﻿VK5AJL - resistance and resistors.

RESISTORS
BASICS

Resistors are the simplest of components but they are used almost everywhere. Their values are most often indicated using colour bands. There are so many references to these on the internet, why bother here. Resistor colour codes are available in Wikipedia.

POWER - VOLTAGE - CURRENT

When a voltage is applied to a circuit with a finite impedance, a current flows. When this happens power can be dissapated by elements in the circuit. These elements are always resistive. Even power radiated by an antenna is radiated by the resistive component. Perfect capacitors and inductors do not dissipate power but there is no such thing as a perfect capacitor or inductance. They have resistive components and it is these components which dissipates power. This might just be the resistance of the wire used to make a coil or capacitor.

The value of resistance is therefore important because of its relationship to power rather than anything else. Resistance and Impedance are two different things. They have the same value only when there is no reactive (capcitive or inductive) component.

The following equations are valid for purely resistive circuits only:-

Where:-
V = potential across a resistance (volts)
I = current through a resistance (amps)
R = value of resistance (ohms or Ω)
W = power dissipated by the resistance (watts)

RESISTANCE OF COMBINED VALUES

RESISTOR TYPES

Resistors as components come in many shapes and sizes. The most common are either carbon or metal film types. There would also probably be more ¼W than ½W used in electronics manufacture but the most common these days would probably be ⅛W or less surface mount.

Care should be taken when selecting a resistor because most are wound in some way. Even carbon and metal film resistors can have windings around the central piece of ceramic used to build the component on. This makes the resistor slightly inductive. (It is inductive anyway because it isn't infinitely short - see reactance). At lower fequencies, this inductance is so low as to be negligible but, as the frequency increases, the effects become more significant.

Another vey important consideration is the positive temperature characteristics of a resistor. As the temperature of the component increases, so does the resistance. For more critical circuits, it is better tp use a much bigger resistor than required and to mount it away from the circuit board.

A more detailed discussion about resistor manufacture is available on Wikipedia.

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