If you’ve connected the circuit up correctly, the LED should now be on. This proves that current is flowing. To calculate exactly what current we can use Ohm’s law. Let’s assume that the total battery voltage of 9 V is dropped across the resistor and that no voltage occurs across the two diodes. In fact, there is voltage across the diodes, but we needn’t worry about it yet, as it is only a small amount. We’ll measure it, however, soon.
Now, with a resistance of 1k5, and a voltage of 9 V, we can calculate the current flowing, as:
The next thing to do is to turn around the diode, so that its cathode is more positive, as in the circuit of Figure 6.5. The breadboard layout is the same with anode and cathode re- versed, so we needn’t redraw it.
Figure 6.5 The circuit again, but with the diode reversed
What happens? You should find that absolutely nothing happens. The LED does not light up, so no current must be flowing. The action of reversing the diode has resulted in the stopping of current. We can summarise this quite simply in Figure 6.6.
Figure 6.6(a) shows a diode whose anode is positive with respect to its cathode. Although we’ve shown the anode as positive with a + symbol, and the cathode as negative with
a – symbol, they don’t necessarily have to be positive and negative. The cathode could for example be at a voltage of +1000 V if the anode was at a greater positive voltage of, say +1001 V. All that needs to occur is that the anode is positive with respect to the cathode.
Under such a condition, the diode is said to be forward biased and current will flow, from anode to cathode.
When a diode is reverse biased i.e., its cathode is positive with respect to the anode, no current flows, as shown in Fig- ure 6.6(b). Obviously, something happens within the diode which we can’t see, depending on the polarity of the applied voltage to define whether current can flow or not. Just ex- actly what this something is, isn’t necessary to understand
here. We needn’t know any more about it here because we’re only concerned with the practical aspects at the moment; and all we need to remember is that a forward biased diode conducts, allowing current to flow, while a reverse biased diode doesn’t.
What we do need to consider in more detail; however, is the value of the current flowing, and the small, but nevertheless apparent, voltage which occurs across the diode, when a di- ode is forward biased (the voltage we said earlier we needn’t then worry about). The following experiment will show how the current and the voltage are related.
Figure 6.7 shows the circuit you have to build. You’ll see that two basic measurements need to be taken with your meter. The first measurement is the voltage across the forward bi- ased diode, the second measurement is the current through it. Each measurement needs to be taken a number of times as the preset is varied in an organised way. Table 6.1, which is half complete, is for you to record your results, and Figure 6.8 is a blank graph for you to plot the results into a curve. Do the experiment the following way:
Current (mA) Voltage (V) 0 0 0.4 0.6 2 5 10 20
Table 6.1 This is half complete, add the results of your experiment
1) set up the components on the breadboard to measure only the voltage across the diode. The breadboard layout is given in Figure 6.9. Before you connect your battery to the circuit, make sure the wiper of the preset is turned fully anti- clockwise,
Figure 6.9 The breadboard layout for the circuit in Figure 6.7
2) adjust the preset wiper clockwise, until the first voltage in Table 6.1 is reached,
3) now set up the breadboard layout of Figure 6.10, to measure the current through the diode — the breadboard layout is designed so that all you have to do is take out a short length of single-strand connecting wire and change the position of the meter and its range. Record the value of the current at the voltage of step 2,
4) change the position of the meter and its range, and replace the link in the breadboard so that voltage across the diode is measured again,
5) repeat steps 2, 3 and 4 with the next voltage in the table,
Figure 6.10 The same circuit, set up to measure the current through the diode
6) repeat step 5 until the table shows a given current reading. Now set the current through the diode to this given value and measure and record the voltage,
7) set the current to each value given in the table and record the corresponding voltage, until the table is com- plete.
Tricky
In this way, first measuring voltage then measuring current, or first measuring current then voltage, changing the posi- tion and range of the meter, as well as removing or inserting the link depending on whether you’re measuring current or voltage, the experiment can be undertaken. Yes, it’s tricky,
but we never said it was a doddle, did we? You’ll soon get the hang of it and get some good results.
Now plot your results on the graph of Figure 6.8. My results (correct naturally!) are shown in Table 6.2 and Figure 6.11.
Figure 6.11 My own results from the experiment
Current (mA) Voltage (V)
0 0 0 0.4 1 0.6 2 0.65 5 0.65 10 0.7 20 0.75
Repeat the whole experiment again, using the 0A47 diode, this time. You can put your results in Table 6.3 and plot your graph in Figure 6.12. Our results are in Table 6.4 and Figure 6.13.
Figure 6.12 Use this graph to plot your results from the second experiment Table 6.3 The results of your experiment
Current (mA) Voltage (V)
0 0 0.1 0.2 0.25 0.3 3 5 10 20
Current (mA) Voltage (V) 0 0 0 0.1 0.5 0.2 1 0.25 2 0.3 3 0.32 5 0.35 10 0.4 20 0.45
Table 6.4 My results from the second experiment
As you might expect, these two plotted curves are the same basic shape. The only real difference between them is that they change from a level to an extremely steep line at differ-
ent positions. The OA47 curve, for example, changes at about 0V3, while the 1N4001 curve changes at about 0V65.
The sharp changes in the curves correspond to what are sometimes called transition voltages — the transition volt- age for the OA47 is about 0V3, the transition voltage for the 1N4001 is about 0V65. It’s important to remember, though, that the transition voltages in these curves are only for the particular current range under consideration — 0 to 20 mA in this case. If similar curves are plotted for different current ranges then slightly different transition voltages will be ob- tained. In any current range, however, the transition voltages won’t be more than about 0.V1 different to the transition volt- ages we’ve seen here. The two curves — of the OA47 and the 1N4001 diodes — show that a different transition voltage is obtained (0V3 for the OA47, 0V65 for the 1N4001) depending on which semiconductor material a diode is made from. The OA47 diode is made from germanium while the 1N4001 is of silicon construction. All germanium diodes have a transition voltage of about 0V2 to 0V3; similarly all silicon diodes have a transition voltage of about 0V6 to 0V7.
These two curves are exponential curves — in the same way that capacitor charge/discharge curves (see Chapter 4) are ex- ponential, just in a different direction, that’s all — and form part of what are called diode characteristic curves or sometimes simply diode characteristics. But the characteristics we have determined here are really only half the story as far as diodes are concerned. All we have plotted are the forward voltages and resultant forward currents when the diodes are forward biased. If diodes were perfect this would be all the information we need. But, yes you’ve guessed it, diodes are not per- fect — when they are reverse biased so that they have reverse
voltages, reverse currents flow. So, to get a true picture of diode operation we have to extend the characteristic curves to include reverse biased conditions.
Reverse biasing a diode means that its anode is more nega- tive with respect to its cathode. So by interpolating the x- and y-axes of the graph, we can provide a grid from the diode characteristic which allows it to be drawn in both forward and reverse biased conditions.
It wouldn’t be possible for you to plot the reverse biased con- ditions, for an ordinary diode, the way you did the forward biased experiment (we will, however, do it for a special type of diode soon), so instead we’ll make it easy for you and give you the whole characteristic curve. Whatever type of diode, it will follow a similar curve to that of Figure 6.14, where the important points are marked.
Figure 6.14 Plotting the reverse bias characteristics for an ordinary diode is not practical, so we give you the characteristic curve here