Components - The Resistor

Resistors are probably the most common component used in basic electronic circuits.

Resistors are use to restrict the flow of electric current, for example a resistor is placed in series with a light-emitting diode (LED) to limit the current passing

through the LED.

Circuit SymbolExample

VCC +

Gnd (0V)

R

LED

The Resistor Colour Code - 4 Band

The Resistor Colour Code

Colour Number

0

1

2

3

4

5

6

7

8

9

±5%

±10%

Commonly used resistors have four colour bands to represent the value of resistance. Three bands represent the value

and the forth band for the tolerance.

The first band gives the first digit of the value.

The second band gives the second digit of the value.

The third band indicates the number of zeros to be added to the value.

This resistor has brown (1), red (2), orange (3) value bands.Indicating that its nominal value is 12,000 or 12k

The sign is usually omitted on circuit diagrams and the value is written as 12k.

The gold tolerance band indicates ± 5% about the nominal value.

The actual value can lie between 11,400 and 12,600.

The Resistor Colour Code

Activity

1. A resistor has colour bands yellow, violet, orange what is its value (nominal).

2. A resistor has brown, black, red value bands and a gold tolerance band. What is the nominal value and value range for this component.

3. You are asked to obtain a 68k resistor, explain how would you recognise this.

4. A 36k and a 27k resistor are connected in series what would the be the nominal value for this combination. If they both had gold tolerance bands what would the be the upper and lower limits.

The Resistor Colour Code - 5 Band

The Resistor Colour Code

Colour Number

0

1

2

3

4

5

6

7

8

9

As component manufacturing techniques became more refined (use of laser trimming) and the use of high stability

materials (metal film) instead of carbon composition.

It became possible to manufacture much closer tolerance components (0.5%, 1% and 2%) allowing the production of

more values in a range.

Three bands represent the digits of value and the forth band the number of zeroes to be added. The fifth band is for the

tolerance.

What value is shown in the diagram?

Preferred ValuesTo produce a sensible range of resistor values you need to increase the size of

the 'step' as the value increases

Standard values form a series which follows the same pattern for every multiple of ten these are known as preferred values.

E6 (20%)6 values for each multiple of ten, 10, 15, 22, 33, 47, 68 and continues 100, 150, 220,

330, 470, 680, 1000 etc. Notice how the step size increases as the value increases.

For this series the step to the next value is roughly half the value.

E12 (10%)12 values for each multiple of ten, 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and

continues 100, 120, 150 etc. (E6 series with an extra value in the gaps)

E24 (5%)24 values for each multiple of ten, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33,

36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91 and continues 100, 110, 120, 130 etc. (E12 series with an extra value in the gaps)

Film Resistors

Electrical energy is converted to heat when current is passed through a resistor, however in most cases this energy is so small that the heat

produced can be ignored.

The amount of energy a resistor can safely convert to heat is dependant on its power rating.

Where the energy is small the most common types of resistor is are the carbon and metal film.

Film resistors have values ranging from tens of ohms to million of ohms.Power rating for film resistors is from 0.125Watt to 1Watt.

Power Resistors

In some cases it is necessary to pass a high current through a resistance which can result in heat being generated in the component.

We must ensure that the component continues to operate to specification and no damage is caused as a result of this current.

The heat generated is also known as power dissipation.

Most power resistors are wirewound (not film) and very high power types are encased in an aluminium cladding that acts as a heat sink or designed to be

mounted on a heat-sink to remove the heat from the component.

Variable Resistors (potentiometer)

Standard single turn Precision multi-turn

Rotary potentiometers

Single turn

Preset potentiometers

10 turn precision

A potentiometer is a resistor that can have its value adjusted from zero to the value stated on its case.

Standard potentiometers are used for amplifier controls by the equipment user and

can have either a rotary or slider wiper. Potentiometers used for volume controls have a logarithmic scale.

Multi-turn rotary potentiometers used for precision instrumentation control applications operated by the equipment user.

Preset potentiometers are mounted directly on the circuit board and used for setting up and calibration by the manufacturer. Presets are not accessible to

the equipment operator.

Construction of a Potentiometer

Construction of a Carbon Track Potentiometer and Circuit Symbol

Terminals

Control shaft

Sliding contact (wiper)

Carbon track {

Variable resistors consist of a resistance track with a connection at either end and a wiper that moves along the track as you rotate the spindle, see diagram.

The track can be made from carbon, cermet (matalised ceramic) or turns of resistance wire.

θ rotation

0 330

Res

ista

nce

Lin

Log

Construction of a Carbon Track Potentiometer and Circuit Symbol

Light Dependant Resistor (LDR)Light dependant resistors use a semiconductor material whose resistance varies

according to the amount of light falling on it. Light provides energy to set free electrons in the semiconductor material (cadmium sulphide) thus increasing

conductivity, (reducing its resistance).

ORP12 Symbol Response

light

Res

ista

nce

A common device is the ORP12 its resistance can range from 10M in darkness to 500 in full light. LDR’s are used

Application of a PotentiometerThe potentiometer is commonly used in amplifiers as a volume control.

This control ‘sits’ between the pre-amplifier and the power amplifier and is used to limit the amount of signal fed to the power amplifier.

Pre-amp Power ampOutput to Speaker system

Input signal

gnd (0v)