The Resistor

We begin by looking in detail at the resistor, used in PLC'S, Drives, Temperature Controllers, Computers - Televisions - Radio - Walkman - Playstation - Automobiles etc etc. In fact virtually every electronic device in the world today uses this component, without the resistor all current control, biasing networks and feedback in amplifiers would be impossible to achieve.
It is interesting to note that the current flow through a resistor is proportional to the voltage across it, this will be shown later R = V/I. Any conductive material has its resistive content dependent upon its length. the longer a piece of wire - the higher its resistance, the shorter its length and/or the greater its diameter - the lower the resistance value will be. However not all resistors are made from wire. 

There are several types of resistor, these differences effect the components Stability, Tolerance and Power rating. The figures below are general and depend upon the manufacturer.

TYPE TOLERANCE POWER (watts) STABILITY OHMIC RANGE

Carbon film

5 – 20% 0.25 - 2 Poor above 150ppm 1-10M

Metal film

0.5 – 5% 0.125 - 0.6 Good 50-100ppm 1-1M

Precision metal film

0.1 – 0.5% 0.25 - 1 Excellent 15-50ppm 20 - 1M

Power oxide

+/-5% up to 3 Reasonable 300ppm 1 - 1M

Wire wound

+/-5% 2 - 1000 Reasonable 100ppm 0.1 - 100K

Carbon resistors are the most generally used resistor type in the world today. They are cheap, have a good range of values available and their power ratings are ideal for Radios and Televisions.
Metal film are used in measurement instruments where stability is paramount, more expensive than carbon but again have a good range of values. 
Wirewound are probably the next most widely used resistor. Wherever current control in high voltage situations eg: 110volts and above is required, the wirewound is more suited. The power rating can support up to 1000watts and their stability at this rating is quite reasonable considering that power generates heat which effects stability. If you look at any resistor specification it will tell you how stable it is over various temperature ranges, you then have to decide which type is necessary for the application and whether some additional heat dissipating device may be required eg: heatsink, fan or just extra ventilation.
Before we move on we should be aware of the terminology used in electronics when discussing resistors:

Values

  • m or milli = milli Ohms (tenths or hundreths of an Ohm)
  • R = Ohms used when discussing value 0.5 - 999 Ohms
  • K = Kilo Ohms
  • Meg or M = Mega Ohms (mega meaning millions)

Power

  • w = watts therefore Kw means Kilo Watts, mw = milli watts

 


Construction
Usually the resistor is made by coating or winding wire onto a ceramic rod. The coating process is done by passing the rod through a vapour which deposits the film oxide onto the rod surface.

< film on surface of the rod

The rod is then machined so that the conductive material resembles a resistive length of wire.

The colored bands on the left side, refer to its value (this value is 1000 ohms or 1Kohm) and the band on the right refers to the components tolerance (Gold is 5%), this means that the value of this component will be within +/- 5% of 1Kohm. (Silver is 10%) for the values study the color code table below.

 

Colour Code:

Band1 - 1st number
Band2 - 2nd number
Band3 - multiplier
some resistors have 4 bands, the 4th being the multiplier.
Brown = 1 Black = 0 Red = 2
 
Therefore value = 1 0 00 = 1000

  COLOUR CODE
Brown   1
Red   2
Orange   3
Yellow   4
Green   5
Blue   6
Purple   7
Grey   8
White   9
Black   0
     

Therefore a marking of Red-Green-Orange would be 25000 or 25Kohms. 

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