two terminals. Rheostats are used to vary current. Just using 2 of the 3 terminals uses very often potentiometers as rheostats


Preset


Circuit Symbol








These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.



Thermistor



This is another kind of resistor. The resistance of these resistance is vary with the heat and use as an input transducer. There are two kinds of thermistors. One has a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. The other one has a positive temperature coefficient (resistance increases as temperature increases).

Power rating
Resistors have a power rating as well as resistance, rating the power of the resistor indicate amount of power the resister can safely dissipate without being destroyed as current pass through the resistor. The resistor is converting electric energy in to heat energy. If the current is allowed to increase, the heat would burn up the resistor. Thus some safe level of heat must be specified and this is what the power rating does.


The manufacturer under specified conditions assigns the power rating of a resistor. These conditions assume free air circulation around the resistor and resistor leads soldered to sizable terminals. Often these conditions are not fully satisfied in the device, which use resistors. Therefore a resistor with a power rating higher than the value calculated for the circuit is used.