 The first lecture is on diodes and as I said this I am sure all of you must have been teaching this course sometime or the other and I was also not sure what to put and in an hour typically kind of our lectures at your place I am sure that our place also is one hour and we roughly have as teachers an idea how much you can how many slides you need or how many pages you need to cover in a in an hour lecture and I have much more than that ok. So, the purpose is not to cover all this, but you know kind of interact. So, please stop me please feel free to stop me maybe I show you a hand or please do that you know if any class and I do not get questions you know get extremely bored and I always feel something is wrong ok especially something is wrong with me that is why people are not reacting yeah. Now as I said the main purpose of this particular course itself is to decide on kind of syllabus is one aspect, but the other aspect is to kind of jointly learn from each other you know how to teach and unfortunately I am sure none of you would have had a training in teaching like a beard course you know we are all launched as teachers from day one and we are expected to learn the art you know while swimming itself we are supposed to learn and therefore I am sure we all have different experiences and based on my own experiences I can say some things which I have helped as I was sharing earlier also I was teaching at IIT Kanpur for last 17 years and I have just come on a visiting postage here for 2 years. Now I have been I taught this particular course several times I mean I have taught this in Kanpur we had at that time a strength of about 200 students so one shot we have to teach is 200 students and it had a lab component also that I taught about 2 times and I have been involved for last 17 years every semester or every year in the either the lab or somewhere else involved. At IIT Bombay also last semester I taught a similar course very similar for 120 students so as you know when IIT is also the number of students are now very large so we have anywhere from 600 to 800 students at a time so we teach in about 5 separate sessions so I taught one such class but here at Bombay we do not have the lab also along with the course that is the main difference and separate lab whereas at Kanpur we had that integrated with the course yeah coming back to diodes so when I quickly go through I also want to share some of my experiences with my own students okay and I find that students are the same there is Kanpur where is Bombay students are students okay and no difference and they may be little bit maybe you know the city makes some difference but I think otherwise students are always students so I am sure you would have the same experience. Now I am sure you are all from different parts of India one of the basic problem in teaching a course like basic electronics is what what is the problem some of some of the problems you have faced so one of the problems I faced is that at least some students would say that we have to know it okay at least I had one student he said I am bored I do not want to attend your classes do not attend no problem but at the same time you would have another set of students say no no we do not know anything assume nothing I had most of the time even here also students said sir assume nothing start from scratch okay the same time you have 50% students say no no we know all this from our boards so we have this problem in our country again we cannot complain we have to solve the problem okay for example some state boards might have especially we look at CBSE they taught everything basic electronics course is over if you have 12 standard 12 standard physics books with you and if somebody understood then do not have but we know that it has been taught so bad or they are so confused so that is where you have to undo some of the damages okay or some state board fortunately have not taught them okay and then you have an advantage you can teach them so I think I am sure you have all this so one of the things I always fun as I said and I am I found this universal last 17 years of experience I can say this and I found it every time one part which students always appreciate that is they want to see things okay and I am sure all of us also appreciate it so always make it a point to and I have not done it today because your teachers I do not should not teach you like three to the kids always bring components to the class okay diodes and they would like to see touch it okay in fact it is not a bad idea to burn some of them you know they should see when you say diode characteristics you know they should understand what it means by a diode characteristics now when you talk about diodes is one of the most basic electronic device and this is something fortunately most of them know about it so we do not need to talk much about diodes but what is more important is we need to dwell on the applications and also how they how it is used so what are the most important concept to a concept concept device I found one of the most important things for students is that they need to figure out that diode is a two terminal device and the moment you say two terminal device you are automatically saying that it is a single port device okay the concept of port because later we will talk about two port devices so they need to know this word you know two port single port so every any two terminal device now the important thing about a two terminal device is it is completely characterized by one equation okay which is the IB characteristics whether it is ohm's law that will give you a resistance if you have a capacitor you have a single equation for an inductor you have a single equation they are all two terminal devices so that is a very very very important concept I always found it helps students tremendously that you can say that if you get a two terminal component take it for granted one equation is enough a simple IB characteristic so that is the importance of the IB characteristic you know sometimes these things they figure I mean they also wonder why you say IB character why not something else so they should know that you know this is a single characteristic and if you equate them to you show that ohm's law which they are very familiar then it