Tutorial 4  Characteristics of Components

We can easily measure voltage and current, using the data to plot voltage current graphs.  We use the following circuit, which you probably did in Year 10 (the 4th Year):

The variable resistor is there to change the voltage and the current.  A variable power supply (like a lab-pack) will also do the job, and a variable resistor is not needed.

Remember that the voltmeter is connected in parallel across the component; the ammeter is connected in series.

The photograph shows a typical experiment:

 

The investigation of voltage-current characteristics lends itself well to data-logging techniques.  The voltmeter and ammeter sensors are wired in exactly the same way as ordinary meters.  They are then connected to the computer. 

From this circuit we take readings of voltage and current plotting them as a graph called a VI characteristic.

We normally put the voltage on the y-axis and current on the x-axis.  This allows us to determine the resistance from the gradient. This is a voltage current graph for an ohmic conductor:

The straight line shows a constant ratio between voltage and current, for both positive and negative values.  We say that the voltage is directly proportional to the current.  This means that the graph is a straight line of positive gradient going through the origin.   So when the voltage is negative, the current is negative, i.e. flowing in the opposite direction.  Ohm’s Law is obeyed.

For a filament lamp we see:

The resistance rises as the filament gets hotter, which is shown by the gradient getting steeper.

 

Question 1

Can you explain why the shape of this graph suggests that a light bulb does not obey Ohm’s Law?

Answer

 

 

Resistive Transducers

A resistive transducer is a device that senses a change to cause a change in resistance.  Transducers do NOT generate electricity.  Examples include:

 

Device

Action

Where used

Light Dependent Resistor

Resistance falls with increasing light level

Light operated switches

Thermistor

Resistance falls with increased temperature

Electronic thermometers

Strain gauge

Resistance changes with force

Sensor in an electronic balance

Moisture detector

Resistance falls when wet

Damp meter

These are called passive devices.  (Active transducers do generate electricity from other energy sources, or have a power supply.)

 

Question 2

Multiple choice question

Question

Question 3

Do the matching exercise

Question

 

A thermistor (a heat sensitive resistor) has a resistance that goes down as it gets hotter.  This is because the material releases more electrons to be able to conduct. 

 

Notice how the gradient is decreasing, indicating a lower resistance.  There is, however, a health warning:

This is called thermal runaway, and is a feature of many semi-conductor components.  At the extreme the component will glow red-hot, then split apart.  

The thermistor is used wherever any electronic circuit detects temperature:

Here we see a thermistor protecting a power supply from too high a temperature.

Question 4

Why does a thermistor not obey Ohm's Law?

Answer

You can investigate how temperature and resistance are related in a thermistor using equipment like this:

 

Light Dependent Resistors

The light dependent resistor consists of a length of material (cadmium sulphide) whose resistance changes according to the light level.    Therefore the brighter the light, the lower the resistance. 

 

 

We can show the way the resistance varies with light level as a graph:

 

 

The first graph shows us the variation using a linear scale.   The graph on the right shows the plot as a logarithmic plot, which comes up as a straight line.  Logarithmic plots are useful for compressing scales.  The picture on the right shows how an experiment can be carried out

 

Maths Window

A logarithmic scale is based on powers of 10.  In a logarithmic graph, the scale does not go 0, 1, 2, 3, etc.  It goes 100, 101, 102, 103, 104... (1, 10, 100, 1000, 10000...)  We say that the scale goes up in decades.  By using a logarithmic scale, we can put a very large range of values in a small space, i.e. a side of A4 graph paper instead of several hundred metres.

 

In the exam

You will see logarithmic graphs.  You will be expected to read values from them.  The axes will be calibrated, but you will need to take care.  If you are asked to sketch a characteristic graph, make sure you make it clear that it is a graph with logarithmic scales.

 

LDRs are used for:

Resistive components can get hot when excessive current is flowing through them, and this can impair their function, or damage them.  This can be prevented by connecting a current limiting resistor in series, as shown in the picture below. 

