TEMPERATURE CO-EFFICIENT OF RESISTANCE The temperature co-efficient of resistance is defined as;

The Fractional change in resistance from 0ºC, per degree temperature change.

and may be represented graphically as shown below.

The graph is reasonably linear for many materials over a moderate temperature range (0º - 200ºC).

The units are ºC because the ohms cancel out in the calculation.

Materials whose resistance increases with increasing temperature have a positive temperature co-efficient of resistance.

Materials whose resistance decreases with increasing temperature have a negative temperature co-efficient of resistance.

Some materials have very small temperature co-efficients of resistance and are used where it is important that the resistance does not change with temperature.

JAR 66 CATEGORY B1

The electrical component used to introduce resistance into a circuit is called a resistor. Resistors can be fixed or variable. Symbols used in circuit diagrams are shown below:

Resistor Type Old Symbol New Symbol Fixed resistor

The physical size of a resistor does not give any clue to the resistance value of the component. This value must be marked on individual components. Two codes are currently used to indicate resistor values: a Colour Code and a Letter and Digit Code.

8.4.1 FIXED RESISTORS Fixed resistors may be:

• Wire wound. Special resistance wire is wound onto a former. The wire wound resistor can dissipate heat easily and is therefore used when larger currents are expected (the larger the current the greater the heat produced).

These resistors are usually larger than other types. The student should note that size does not indicate resistance value, but depends upon the heat to be dissipated.

• Carbon Composition, Metal Oxide and Metal Film. Resistors made from carbon composition or from metal films and oxides are usually small. They are therefore used where the currents are kept small.

JAR 66 CATEGORY B1

The current method of colour code marking of resistors is the Band System.

Close to one end of the resistor are four coloured bands (there may appear to be only three, in this case the forth band is ‘no colour’

– see diagram below). They are known as bands 1 – 4. Bands 1 and 2 give the first two numbers of the resistor value, band 3 gives the multiplication factor, i.e. the number of zeros, the fourth band gives the tolerance, which indicates how close the actual value may be to the stated value.

Colour First band

Certain resistors remain very close to their stated value, despite temperature

JAR 66 CATEGORY B1

High value resistors. High value resistors may have three significant figures. If the colour code is used here, the first three bands represent figures, the fourth band is the multiplier and the fifth band is the tolerance. For example, a resistor of value 249,000Ω + 1% would be coded as shown below:

First band Red is 2

Second band Yellow is 4

Third band White is 9

Fourth band Orange is 3 zeros

Fifth band Brown Tolerance + 1%

Note: To avoid possible confusion, the fifth band is 1.5 times to 2 times wider than the other bands.

8.4.3 PREFERRED VALUES AND TOLERANCES

In practical electrical circuits the precise value for a resistor is not usually critical.

It is more economic to produce large tolerance resistors than low tolerance ones.

The number of resistor values required to cover a given range of resistance depends on the tolerance of the resistors being used. An example of resistor Preferred Values for 10% is given in the table below.

1 10 100

Note that the upper and lower tolerance resistance limits of each preferred value cover the complete range;

eg. 2.2KΩ + 10% = 1.98KΩ to 2.42KΩ

JAR 66 CATEGORY B1 MODULE 3 (part A)

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8.4.4 LETTER & DIGIT CODES

In this code the numbers are printed on the body of the resistor to indicate its value. In addition, letters are used to indicate the multiplying factor (eg, MΩ) and the tolerance as shown below.

Multiplying Factor Tolerance %

X1 R

(resistor)

Ω 0.1 B 5 J

X10³ K KΩ 0.25 C 10 K

X10⁶ M MΩ 0.5 D 20 M

X10⁹ G GΩ 1.0 F 30 N

X10¹

2 T TΩ 2 G

The position of the multiplying letter is also used to indicate the decimal point position.

eg. 470R is 470Ω

4K7 is 4—7KΩ

R47 is 0—47Ω

4R7 is 4—7Ω

The tolerance letter is added on the end.

eg. 1M5 B is 1—5MΩ + 0.1%

2K2 N is 2—2KΩ + 30%

Other markings may also be used in the code to represent date of manufacture.

They are placed after the value and tolerance markings.

8.4.5 POWER RATING

Resistors are rated according to their resistance value and also to the rate at which they can dissipate heat. Rate of heat dissipation is measured in watts.

(The watt will be discussed later in the course). The higher the wattage rating the more current it can carry.

JAR 66 CATEGORY B1 MODULE 3 (part A)

ELECTRICAL FUNDAMENTALS

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8.4.6 POTENTIOMETERS

A variable resistor arranged so as to control voltage in a circuit is called a ‘Potentiometer’ and controls the potential difference between two points in a circuit. It is used to ‘tap off’ part of the supply or signal voltage for connection to a load. See diagram.

8.4.7 RHEOSTATS

Variable resistors can be made to vary either current or voltage. A variable resistor arranged to control current is called a ‘Rheostat’ and controls the current by varying the resistance in the circuit. See diagram.

8.4.8 VOLTAGE DEPENDENT RESISTORS

Some components do not obey Ohm’s law, that is the current flow through them does not vary linearly as the applied voltage is varied. These elements are known as non-linear resistors or non-linear conductors. Transistors, diodes and voltage dependent resistors all fall into this group.

The current through a voltage dependent resistor increases at a progressively rapid rate as the voltage across it increases, such a device is used for protecting circuits against voltage surges or as a voltage stabiliser.

8.5 THERMISTORS

Insulators and semi-conductors behave in a different way when the temperature increases, because their resistivity decreases. That is: the resistance of an insulator and of a semi-conductor decreases with temperature increase, (their resistance-temperature coefficient is negative!). This feature can be used to advantage as the following example shows.

JAR 66 CATEGORY B1 MODULE 3 (part A)

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One example of this effect occurs in a thermistor, which is a thermally sensitive resistor whose resistance alters with temperature; a negative temperature coefficient (n.t.c.) thermistor is one whose resistance reduces with increase in temperature. A thermistor is used in the cooling-water temperature-measuring circuit of a car or lorry; it is inserted in the cooling water and connected in series with the battery and temperature gauge. As the water temperature rises, the resistance of the n.t.c. thermistor falls and allows more current to flow through the temperature gauge; this causes the gauge to indicate variations in water

temperature.

JAR 66 CATEGORY B1 MODULE 3 (part A)

ELECTRICAL FUNDAMENTALS

engineering uk