makes sense there is a current there is a voltage so you are relating the voltage to the current so that is the importance of the diode equation now coming to application as I said diode fortunately is something which students generally understand well because they have studied it and they have some idea but it is a very simple device also but again what is most important is that the application we need to especially basic electronics course I would say from A to Z you have to emphasize on application and show them and always try to relate to some gadget which they are familiar with and so on then it makes and then they keep interest they their interest especially basic electronics course is a course taught to everybody whether it is a civil engineer mechanical engineer anybody you teach it everybody that is why and it is very important at the same time today basic electronics course is extremely important why why is it very important today just maybe not so 10 years ago why is it so important today why is it very important for all engineers to know basic electronics why yeah today you you do not have a single equipment which does not have electronics okay not a single equipment so this is another reason why you need to motivate in fact I always I happen to teach always non-electrical students always even last semester I taught mechanical students and that is the time it is difficult to teach them so I always motivate them by saying that you are very lucky and they are very surprised why they lucky that you are very very lucky tell them that you have the most sophisticated new people are going to handle the most sophisticated electronic equipment and they are very happy when to hear that coming from an electrical engineer I said we are very unlucky people they are even more happy and say we are just have three equipment that is all we have a CRO a functionator and the power supply that is all we have and they are very happy and they laugh but then I tell them but we designed your equipment then that may complete the story and I think that is very important to keep them and this very true today you walk into any department you would see that the kind of equipment which is used in other non-electrical departments are much much much more sophisticated than the equipment you would see in your own department so which is very good now again when we come look at the diode equation I mean again one of the biggest problem today I am maybe I think is more in IITs maybe than less at your place that students are extremely good with maths okay but what they lack is what something else they lack what is it what it means okay and I always know in IIT system I know it very well if I want to trick a student very easy write a question without writing an equation just write in words they are sunk okay right one equation they'll solve it just like that don't write an equation but write everything in words you can just trap them completely what I'm trying to say is our present generation unfortunately you know have been taught equations without telling them what it means and here like a diode equation you need to really tell the significance of each and every term there and that's extremely important and then it makes sense you know and that's true for engineering that you know and that equations they have their domains sometimes unfortunately students are taught to think that you know here is everything you know and so some of the things they must understand and looking at the diode equation the there are a few things again very common mistakes both students and teachers make for example the diode current is related to the IS so diode current I is equal to IS times e to the power V by VT minus 1. Now what is the relative significance where IS is the saturation current V is the voltage across the diode I is the current flowing through it VT is the thermal voltage given by kT by Q now what is the relative significance of these two term that's the most important thing now let's look at the forward bias region now in the forward bias region the first term is what is important now since V, VT is typically at room temperature the order of 25 to 26 millivolts and V is about let's say 5 to 10 times VT then you will see that even though IS is an extremely small number typically the range from here from 10 power 12 to 10 power 15 we'll come to that in a minute again a very very common mistake made now because of this once V the voltage across the diode becomes about 10 times VT then you see that the it shoots up and you get a very very large current here now this is something they need to appreciate that that you know that number V by VT that ratio is what makes a difference and that need to be a order of 10 or 20 now another very very very important thing to remember when we look at the value of IS now very very common mistake made is about the value of IS what do you think would should be the typical value of IS and why paranormal diode let's say a diode which you would use in a lab in a lab experiment what should be the value of IS I'll show you an actual simulation then we'll see but before that nano ampere for silicon and micro ampere for germane okay now today by the way germanium diodes we don't use at all okay so we'll but fine let's now let me I want to show you the same equation I just kind of calculated and now and let's see the effect of IS then you'll be shocked you'll be shocked to see this okay so that's why now what I have here is a graph where I have used 4 values of IS okay same diode equation 1 is equal to you know 10 power minus 6 10 power minus 9 10 power minus 12 and 10 power minus 15 now let's look at 10 power minus 6 look at a value like 0.6 volts okay at 0.6 volts this is an amp mind you 10,000 amps 0.6 volts 10,000 amps is there something wrong so what is wrong so a 10 power 6 value for IS is what good or bad you should never ever use that value okay this is just I just plugged in numbers and calculated you can do it yourself okay must be shocking so you see the effect now look at the same 0.6 let's look at if you use 10 power minus 9 that current is somewhere or the order of from the like 20 amps still amps sorry about 10 roughly 10 amps okay same 0.6 come down to 10 power minus 12 that current becomes now 10 milliamps