 There is a small numerical that's there. Let me project it on the screen. Just give me one moment You can see this Yes, sir. So in current is 10, what's the voltage? When current is 10 see It's 0.8 and when current is 20 the voltage is 0.9 Okay, what sorry when When the when the current is 10 it is 0.7 and when current is 20 it is 0.8 So first answer is 10 ohms On the next picking Yes, sir others How more what is your answer message? Okay, so For the first game See at 15 milli ampere I need to get the value of resistance Okay, now here it is rapidly changing curve Okay, so that is why I'm trying to find here Dynamic resistance delta V by delta I so delta V across 15 milli ampere is 20 minus 10 that is 10 milli ampere or that is delta I sorry Delta I is 10 into 10 is so minus 3 That is 10 is so minus 2 and Delta V is 0.8 minus 0.7. That is 0.1 Okay, so 0.1 divided by 10 is so minus 2 it will come out to be 10 ohms fine Understood Okay, now let's see with respect to minus 10 volts now for minus 10 volt You can see that whatever you do current almost remains minus 1 micro ampere only Okay, so dynamic resistance Delta V by delta I You will get that to be infinite And that is in a way true also it is it close to infinite because changing voltage is not changing the current Okay, so rather than finding the dynamic resistance We can you know give a rough estimate about how much is a ratio between voltage and current over here Fine, so that will be 10 divided by 10 is so minus 6. It will come out to be tennis for 7 ohms Fine So when it comes to reverse bias in that zone where the curve is flat you just take a ratio of voltage and current because Delta I is close to zero and You can't have Delta V by Delta I that becomes indeterminate. All right, so let us move to next topic So till now we have only learned about How this PN junction actually functions, okay Now we are going to learn about the application of the PN junction we are going to learn about some of the instruments that we can make out of the Characteristic what we have observed just now VI characteristics so this is a very special characteristic and you will slowly understand why The behavior of voltage and current the way it is shown here is very special Okay, and we can exploit it in many different ways So first first application of PN junction is as a rectifier fine Do you know what is rectifier? No idea So rectifier is a device that converts alternating current into direct current Are you hearing rectified terms for the first time in your life? You already learned fine, so we are going yes, I'm hope that's correct fine, so first We are going to learn about the Half-wave rectifier. What is half a rectifier? Why it is called half a rectifier? This thing will be clear when we discuss this particular Device okay, let's first see the construction of half a rectifier Yes, this is how it will look like all of you draw it with me the almost every Device which you use at your home it Actually runs on DC only okay your TV your fridge What happens inside the TV of fridge or radio whatever you use Inside it there will be a rectifier that converts your AC current into DC current Okay, then only that device runs So that is why this has a Good Importance and also when you put your mobile phone for charging okay You you use an adapter for computer also you have an adapter Find what does that adapter do that's adapt that adapter converts alternating current into direct current fine, so this is what this Rectifier device does it converts altering current into direct current Okay, so we are going to just look at the block diagram Of course the picture is not as simple as it looks but then we can at least understand the principle of how it works Draw it with me and also one more thing the transformer is One of the most integral part of any electronic circuit Okay, because from outside you supply 220 volt But then suppose as an output you just want 5 volt and 2 ampere So there has to be a transformer inside the device fine, but that that is why you know I am showing here this dotted line as A transformer this is not a transformer that is connected Outside your home. Okay. This is a transformer of a device only small transform will be there Okay, so this is primary circuit on the left hand side This dotted line encloses a transformer fine Then this one is a secondary coil secondary Okay, then you have point a and point b Across which you have connected a load resistance RL Okay, now the the primary input is nothing but your AC voltage Okay, so how the AC voltage will be it will be a sinusoidal curve which comes in your home It will vary like this Isn't it this is the primary voltage now Let's see what this load resistance will feel now this load resistance is your device Okay, suppose you're connecting mobile phone fine, so mobile phone is the load resistance Fine, so load resistance is an equipment So I am going to see what this load resistance will feel So let's define the sign convention first according to sign convention what I'm doing is I'm assuming that Whenever there is a positive cycle of the voltage. I am saying voltage of a is more than voltage of b Okay, and when it is a negative cycle voltage of b is more than a Fine like this I can denote Now tell me When it is positive cycle the diode is forward bias or reverse bias It is forward bias and and when it is in a negative cycle diode is in reverse bias Okay, so during the positive cycle It will conduct electricity Okay, and the current will go from x to y But when it comes to negative cycle the diode becomes Reverse bias and it no longer conducts electricity and I can say that no current to the load resistance Fine, so what I get the voltage across RL is something like this. I'll get this Then the middle section will be out Fine, so I'll get only positive cycle negative is gone Okay, and since The current is not reversing its direction. It remains positive only throughout. I can say that it is direct current fine, but then It is