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Specification and Selection of Resistors


Resistors are used in electricalcircuits to:

  • Reduce the voltage applied to another component(s)
  • Limit the electrical current flowing through another component(s).


A "package" of resistance made up into a single unit is called a resistor. Resistors having the same resistance value may be considerably different in size and construction. The flow of current through resistance causes the conductor to become heated; the higher the resistance and the larger the current, the greater the amount of heat developed. Resistors intended for carrying large currents must be physically large so the heat can be radiated quickly to the surrounding air. If the resistor is unable to dissipate the heat quickly it may reach a temperature that will cause it to melt or burn.

Types of Resistor

Several types of resistors are available including:

“Fixed” value small components with wire leads are commonly either carbon film or metal film types. These range in size from 0.125 Watt to 2 Watts. Fixed Resistor
Larger fixed values resistors manufactured using wire wound resistance; available in the range 2 Watts to 100 Watts. Large Fixed Value Resistor
Resistor Network components that comprise a number of separate film resistors contained within a single integrated package. Resistor Network
Adjustable “potentiometer” and pre-set types, comprising a sliding resistance on an exposed section of resistance winding or film. Ajustable Potentiometer

Specification and Calculation of Resistance

Three (3) separate criteria determine the suitability of a resistor for use within any circuit:

Resistance Value

The resistivity of fixed value resistors are measured in "Ohms" - designated as

Resistance formula

Resistance is calculated using the "Ohms Law" formula:

where Voltage = voltage across the resistor expressed in “Volts”

and Current = current flowing through the resistor expressed in "Amperes" (or "Amps")


The resistivity of a resistor is manufactured to be within a specified “tolerance” of the component’s nominal value. Modern fixed resistors are available with tolerance limits of ±1%, ±2%, ±5%, or ±10%.


Power formula

The amount of power dissipated as heat (or energy) is measured in "Watts", and is calculated using the formula:

where Voltage = voltage across the resistor expressed in “Volts”

and Current = current flowing though the resistor expressed in "Amperes" (or "Amps")

Resistor Colour Code

"Fixed" value carbon film or metal film types are generally colour coded to signify their value and specification. The original method of colour coding components used a 4 Band code, but this is being superseded by a 5 Band colour code.

Resistor Colour Code


We offer you a neat little programme you can download which will illustrate any desired resistor value.

Programme screenshot

To install the programme simply download this .zip file, extract all the files from it to a folder of your choice. This is referred to later as "Your Folder". If you want an Icon on your Desktop simply right click on the Icon for CreateResistor.exe in your folder, select Create Shortcut, and drag the created shortcut Icon onto your desktop.

CreateResistor does not use any registry resources and can simply be removed by deleting the installation folder and its contents.

CreateResistor is © Trevor Stockill 2010. This version (5.01) is supplied free of charge to members and prospective members of MERG.

This programme is provided as is. No Warrantee is given or implied as to its accuracy or suitability for any use or purpose.

Page author: C.J. Dadson

Last updated: 14th March 2010

Re-formatted: 28 June 2010

Copyright © 2010 Model Electronic Railway Group