 

Worked Example

At a certain light level, a light dependent resistor has a resistance of 100 ohms.  It can only handle a current of 10 milliamps before it risks heating up.  If the LDR is connected to a 20 V supply, what value resistor should we place in series?

We know two things about a series circuit:

  • The current is the same all the way round;

  • The voltages add up.
     

So we can say that the current through the series resistor will be 10 mA.

Therefore we can work out the voltage across the LDR:
 

Remember that 10 milliamps = 0.01 amp.
 

V = IR = 0.01 A × 100 ohms = 1 volt.
 

Therefore we can say that the voltage across the series resistor is 20 - 1 = 19 volt
 

So we now work out the resistance with R = V/I = 19 volts ÷ 0.01 = 1900 ohms.

 

 

Question 5

Answer the interactive question

Question

 

 

 

Diodes

Diodes are semi-conductor devices that allow electric current to flow one way only

The circuit to measure the characteristic of a diode is like this, based on a potentiometer.

The potentiometer allows a range of values from 0 volts to the battery voltage.  We will look at the potentiometer in more detail in a later tutorial.

The diode characteristic graph looks like this:

The diode starts to conduct at a voltage of about +0.6 V.  We call this forward bias.  Then the current rises rapidly for a small rise in voltage. If the current is reversed (reverse bias) almost no current flows until the breakdown voltage is reached.  This usually results in destruction of the diode.

Question 6

(Harder) Can you use the graph to explain why a diodes allows a current to flow one way only?

Answer

 

 

 

The E24 Series of Resistors

Electronic engineers use standard values of resistor which are given by the E24 series of resistors.  If we work out that we need a resistor of 460 ohms, it is usually OK to use a 470 ohms resistor.  If it's critical to have 460 ohms, we can always use a 430 ohm resistor in series with a 30 ohm resistor.  Often a resistor is wired in series with a small variable resistor, called a trim pot, to adjust the voltage across it if a critical value is needed.  The trim pot is adjusted with a screwdriver until the desired value is achieved.

 

 

E24 series

10

11

12

13

15

16

18

20

22

24

27

30

33

36

39

43

47

51

56

62

68

75

82

91

 

This set of values is marketed in decades, or powers of 10:

 

Power of 10

0

1

2

3

4

5

6

×

1

10

100

1000

10000

100000

1000000

 

So we can have values such as:

            47 W, 1500 W, 120 W, 68000 W, 5600000 W, etc.

 

 

Identification of Resistors with the Colour Code and BS 1852 Code

Resistors can make a circuit rather colourful, but that's not really the intention.  The colours have significance.

 

 

The values of the bands are shown in the table below:

 

Colour

Black

Brown

Red

Orange

Yellow

Green

Blue

Violet

Grey

White

Gold

Silver

Value

0

1

2

3

4

5

6

7

8

9

 

 

Multiplier

100

101

102

103

104

105

106

107

108

109

10-1

10-2

Tolerance

(%)

 

± 1

± 2

 

 

± 0.50

± 0.25

± 0.10

± 0.05

 

± 5

± 10

For example a resistor marked red orange yellow gold would give:

 

            23                                 Multiplier 104 (= 10000),                           tolerance ± 5%

 

            23 × 10000 = 23 × 104 W = 230 kW

 

A tolerance of ± 5 % means that the resistor would have a value between 220 kW and 240 kW.

 

Question 7

Answer the interactive matching question on the resistor colour codes.

Question

 

Some resistors have their values marked not in colours but in numbers and letters.  They use the BS1852 resistance code.

Instead of the W the resistors are marked with the letters R, K or M.  The R, K or M denotes the decimal point:

  There are other suffix letters that denote the tolerance:

 

Letter

Tolerance (±%)

F

1

G

2

J

5

K

10

M

20

Be careful not to confuse the letter K with the K for kilohms.  68KK = 68 kW ± 10%

 

Question 8

Answer the gap-fill exercise.

Question