you see how drastically is with exponential dropping that makes sense okay 10 power yeah 0.6 volts 10 milliamps makes sense the normal diodes which we have made measurements so this is why you should always remember IS value never ever come below 10 power minus 12 okay the other will be a gross error so this something very important in fact good diodes would have typically 10 power minus 15 look at 10 power minus 15 your current is to the order of 0.1 milliamp the value which we very commonly use is 0.7 okay let's look at 0.7 but 0.7 forget about the 10 power minus 6 current goes to you know huge but even at 0.7 a 10 power minus 9 value of IS gives you current of the order of 150 amps okay which we have never ever seen that kind of current in the lab whereas 10 power minus 12 gives you even that gives you 1 amp okay now I'm sure when you make measurements in the lab so this this simple equation if you just calculate will make sense okay now you would have seen that when you do a diode experiment in the lab which diode do you generally do you use what kind of diodes we have generally you know normal diodes in the lab what what kind of IN 4001 okay this one diode what diode is that it's a it's a power diode okay it's meant for power supply applications now the IN 4001 has 1 amp and 1000 volts VIB okay it's meant for power supply application now you would see that you know I've done measurements and you would be shocked and you will be again doing some doing some measurements in the lab you would see that in IN 4001 if you make a measurement for 10 milliamps of current you would see that the forward voltage is hardly 0.3 volts now everything is make sense okay it makes sense now so that's why don't take 0.7 you know as a kind of a value which is there you know that value itself is a approximation some of these things students also need to know I mean some of these simple numbers if you tell this to students they also understand it okay very very important and I'm sure if you use a signal diode okay signal diode is very different from a power diode signal diodes would how do they look like typically they're very on the first of all they'll be very small very small diode sometimes they kind of glass type they have okay I will show you some actual measurements also we'll actually show you some of these measurements over here and or sometimes you have like a transistor kind of a can type also SH some number they have they are all signal diodes now those diodes would show you very different characteristics okay now those signal diodes would have typically IS of the order of 10 power minus 12 or minus 15 that range but 10 power minus 12 is a good number okay don't come below that if at all you have to come down come down only for power diodes and power diodes we will seldom use for such calculations so these are extremely important thing to remember okay and at least for our own sake you know keep this numbers at mind but tell me why what is the problem IS is what is the saturation current but if you actually make a measurement somebody might say that you actually get much higher current so the forward region we said it's only forward bias current but the reverse region reverse bias and the same equation you would see that the first time when again your V becomes negative your negative that e to the power minus V by VT becomes a almost close to 0 okay now we see that I is approximately equal to minus IS okay so that's why this significant those two terms which actually is very very interesting but if you actually make a measurement in the lab what do you think you will get just take a diode and measure okay and you would be shocked that even if I have a diode with IS is equal to 10 power minus 12 if you when you make a measurement in the lab you will not get it anywhere there for 10 power minus 6 that come to our head because of this make a measurement you will get typically 10 power minus 6 or 10 power minus 10 what are the various what is the problem why why do you see that why do you get a much higher current so that's why this 10 power 6 is there in our mind because we have seen it why now the the problem is the following IS is something you know highly purely at the device level but actually when you make a measurement the there is another current called the leakage current okay in a real diode you have a much higher reverse current the main reason is because you have leakage effects and this leakage current is directly promote proportional to the area okay area of the so therefore and unfortunately the strictly speaking reverse bias should be what is a constant which means supposed to have a flat line when actually make a measurement you see that what do you actually see to make a measurement in fact is very difficult to make this measurement why device that very difficult because the kind of impedance the meter multimeter or whatever if you make a measurement of the multimeter you make error okay you need a much higher impedance for the meter you need okay now the problem here is the leakage current is directly proportional to voltage so actually what you will see is not this okay but you would see a combination of this plus a leakage current which is directly proportional to the voltage okay so these are things which are important for us also to realize that when you actually make a measurement you would find that the leakage current is much higher than the IS value so this is why it's very very important to you know and if you are making reverse current measurements you have to be aware of all this okay otherwise you will make a gross error gross error I hope this is something to think about okay and the best thing I would say is to just sit with me this when we be when tomorrow onwards you will be doing labs when you go to the lab please make some of this measurement okay and see the limitation of a multimeter okay you must see what do you think what is the typical impedance of a multimeter input impedance what is the impedance typical one make any other answers ten make any other answers typically all multimeters you buy today would give you at least 20 make okay one make is somebody else's impedance pitch one another instrument which is used for measurement CRO CROs have very bad so never ever makes measurements