Resistors

Learn Everything about Resistors

Resistors

 

Resistors are the most commonly used component in electronics and their purpose is to create specified values of current and voltage in a circuit. 

A number of different resistors are shown in the photos. (The resistors are on millimeter paper, with 1cm spacing to give some idea of the dimensions).  

The Image 2.1a shows some low-power resistors, while Image 2.1b shows some higher-power resistors. 

Resistors with power dissipation below 5 watt (most commonly used types) are cylindrical in shape, with a wire protruding from each end for connecting to a circuit (Image 2.1-a). Resistors with power dissipation above 5 watt are shown below 

(Image 2.1-b).

 

Fig. 2.1a: Some low-power resistors

 

Fig. 2.1b: High-power resistors and rheostats

The symbol for a resistor is shown in the following diagram (upper: American symbol, lower: European symbol.)

Resistor symbols

The unit for measuring resistance is the OHM. (The Greek letter Ω - called Omega).

Higher resistance values are represented by "k" (kilo-ohms) and M (Meg ohms). For example, 120 000 Ω is represented as 120k, while 1 200 000 Ω is represented as 1M2.

The dot is generally omitted as it can easily be lost in the printing process. 

In some circuit diagrams, a value such as 8 or 120 represents a resistance in ohms. Another common practice is to use the letter E for resistance in ohms. 

The letter R can also be used. For example, 120E (120R) stands for 120 Ω, 1E2 stands for 1R2 etc.

Resistor Markings

Resistance value is marked on the resistor body. Most resistors have 4 bands. The first two bands provide the numbers for the resistance and the third band provides the number of zeros. The fourth band indicates the tolerance. Tolerance values of 5%, 2%, and 1% are most commonly available.

 

The following table shows the colors used to identify resistor values:

 

COLOR	DIGIT	MULTIPLIER	TOLERANCE
Silver		x 0.01 	±10%
Gold		x 0.1 	±5%
Black	0	x 1 
Brown	1	x 10 	±1%
Red	2	x 100 	±2%
Orange	3	x 1 k
Yellow	4	x 10 k
Green	5	x 100 k	±0.5%
Blue	6	x 1 M	±0.25%
Violet	7	x 10 M	±0.1%
Grey	8	x 100 M
White	9	x 1 G

Figure 2.1.1-Resistor value table

 

Resistor Dissipation

If the flow of current through a resistor increases, it heats up, and if the temperature exceeds a certain critical value, it can be damaged. The wattage rating of a resistor is the power it can dissipate over a  long period of time.

Wattage rating is not identified on small resistors. The following diagrams show the size and wattage rating

Most commonly used resistors in electronic circuits have a wattage rating of 1/2W or 1/4W. There are smaller resistors (1/8W and 1/16W) and higher (1W, 2W, 5W, etc).

In place of a single resistor with specified dissipation, another one with the same resistance and higher rating may be used, but its larger dimensions increase the space taken on a printed circuit board as well as the added cost.

Power (in watts) can be calculated according to one of the following formulae, where U is the symbol for Voltage across the resistor (and is in Volts), I is the symbol for Current in Amps and R is the resistance in ohms:

P = V.I

P = I² .R

P=V²/R

For example, if the voltage across an 820 Ω resistor is 12V, the wattage dissipated by the resistors is:

P = V²/R = 12 ²/820 = 0.176W = 176 mW.

A 1/4W resistor can be used. 

Nonlinear Resistors

Resistance values detailed above are a constant and do not change if the voltage or current-flow alters. 

But there are circuits that require resistors to change value with a change in temperate or light. This function may not be linear, hence the name NONLINEAR RESISTORS.

There are several types of nonlinear resistors, but the most commonly used include: NTC resistors (figure a) (Negative Temperature Co-efficient) - their resistance lowers with temperature rise. 

PTC resistors (figure b) (Positive Temperature Co-efficient) - their resistance increases with the temperature rise. 

LDR resistors (figure c) (Light Dependent Resistors) - their resistance lowers with the increase in light. VDR resistors (Voltage dependent Resistors) - their resistance critically lowers as the voltage exceeds a certain value.

 

Symbols representing these resistors are shown below.

Resistance adjustments are made via a screwdriver. Exception is the trim pot on the lower right, which can be adjusted via a plastic shaft. 

Particularly fine adjusting can be achieved with the trim pot in the plastic rectangular casing (lower middle). 

Its slider is moved via a screw, so that several full turns is required to move the slider from one end to the other end.