RESISTORS

RESISTOR COLOR CODES:

fig:1

Resistor values are always coded in ohms ( symbol Ω), capacitors in picofarads (pF), and inductors in microhenries (µH).

• band A is first significant figure of component value
• band B is the second significant figure
• band C is the decimal multiplier
• band D if present, indicates tolerance of value in percent (no color means 20%)

how to calculate:

1# always start from opposite end of tolerance band

2#calculate by formula

resistance value :color1 color2 X 10(power color3)

3#(optional) finally to that value add this tolerance

+/- (5/100)*R(for gold)

+/-(10/100)*R(for silver)

+/-(20/100)*R(no color)

let us make this a little clear with an example take example as fig:1

1st color is red means '2'

2nd color is violet means '7'

3rd color is green means '5'(multiplier)

4th color is tolerance i.e gold here

resistance value is: 27x10(power)5

i.e 2700000=2.7 mega ohms

for accurate value add tolerances also

i.e 2.7M +/- (5/100)*2.7M

2.7M+/- 135000

so the resistance may vary from 2835000 to 2565000 ohms

Resistors manufactured for military use may also include a fifth band which indicates component failure rate

Tight tolerance resistors may have three bands for significant figures rather than two, and/or an additional band indicating temperature cofficient, in units of ppm/k.

All coded components will have at least two value bands and a multiplier; other bands are optional

The standard color code per EN 60062:2005 is as follows:

Color	Significant figures	Multiplier	Tolerance		Temp. Coefficient (ppm/K)
Black	0	×100	–		250	U
Brown	1	×101	±1%	F	100	S
Red	2	×102	±2%	G	50	R
Orange	3	×103	–		15	P
Yellow	4	×104	–		25	Q
Green	5	×105	±0.5%	D	20	Z
Blue	6	×106	±0.25%	C	10	Z
Violet	7	×107	±0.1%	B	5	M
Gray	8	×108	±0.05%	A	1	K
White	9	×109	–		–
Gold	–	×10-1	±5%	J	–
Silver	–	×10-2	±10%	K	–
None	–	–	±20%	M	–
Any temperature coefficent not assigned its own letter shall be markd "Z", and the coefficient found in other documentation. For more information, see EN 60062

example2#:

As an example, let us take a resistor which (read left to right) displays the colors yellow, violet, yellow, brown. We take the first two bands as the value, giving us 4, 7. Then the third band, another yellow, gives us the multiplier 10⁴. Our total value is then 47 x 10⁴ Ω, totalling 470,000 Ω or 470 kΩ. Our brown is then a tolerance of ±1%.

Resistors use specific values, which are determined by their tolerance. These values repeat for every order of magnitude; 6.8, 68, 680, and so forth. This is useful because the digits, and hence the first two or three stripes, will always be similar patterns of colors, which make them easier to understand.

There are many mnemonics to remember the color code here are some of them :

Black Brown Red Orange Yellow Green Blue Violet Gray White

B. B. R O Y of Great Britain has Very Good Wife.
Bill Brown Realized Only Yesterday Good Boys Value Good Work
Bak B R O, Yeah Greasy Blubber's Very Grabable, Why?
Bad Boys Run Our Young Girls Behind VictoryGarden Walls.
Big Boys Race Our Young girls But Violet Generally Wins
Bye Bye Rosie Off You Go Bristol Via Great Western
Big Brown Rabbits Often Yield Great Big Vocal Groans When
Bye Bye Rosie Off You Go Birmingham Via Great Western

thanks to thanujakothamasu for reminding me this mnemonics

VARIABLE RESISTANCES:

Variable resistors consist of a resistance track with connections at both ends and a wiper which moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight track versions, usually called sliders, are also available.

Variable resistors may be used as a rheostat with two connections (the wiper and just one end of the track) or as a potentiometer with all three connections in use. Miniature versions called presets are made for setting up circuits which will not require further adjustment.