such measurement with a CRO never if you use a voltage probe a 10 is to 1 voltage probe then the impedance becomes 10 make okay so never ever in fact multimeters are much better in making this measurement because multimeters would most of them would give you at least 20 make if you have a very sophisticated multimeter would give much better that okay so this something very very very interesting now when you talk about diode model I am sure again we do this in the I am sure you are doing this and again some some in size yes yeah I am not a device man but let me I will try my best okay now when you talk about a basically when you take a device you know you have this minority carries and majority carries IS is the current due to minority carries that is precisely why it is extremely small okay so that is basically IS itself is highly dependent on the doping of that particular diode now so typically the normal diodes we talk about the signal diodes we talk about the doping levels are very very small okay that is why the currents are very very small where in power diode because you need much higher currents also I mean it is a not only doping the junction old areas also very large in a power diode much the size also we see that much higher so IS actually is a function of many things one is the doping one is the area so it is a lot of these parameters decide the value of IS but anyway for a signal diode the junction area is extremely small so that is the another reason why the current for a signal diode is very very very small so IS is a function of of many many things good is a good question see one very very very very very good question I like it very good question see again this another thing we all do in engineering all the time we talk about models what are models a very good question that's a very good question it's a very common question in our certain source of phase we must be clear first ourselves okay now what are models why do we use models what are models take for example ohm's law okay IS ohm's law correct what do you think I would say yes or no it's not completely correct yes when is it correct when is it wrong it is correct as long as you make measurements for low voltages and as long as you keep your you know the sorry yeah see see normal room temperatures it's it will work very well similarly if you think about the the frequency okay you need to keep the frequency also very low as we keep increasing frequency and temperature also to some extent you would see that ohm's law fails okay so then why do we use ohm's law why I'm asking this question is to bring the issue of models why do we use models why do we need models why do we ohm's law it's a model it's a model it's a way of finding given a resistance how do I find current so it's a model now every model has some assumptions and some limitations okay so when you talk about a diode and when you say IS okay that is a current which flows when you apply a voltage it's there is nothing sitting inside in fact a diode is a passive device what is the meaning of a passive device it has nothing inside okay as opposed to a battery or something now strictly when you actually when you apply a voltage there like you to apply a forward voltage you have forward a current flowing there due to the majority carriers so you have a much larger current similarly when you apply a reverse bias what is happening is essentially in one sense you see ohm's law strictly speaking it's not valid there okay maybe for some very limited range ohm's law work so what is happening is you have a semiconductor material okay which actually I mean you cannot directly model as a resistant but the point is IS therefore I mean if you look at the there's an expression for you can actually write an expression for IS in terms of the doping and area okay there are standard equations are available but so the point is this particular IS is applicable only when you apply a voltage there is nothing sitting inside so it's a it's a it's something which you physically relate to and actually you can mod so that's why what I'm trying to say is when you make an equation like this this equation was I mean by Shockley okay very famous person I'm one of the persons involved in design of the invention of transistor also now this is something you observed and then they try to relate okay and the diodes which they used where point contact not the junction diode because the technology was not well developed in the early 40s okay so they observed some of these effects and they found a model and they found it is working clear okay yeah by the way brings to another very important issue when we talk about another very very very important thing is the issue of textbooks okay and I know one of the most commonly used textbooks in in unicities is what? Milman and Halkias if possible please throw it out of the window okay the simple reason is Milman and Halkias a book I studied when I was engineering student I graduated in 1978 this is a book which I used okay that's the time it book came there is no other book available in the whole world that time it's a very book good book at that time but today that's a very bad book do you know why please don't use that book today why no no no no then I would have used it we should not be biased against Milman and Halkias what is the problem see that is a book written with the technology knowledge of the 60s okay if you say that semiconductor has not changed since 60 something is wrong so you should use one of the current book one of the best books you can use is Satter and Smith okay Michael electronic engineering available cheap very accurate precise and extremely readable Milman and Halkias is not at all readable okay but if you if you if you are all involved in syllabus please remove Milman and Halkias from the syllabus okay you will do a big service to students I'll I'll this notes will be available to you Satter and Smith I'll bring the I wanted to bring that book I'll bring you this book and show you this is called micro electronic circuits by Federa and Smith it cost about 300 rupees thick book extremely good book no all all in fact starting from diodes transistor no better book I have found even today okay diodes transistor op-amps in fact you are not only basic electronics course