This video is about diodes. I built this circuit to test this diode so as I can measure the current through the diode and the voltage across the diode and plus is on this graph using my data logger. This is called the transfer characteristic. Let's have a look and see what happens. So I'm going to turn the voltage up and to start with not so what happens I go from 0 to 0.6 volts very little current and then as I continue to turn the power supply up suddenly the current rises very very steeply and the voltage across the diode doesn't increase very much more. That's the classic shape we would expect for a diode. But hold on I hear you say, when I sketch this graph I'm always told that this voltage here is 0.7 volts and as you can clearly see on my display it's 0.9 volts so what's wrong? Well this graph that we often draw like this with 0.7 volts assumes a current of only 10 milliamps or so, maybe 100 milliamps. On my graph I've gone right up to 1 amp. If you make a large current through a diode then the voltage does increase above 0.7 it goes 0.8, 0.9. But for small currents this is the correct graph and 0.7 volts is the correct voltage. This graph I've drawn here is only looking at this part of my data logger. To illustrate my point about a diode having a fixed voltage of 0.7 volts across it when it's conducting I built this circuit including a 1 kilo ohm resistor just here to limit the current to just a few milliamps. I'm recording the supply voltage on my red volt meter which is this one just here and I'm recording the diode voltage VD on the 1 after 1 in green which is this one just here which seems to have an extra 0.02 added on for some reason. Let's see what happens when we turn the supply voltage up. Well to start with when the voltage is low you'll notice that the supply voltage and the diode voltage are the same. The reason is the diode is not yet conducting it needs 0.7 volts to conduct. No current flows around the circuit there is no potential difference across the resistor therefore this voltage and this voltage are the same as you can see. However if I turn the supply voltage up further and make it go to maybe 1 volts or 1.2 volts you'll see now that the diode voltage is no longer equal to the supply voltage it's remaining at 0.6 volts. That's because current flows around the circuit and there's some voltage drop across the resistor. And if I keep increasing the voltage to 2 volts and 3 volts you'll notice that the voltage across the diode remains pretty steady at about 0.7 volts which is what we're expecting on our previous graph. And even if I go right up to 8 or 9 volts the voltage across the diode doesn't really rise much above 0.7 volts. Why is this? Well if we take our graph from before what we're effectively doing is we're increasing the voltage across the power supply which is increasing the current and as the current goes up and up and up this voltage here is hardly changing even when the current increases to 10 milliamps which would at 10 volts I would get 10 milliamps through my 1 kilo ohm resistor even at 10 milliamps I'm only just above 0.7 volts. So that means that in this circuit even though my input voltage changes dramatically my output voltage the voltage across the diode stays almost fixed. It's a fixed voltage reference which would be very useful in a comparator circuit or something like that.