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.
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.
Can you explain why the shape of this graph suggests that a light bulb does not obey Ohm’s Law? |
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.)
Multiple choice question |
||
Do the matching exercise |
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:
As the current goes up, the thermistor gets hotter.
As it gets hotter, it allows more current to flow;
Therefore it gets hotter and so on.
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.
Why does a thermistor not obey Ohm's Law? |
You can investigate how temperature and resistance are related in a thermistor using equipment like this:
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 10^{0}, 10^{1}, 10^{2}, 10^{3}, 10^{4}... (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:
Smoke detection
Automatic lighting
Counting
Alarm systems.
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:
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. |
Answer the interactive 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.
(Harder) Can you use the graph to explain why a diodes allows a current to flow one way only? |
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 |
Identification
of Resistors with the Colour Code and
BS 1852 Code
Answer the interactive matching question on the resistor colour codes. |
Answer the gap-fill exercise. |