your other analog electronics course this is all that you don't teach anything else too good a book okay and the good thing is fifth edition if a book has five editions it tells something okay that's the book will follow all over the world and I have never found a better book okay and last semester we use another book here called Bob Roe LS Bob Roe again by Oxford University Press a good book but that's a only one edition plus and minus simple book but I would strongly recommend all of you to you know I have this Satter and Smith because you know beautifully tells all this and lots of problems lots of problems this is another very important issue now when we talk about models okay again very important to have a models when you have a diode we have that equation now the purpose of a model and all engineering models is to be as accurate as possible but accuracy is not the big issue what is the more what is the big issue in all models this is a very very important concept not only for us but for students also because all subjects they learn models here also we are talking about models what is the purpose of a model some you have an equation that equation why are you not happy with that equation why do you need a model the equation is too complex okay for an actual application you would like to have a model which is simple at the same time really kind of reasonably accurate okay why is accuracy not important especially in electronic circuit in our courses why is it not important sorry yeah see one of very important thing you must remember is that's why again I said Milman Halki as you know I mean it's a book which was written a long time ago when there was nothing available but today you have extremely good simulation tools available the purpose of teaching a course is not to get accuracy to this third decimal point but is to help a student to design or to do a hand calculation in hand calculation you are not looking for accuracy you are looking for a for a range I always tell students you must always have the range in mind not the accurate number okay they should know that if current is in 100 amps something is wrong in a diode okay that they should get it may be within 10 milliamp 20 that milliamp microamp the range is very very important okay accuracy you have a plenty of free simulation tools which they could use okay so when you talk about exponential model this is directly picked up from the Shockley equation therefore will be very accurate but what will be the problem what is the problem this is never used but it's very accurate what is the problem too complicated for example you have a simple circuit like this now you apply a voltage find the current and the voltage you have one equation to unknown okay and you somebody might say that this can be solved either graphically or iteratively but fortunately thing is just by writing two three equations you can write two equations one for the current this is the being forward because the forward bias you can write just the simpler one and the second equation is because you write a symbol you know ohm's law if you apply you can write it iteratively solve this you will get extremely good accuracy so in two iteration I have done the iteration here iteration number one you assume the Vd to be 0 which will give you an id value of battery divided by the reason 5 million now put that back into the previous equation now you find out what is Vd give you 558.8 0.3 millivolts second iteration take that value plug in here find the new value of id and you get 4.44 million plug it back here find the thing and you see it has converge just in two steps so a simple iterative method with simple calculator is all that you need but still this is too complex because of this this is not used at all okay and most of the diode circuit you don't need this kind of accuracy you don't need it so this very important when you talk about model what is important is so when we talk about we have at least three more models the piecewise linear model this is again reasonably good accuracy a constant voltage drop model where you just take the diode with just a voltage okay and you also have an ideal model which is the simplest one now the piecewise linear model essentially what you have is you assume the diode to be a ideal diode and in series with a kind of a battery and a Rd now the most important thing to remember here is again is that there is no battery sitting here you have a voltage source so the current is flowing what into the battery which means it only can dissipate power this is very important in a model because this is a very common doubt okay you have a battery sitting there is it delivering power no it's only a representation you can only dissipate power you cannot give power okay so it's only an equivalent circuit so this is a model that's the important thing to remember about model models are made for our convenience and they always have limitations and they have to be used within those assumptions not outside that now I've just I plugged in the piecewise linear model and the exact diode equation and you can see that pretty good pretty good approximation okay but again what is the problem even this is most of the time too complex okay most of the problem in a diode problem this is not but this is very very good you will get results very close to what we actually observe so this if you want accuracy then you should go for piecewise linear now the issue is here I used to the series resistance as 10 ohm shouldn't be 5 ohm should be 15 ohm that depends depends on the kind of current the operating point you want so this is where you may have you want a lot of accuracy you may have to play with it yeah the constant voltage model is kind of very very commonly used and all that you are saying is you have you assume that the diode doesn't contact up to some voltage so up to 0.7 it doesn't contact and all of a sudden you have infinite current okay it looks very very very ideally see but this is a very very good model okay especially when you talk about clipping circuits you don't want lot of complexity so this is a good model okay and because of this the output result which you are going to get will not be very accurate but that's fine if you are not satisfied with the accuracy you can first use