very lossy proposition because Half the power is readily lost Are you guys clear about it? Yes, sir. Yes, sir. Okay, so this is half a rectifier now Why to lose so much power Can we devise an instrument which converts entire cycle into direct current? Okay, so device that converts entire cycle into direct current that device is called full wave rectifier Okay, so in half a rectifier We have only one diode in full wave rectifier. We will have two diodes Okay, let's see how the construction of full wave full wave rectifier Will be Right down full wave rectifier So I'll just draw the circuit diagram first and then we'll discuss about it You can also draw with me. It had it has two diodes like this This is let's say diode one That is diode two There's dotted line It Encloses the transformer So there is a center tap that is there in the transformer So you basically take the feed from the middle of the coil from the secondary Secondary coil of the transformer from the center of it you take an input This is center tap Okay from the center tap Goes like this And you are connecting a load resistance like this This is X and that is Y Fine. Now, let's see how it functions So first I will draw the input Waveform for the voltage this is the input waveform okay, and Let me just draw an axis Here where I plot the output one so again the sign convention remains the same positive means forward bias and negative means reverse bias This positive that is Now, let's see here when it is a positive cycle Let's say this is point a This is point B and that is point C Can I say that VA is more than VC? Yes or no For a positive cycle is VA more than VC. Yes, sir Right that that's how we have you sign convention. So for negative cycle This is for positive cycle And for negative cycle VC is more than VA. Okay. This is for negative cycle Now, let's say there's a positive cycle that is going on hence diode number one will be forward bias and diode two will be reverse bias Okay, so the current when it goes on like this This path is closed because that is reverse bias diode the current will just travel like this and That's how it completes the loop In a positive cycle and when it comes to you know negative cycle Diode one becomes reverse bias. Okay, and diode two becomes forward bias in negative cycle we have VC more than VB so the current will go like this and It will again go from X to Y only and that's how it completes the loop Fine, so one thing you might have noticed here that no matter Whether it is positive cycle or negative cycle the current always Current always move from X to Y So what does it mean? Potential of X will be more than potential of Y all the time okay, so In both the cycles for sure negative VX is more than VY right, so the load is feeling a direct current in a way Fine, so that is the reason why you will see this kind of Voltage across the load any doubt so why so why do you connect to the ground? this one Yeah, see you have to take a reference somewhere. It's like Finding potential energy right an object at a height H The potential it is mgh, so you have assumed that it is zero potential energy the horizontal line So like that I I have to assume some potential to be zero in order to find out all the potentials Related to it. It's a common practice. It's Nothing too great about it just it just gives you a convenient way of Identifying the voltages fine They're answering Okay, so load resistance will always feel a positive voltage only This is the full wave rectifier because you have captured the entire wave and converted entire wave as direct current that Half a rectifier was incidental Rectifier because you have made alternating current into direct current just by removing the negative part of it But this is the actually a full working rectifier where you convert You know that also to to come out to be a direct current Right. So this is how you convert AC to DC using full rectifier But then even though the direction of current or voltage is not changing. This is not something which you desire okay, what you desire is You need a constant voltage a constant direct voltage you want Even though it is not fluctuating The voltage but then the magnitude of the voltage is keep on changing direction is not changing But the magnitude is changing. I want a constant magnitude Voltage right so in order to achieve that What we do is we connect a capacitor fine We call this a filter capacitor. So first let me draw a capacitor here, so This is how you connect you connect a capacitor Parallel to the load resistance so write down capacitor connected parallel to Load resistance Okay, this capacitor will act like a filter capacitor Fine now see what will happen because of this The capacitor as the voltage increases Okay, as a voltage increases capacitor will get charged Right now first of all understand that this plot is the plot for the Potential difference between X and Y. Okay, so what will happen here is that when the potential across the capacitor is Decreasing so whatever charge capacitor has stored during the charging or when the capacitor Voltage was increasing capacity will Gain the charge right so whatever charge it had gained earlier When the potential goes down the capacitor will discharge and we'll just throw away the charge out Okay, because you're decreasing the voltage so when the capacitor throws away the charge this current Will sort of compensate for any decreasing current in the load resistance Fine so in a way the current in the load resistance largely remains constant Fine, so if current in the load resistance remains constant even the potential will remain almost constant So every time charging happens it will slowly get charged and when discharging starts Capacitor discharges and make sure the current through X and Y Remains constant and that will create sort of a Uniform voltage although still a processing is required, but more or less you get I Mean somewhere close to where you want it to be all of you clear about it any doubt