This Chapter describes fixed value resistors and outlines some of the important uses of resistors in electronic circuits.

What do resistors do?

Resistors limit current. In a typical application, a resistor is connected in series with a Light Emitting Diode (LED):

Enough current flows to make the LED light up, but not so much that the LED is damaged. Later in this Chapter, you will find out how to calculate a suitable value for this resistor. (LEDs are described in detail in Chapter 5.)

The 'box' symbol for a fixed resistor is popular in the UK and Europe. A 'zig-zag' symbol is used in America and Japan:

Resistors are used with transducers to make sensor subsystems. Transducers are electronic components which convert energy from one form into another, where one of the forms of energy is electrical. A light dependent resistor, or LDR, is an example of an input transducer. Changes in the brightness of the light shining onto the surface of the LDR result in changes in its resistance. As will be explained later, an input transducer is most often connected along with a resistor to to make a circuit called a potential divider. In this case, the output of the potential divider will be a voltage signal which reflects changes in illumination.

Microphones and switches are input transducers. Output transducers include loudspeakers, filament lamps and LEDs. Can you think of other examples of transducers of each type?

In other circuits, resistors are used to direct current flow to particular parts of the circuit, or may be used to determine the voltage gain of an amplifier. Resistors are used with capacitors to introduce time delays.

Most electronic circuits require resistors to make them work properly and it is obviously important to find out something about the different types of resistor available, and to be able to choose the correct resistor value, in , , or M, for a particular application.

Fixed value resistors

The diagram shows the construction of a carbon film resistor:

During manufacture, a thin film of carbon is deposited onto a small ceramic rod. The resistive coating is spiralled away in an automatic machine until the resistance between the two ends of the rod is as close as possible to the correct value. Metal leads and end caps are added, the resistor is covered with an insulating coating and finally painted with coloured bands to indicate the resistor value.

Carbon film resistors are cheap and easily available, with values within �10% or �5% of their marked, or 'nominal' value. Metal film and metal oxide resistors are made in a similar way, but can be made more accurately to within �2% or �1% of their nominal value. There are some differences in performance between these resistor types, but none which affect their use in simple circuits.

Wirewound resistors are made by winding thin wire onto a ceramic rod. They can be made extremely accurately for use in multimeters, oscilloscopes and other measuring equipment. Some types of wirewound resistors can pass large currents wihtout overheating and are used in power supplies and other high current circuits.

Colour code

How can the value of a resistor be worked out from the colours of the bands? Each colour represents a number according to the following scheme:

The first band on a resistor is interpreted as the FIRST DIGIT of the resistor value. For the resistor shown below, the first band is yellow, so the first digit is 4:

The second band gives the SECOND DIGIT. This is a violet band, making the second digit 7. The third band is called the MULTIPLIER and is not interpreted in quite the same way. The multiplier tells you how many noughts you should write after the digits you already have. A red band tells you to add 2 noughts. The value of this resistor is therefore 4 7 0 0 ohms, that is, 4 700 , or 4.7 . Work through this example again to confirm that you understand how to apply the colour code given by the first three bands.

The remaining band is called the TOLERANCE band. This indicates the percentage accuracy of the resistor value. Most carbon film resistors have a gold-coloured tolerance band, indicating that the actual resistance value is with + or - 5% of the nominal value. Other tolerance colours are:

When you want to read off a resistor value, look for the tolerance band, usually gold, and hold the resistor with the tolerance band at its right hand end. Reading resistor values quickly and accurately isn't difficult, but it does take practice!

More about colour codes

The colour code as explained above allows you to interpret the values of any resistor from 100 upwards. How does the code work for values less than 100 ? Here is the code for 12 :

brown red black

The multiplier colour black represents the number 0 and tells you that no noughts should be added to the first two digits, representing 1 and 2.

What would be the colour code for 47 ? The answer is:

yellow violet black

Using this method for indicating values between 10 and 100 means that all resistor values require the same number of bands.

For values bewteen 1 and 10 , the multiplier colour is changed to gold. For example, the colours:

brown black gold

indicate a 1 resistor, while the colours:

red red gold

refer to a 2.2 resistor.

Metal film resistors, manufactured to 1 or 2% tolerance, often use a code consisting of four coloured bands instead of three. The code works in the same way, with the first three bands interpreted as digits and the fourth band as the multiplier. For example, a 1 metal film resistor has the bands:

brown black black brown (+brown or red for tolerance)

green blue black red

It is worth pointing out that the multiplier for metal film resistors with values from 1 upwards is brown (rather than red, as in the three colour system), while the multiplier for 10 upwards is red (instead of orange).

You are likely to use low value resistors and metal film resistors on some occasions and it is useful to know how to read their codes. However, most of the resistors you use in building electronic circuits will be carbon film types with values indicated using the three band colour code. It is this system which you should master first.

Free download Color Code Convertor Program.

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