this and then go back to the previous one so very often and this is a skill we also need to pass on to the students that given a circuit they should learn skills to divide and conquer okay they should know how to and that's kill they have to learn so that's where when looking at a circuit they should be able to figure out forward bias road bias which one to do I remove this or that that skill they had to develop so this kind of a model very useful just to see what is happening in the sector once I understand that then I can figure out what to do next yeah the ideal diode model again is very very often used this is especially used when you are talking about the voltage drop across a diode 0.6 to 0.8 volt being negligible like in a power supply circuit you are by your hands over may be 12 sin omega t huge so 0.6 volts is something you don't want to bother about it so such cases you just ignore yeah now coming back to diode applications quickly go through a few applications now rectifier application is the most common application I would strongly or I don't know how much flexibility you have in your curriculum or how much your college will be happy about it or unhappy about it to give some small projects to students but something which is extremely good today if some students can make a simple power supply okay maybe in groups okay the amount of learning they will go through is tremendous okay they get to see a transformer you know they they get to understand some understand of a electric engineering they see a transformer they see the voltage you know they see the connection between peak voltage and RMS voltage all this confusions are done so nothing to beat an experiment I think it's a good idea I don't know like unfortunately one of the biggest problem even in IITC phase when we get our M Tech students a good number of those students have never ever experimented at least but I am I'm assuming all of you are from very good institutions I'm sure that's not a problem definitely at your place but many colleges I'm sure you may be aware of such colleges experiments are rigged up and kept there students are asked to take what take readings okay that's the silliest thing that can happen okay no student will ever learn anything if you do that and we get into our M Tech good students who have come for that background and we have to teach them all basic electronics okay before they are of any use to us so please remember especially electronics is a course fortunately you can do simple as I said all that you need is a multimeter I may an oscilloscope sometimes you have multimeter all that in a multimeter a power supply and a functionator you can teach the entire basic electronics just with that and this amount of learning students go through why I'm saying this because when you find the kind of basic questions students have even after graduation electronic students then you are amazed something is wrong I think you have a very big role to play and that's where I think we need your feedback also you know what are the things where we should emphasize you know then accordingly especially for the June the lectures then those issues we could take care so we need a lot of feedback from you yeah so this is a clipping circuit you will be doing some of this in the lab now how do you analyze this kind of circuit any circuit so and before we actually given a solution there but anyway the most important thing whenever you get a circuit like this from the point of your student needs to figure out what is happening in the diode more than what is the result okay first he has to figure out how the diode is connected now diode is connected in this fashion which means that for the positive half diode will be off okay which means this is an open circuit okay so this can be let's say divide and conquer strategy is what we do always do in engineering okay so that entire branch can be removed for the positive half and that becomes a very very simple simple resistive network so you get output will be 0.75 of input very very simple now the reverse what happens again because the battery connected in series here till this branch in the voltage across this particular branch has to exceed 3 volt then only so some of these basic things if you understand then it can be solved so I think that's very very very important this kind of circuit here which I have given here I am sure you have done this kind of thing and what I have done here is I just done the same thing simulated it and see how it look like so this is the input here and this is output yeah now yeah coming back to so diodes as I said you know something which students really understand well and it's basically there are not not many conceptual problems here but when you come to synodiode yes that's where lot of problems students have lot of problems why the reverse you why positive why negative lot of lot of lot of confusion so this is where we need to really emphasize again as I said an experiment is something which is very very very useful so synodiodes are primarily reference diodes and used in as voltage regulator and typically the the synod voltage would maybe anywhere from a few volts to a few hundreds of volts now today especially lot of these things you can relate to their charger mobile charger in their hand okay and again if you can ask students to maybe design a simple voltage regulator something little not you are not worried about the actual output of what they got but more than that the process they go they are gone through and only if they burn a few components they learn something okay and only thing it should be a measured not the lab should not burn okay maybe a component can burn yeah so again a sinner simple sinner regulator because again a sinner when you model it you have to model both the positive forward bias region and the reverse bias region okay and those two reasons will be very different we'll see that we'll show you I'll just show you an actual measurement of a thin and out yeah so what I have done is we have connected a actual measurement now this is using lab you which is a very good software but unfortunately as far as the current right we said for education institution is very cheap you get a 4 lakh license for about 100 users but still it's a costly but the good