Variable resistors are often called potentiometers in books and catalogues. They are specified by their maximum resistance, linear or logarithmic track, and their physical size. The standard spindle diameter is 6mm.

The resistance and type of track are marked on the body:
4K7 LIN means 4.7 k linear track.
1M LOG means 1 M logarithmic track.

Some variable resistors are designed to be mounted directly on the circuit board, but most are for mounting through a hole drilled in the case containing the circuit with stranded wire connecting their terminals to the circuit board.

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Linear (LIN) and Logarithmic (LOG) tracks

Linear (LIN) track means that the resistance changes at a constant rate as you move the wiper. This is the standard arrangement and you should assume this type is required if a project does not specify the type of track. Presets always have linear tracks.

Logarithmic (LOG) track means that the resistance changes slowly at one end of the track and rapidly at the other end, so halfway along the track is not half the total resistance! This arrangement is used for volume (loudness) controls because the human ear has a logarithmic response to loudness so fine control (slow change) is required at low volumes and coarser control (rapid change) at high volumes. It is important to connect the ends of the track the correct way round, if you find that turning the spindle increases the volume rapidly followed by little further change you should swap the connections to the ends of the track.

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Rheostat

Rheostat Symbol

This is the simplest way of using a variable resistor. Two terminals are used: one connected to an end of the track, the other to the moveable wiper. Turning the spindle changes the resistance between the two terminals from zero up to the maximum resistance.

Rheostats are often used to vary current, for example to control the brightness of a lamp or the rate at which a capacitor charges.

If the rheostat is mounted on a printed circuit board you may find that all three terminals are connected! However, one of them will be linked to the wiper terminal. This improves the mechanical strength of the mounting but it serves no function electrically.

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Potentiometer

Potentiometer Symbol

Variable resistors used as potentiometers have all three terminals connected.

This arrangement is normally used to vary voltage, for example to set the switching point of a circuit with a sensor, or control the volume (loudness) in an amplifier circuit. If the terminals at the ends of the track are connected across the power supply then the wiper terminal will provide a voltage which can be varied from zero up to the maximum of the supply.

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Presets

Preset 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. For example to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets.

Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.

Multiturn presets are used where very precise adjustments must be made. The screw must be turned many times (10+) to move the slider from one end of the track to the other, giving very fine control.

Preset (open style)	Presets (closed style)	Multiturn preset

GENERALLY ALL VALUES OF RESISTANCES ARE NOT AVAILABLE DO U KNOW WHY???

different people say different reasons for this but my reason is very straight..

i think this is reason!!!

1k resistor is available but 500 ohm resistor is not available, this is from the fact that 500 ohms can be obtained

from connecting two 1k 's

i shall make my point little bit clear with another example 2.2k and 2.7k are available but why 2k is not available???

because it can be obtained by connecting two 1k's in series

not believing my words!!!! come on just check out by taking different examples and assess ur self!

*Resistors are available for different power levels:

like 0.25w,0.5w,1w,2w,5w etc based on the circuitry demand and application each of them are preferred!!

*Resistors are the contributors of NOISE in communication systems

NOISE can be two types in a communication system

1.external noise (due to transmission medium)

2.internal noise(due to resistance)

internal noise is produced by resistors which is also called THERMAL NOISE or WHITE NOISE

or JHONSONS NOISE

this noise is the random noise generated in a resistor or a resistive component of a complex impedance due to rapid and random motion of molecules,atoms and electrons

according to kinetic theory of thermodynamics the temperature of a particle denotes its internal kinetic energy. this means that the temperature of the body expresses the rms value of velocity of motion of particles in body. as per this theory kinetic energy of particles become apprx zero(zero velocity) at absolute zero.

therefore noise power produced in the resistor proportional to absolute temperature and also

the noise power is prop to bandwidth over which noise is measured

therefore expression for maximum noise power output of a resistor may be given as

p=kTB

where 'k' is boltzmann's constant

'T' is absolute temperature

'B' is bandwidth in hz