RESISTORS
(Background information is available in the downloadable booklet at: http://www.southwest.com.au/~jfuller/electronics/electronic1.htm )

In terms of electrical conductance, substances are described as: "conductors", "insulators", or "semi-conductors". Conductors allow electricity to flow through them. Insulators prevent electricity from flowing through them while semi-conductors are poor conductors, or poor insulators. They allow the partial flow of electricity.

Typical electrical conductors are mercury, iron, copper, gold and silver (in fact apart from some special alloys like a mixture of Nickel and Chromium, ALL metals are good conductors of electricity).

Non-conductors, or insulators are generally the non-metals like the gases, sulphur, etc. Paper, rubber and most plastics are non-conductors (or "insulators") of electricity.

The most common "semi-conductors" are Carbon and Silicon.

Resistors are devices which make use of poor conductors to limit the flow of electricity through a circuit. Resistors are generally made of substances which only partially conduct electricity such as carbon, special alloys and some metal oxides. A high value of resistance will allow less current to flow than a low resistance.

Symbol for a Resistor

The unit of resistance is the "ohm" (pronounced "owm" as in show). The symbol for resistance is the Greek letter omega:
A light globe has a resistance in the order of tens of ohms. Your skin has a resistance in the order of millions of ohms.

When current flows through a resistor it heats up due to the voltage drop across the resistor. The amount of heat a resistor can handle is indicated by its "power rating" or "wattage". Common carbon resistors have a power rating of half a watt. Where larger ratings are required the resistor may be made of a spiral of Nickel-Chromium alloy which is able to handle much higher currents and much more heat. These resistors are usually termed: "wire-wound" resistors. They are physically much larger than Carbon and metal-film resistors.

Resistors are constructed to provide predetermined resistances. Most common resistors are guaranteed to be within 5% of their marked value. ('Metal-oxide' resistors with a blue body are guaranteed to meet their marked value plus, or minus 1%.)

In the early days of electronics, resistors were large enough to have their resistance printed directly onto the body of the device. Modern resistors, however are far too small to allow values to be marked and use a "colour code" consisting of 'bands' painted onto the device. Each colour and its position represents a specific value.

The values represented by each colour are:

 
Reading Resistance Values:
On 5% resistors (ie the ones that don't have a blue body) the first two bands represent the first two digits in the number. The third band represents the "multiplier" ie the number of zeros to be added after the first two numbers. The value is in 'ohms'.

eg1	The digits are red, red - therefore: 2, 2 The multiplier is orange - therefore: 000 (ie three zeros) The value is therefore: 2 2 0 0 0 ohms, or 2.2 thousand ohms, or 2.2kohms, or 2.2 k, or 2k2.

eg2	The digits are brown, black - therefore: 1, 0 The multiplier is black - therefore: nil (ie no zeros) The value is therefore: 10 ohms, 10. The common mistake is to interpret this combination as 100. The last band says there are ZERO zeros after the first black.

1% Tolerance Resistors
High accuracy resistors are made using a metal-oxide film, rather than Carbon. These resistors have a blue body and four colour bands instead of three. The same colour code system applies, but there are three 'digit' bands and one 'multiplier' band.

On metal-oxide resistors the Tolerance bands are: Brown = +/- 1%, Red = +/- 2%

First digit is red - therefore: 2   Second digit is red - therefore: 2   Third digit is orange - therefore: 3   The multiplier is Brown - therefore: 1 zero The value is therefore: 2230 ohms, or 2.23k

Standard Resistor Values - The E12 Series:
With 5%Tolerance resistors, there is no point in making values closer than 10% apart. If a 1000ohm resistor can only be guaranteed to be somewhere within the range 950ohm to 1050ohm, there is no point trying to market a 980ohm resistor, for example.

To cater for the 'overlap' in resistor values, a standard 'series' of values is used. The most common series is called the "E12 series" and was developed in the days of +/- 10% values. (You can confirm the 'overlap' status of this range for yourself.) The E12 series is:

1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2

in 'tens' this would become:

10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82

in 'hundreds:

100, 120, 150, 180, 220, 270, 330, 390, 470, 560, 680, 820

and so on ....

When designing projects using the E12 series resistors the 'nearest' value in the series is chosen. Since the manufacturers only guarantee their product to be within +/- 5% of the marked value there seems little point in worrying about precisely calculated values.

NOTE: You can purchase exact values down to the several decimal places, but the cost is significant. A special value resistor for a digital multimeter, for example, may cost up to $10 !!
 
If guaranteed accuracy is required, the 1% 'metal-film' resistors are used. These values are close enough for all but high precision applications.

The following Modules in this Course will discuss the applications of Resistors in electronic circuits.

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