thing about lab use very easy to do things lot of things can be done easily now what I what we have done is we have connected a sinner diode and the characteristics see the character is running and see that's hope hope it works yeah so you can see this is a sinner yeah so this is nice to see see you can see this is a sinner diode now very important thing the room for in a sinner diode is the forward region is exactly like a normal diode and the look at the slope this slope and this what is the main difference here we model this before the which one has more reasons forward region or reverse reverse okay so this is very very important and this tells you a lot about a sinner in a sinner diode the is extremely important the way the voltage which you mentioned there is mentioned for a current okay so you can see here depending on the current like for example if you are talking about 1 million minus 1 million the corresponding voltage is around minus 2.8 volts okay but depending on that particular current here you can get different different values so now putting putting it back if you want a extremely accurate value of voltage in a sinner diode what should you do sorry no no no listen to the question again look at the graph sinner diodes are used all over the world for as voltage regulators okay now if I want to get an extremely constant sinner voltage what should I do then I design my circuit what should I do just looking at the graph no no no no my question is what should I do what should I do sorry no no I have no liberty I somebody has bought me as you know I have to use it that's fine I'm talking only about this my question is where should I operate now what should I do yes that is under taken for granted but then but you are all missing one big point the important thing is if you want I asked the point I used to I told you something you I want very good accuracy what should I do I can't do anything that's a given to me no can't change the diver sorry there's something with the current no on operate at a constant current that word I was looking for nobody said it ticket so that's the important thing so if you want a very accurate sinner voltage see the exact voltage is not very important once I have a fixed voltage I can do whatever I want to do that that's what this graph says here okay this graph is telling is if I have if I can if I if I pass 1 million current to this I get some voltage so I must have a constant current source passing only 1 million okay of course it has to be the reverse then I get that voltage which is steady okay and another very interesting thing about sinner is any other property of sinner as opposed to normal diodes will put a diode also and show you okay another diode this is a sinner diode any other any other interesting about I see a normal diode has another today is a very very very important use of a normal diode one is rectifier we said clipper clamp or anything else silicon simple diode has a very big use today sorry good very good temperature temperature sensor extremely good temperature sensor now how is it used again same thing if you pass a constant current through a silicon diode okay for every degree rise in temperature the junction voltage will decrease by exactly 2 millivolt you may be aware of an IC called LM 35 LM 35 just uses it but only one difference the output will be exactly 10 millivolt per degree centigrade they linearized it that's all they're done so all that you do the three terminal device connect any voltage to the input output be exactly 10 millivolt into your degree centigrade so if it gives you 290 millivolt 29 degree centigrade it's very commonly available 20 rupees but fantastic thing okay extremely good thing all that it does is it has a constant current source pass pass that to a diode and just take the voltage across it amplify it and linearize it that's all he has done okay all based on this principle so this is a very very very simple simple very good thing we have put a normal diode let's let's now use a see how a normal diode characteristics look like yeah reverse region no current and then it shoots up okay so this is a normal diode again you can see the forward characteristics of this and the previous synodiode where what similar another important thing is look at the voltage forward voltage is how much this is a silicon diode how much roughly around 0.6 0.6 0.7 okay based on that previous IS value what should what do you think we if you guess back what may be the IS value 10 power minus 12 somewhere between 10 power minus 10 power minus 15 okay that's very very evident here okay so this one thing now one more thing before I disconnect it now there are a few more types of special diodes we'll talk about them one such special diode is an LED I'll put it up right now now the most common problem if you any idea what what if you do the measurement of an LED how that will be different compared to this okay good now the current voltage may not be that high but you are right sometimes can be 2 plus but it will be definitely more than 1.5 volts okay depending on the type of a LED you will never be 0.7 that's the most important thing do the unfortunately this number 0.7 has got stuck into a head okay and we have to get out of it and that 0.7 or that voltage is a kind of it depends on the material you see the germinium is 0.2 if it is gallium arsenide and gallium phosphate that kind of material you have different different different values so we'll do an experiment right now this is a LED yeah same reverse characteristic but look look for the forward four characteristics can you see it's somewhere around 0.1 point that we expand that you can see that around 1.8 okay very different from the previous one so this something very very important to remember but an LED the forward voltage is quite high somewhere around 1.8 it can even be greater than 2 depending on the material but we use a blue LED will be greater depends on the material they are all different materials and LEDs LEDs are made of all all diodes which we use in the lab the power diodes or the signal diodes they are all most of the time made of silicon okay now as a material silicon is a indirect bandgap material okay whenever you inject carriers there's always recombination taking place okay when for example electrons are injected into the into the let's say p region there's recombination taking place all the time now recombination can when you when recombination takes place we are actually giving out energy it can be give out either radiatively or non radiatively okay in a silicon the chance of having a radiative recombination is negligible whereas if you have special semiconductors like direct semiconductors which are one example is gallium at night now such as most of the three five semiconductors come in that category gallium at night aluminum gallium at night indium gallium at night all this all of all these are direct bandgap material these materials if you make a diode out of this material whenever you pass a forward current it will emit light because the recombination that takes place has at least 50% of it will be radiative recombination and the remaining 50 may be non radiative so LED the principle of an LED is just to have a p-n junction made out of direct bandgap semiconductor and depending on the material you get different color okay that is the principle of an LED now let me go down yeah we are talking about other diodes now another LED we just now talked about it LED is an extremely interesting extreme again this something you could enthuse students the cost of an LED is only 50 paisa today very very very cheap very cheap but the kind of things you can do then LED is amazing okay we will talk about one such thing now LED is also called an electro optic converter the it can convert the electrical current into an optical signal in fact LED if you look at the light output you would see that the light output is directly proportional to the current flow into it so it is used in fiber optic communication as a source now let us look at photo detectors I have a purpose in this photo detectors now now photo detectors LED is always used in which bias forward bias very very important remember now photo detectors on the other way they are always using the words bias now again any radiation being a semiconductor semiconductors have a problem because any radiation especially heat okay would increase the number of carriers now in a forward bias condition if you put some heat the effect of that excess carriers is negligible but if it is a reverse bias the current is very very small now if you put any any radiation okay especially if you open the junction area put some radiation light or anything you would see that the current shoots up okay very high now the principle of photo detectors is nothing but all that is done in a photo detector is you make a large depletion region okay make that large so that most of the light falling on it falls in the depletion region now what happens is the number of carriers will shoot up so in a photo detector and most of the time the visible light you can get you can use a silicon for that purpose now in a the current which is flowing because it's a reverse junction there is no light it will be nothing but our reverse saturation current and as I said earlier even though the reverse saturation current IS may be 10 power minus 15 but because of leakage current typically you the best you can get rather it will be about 1 nano amp you can't get less than that okay most of the diode in a photo diode also the typical current the reverse current is 1 nano amp and that current has a name what is the name of the current dark current intensely done to say that that's the current flowing there is no light and when light flows you have a you have what is called a photo current now you can think of another very interesting thing about a photo detector is because it's used in the reverse bias mode you can think of this like a current source okay and if you model it just be modeled as a current source I have supposed to a diode where it has to be modeled as a voltage source you saw that all diodes have been whereas here the photo detector you have to model it as a current source because what is the property of a current source how do you model a current source which model would you use the model or not a model not a model not a model you have a current source and a parallel and that recents is ideally infinite okay so here a photo detector you can think of as a control current source with a recents in parallel now a beautiful device can be made out of a LED and a photo detector called optocoupler now this something all non-electrical students must see okay or we should tell them now optocouplers are extensively used now the biggest use of optocoupler is if you want to transmit a signal from one system to another system without interconnecting them what is the problem if you connect two systems together sorry wire what is the problem impedance that's let me ask a simple question very often you know you try to take a laptop somewhere and you want to print or something most people are scared to connect why see what happens is one very very common mistake we all make is about ground what is ground what is the definition of ground let me ask a question for an a for an aeroplane where is ground which is ground for aeroplane body what about a car chassis okay the point is reference ground is nothing but a reference point now because of the same thing two systems you bring together the chances of having exactly the same ground is almost you know zero this is extremely dangerous now if you connect two systems with two reference points and short them together what will happen sometimes you can have fire also okay now this is why air if you have you are talking about a high voltage system and if you count to connect that to another system through a wire you can have disaster so most such applications you would optocoupler optocoupler that's the beauty of optocoupler you don't have to connect okay you have a electrical isolation all that you do is you put your signal here this light shines other end if you pick it up so both are sitting at their own ground level without any problem extremely useful and a very very interesting device optocoupler okay so especially non-electrical students must see this why expertly mechanical engineers you would they would burn a few things okay just because of this ground is such a complicated concept and a lot of things have burnt just because they don't understand what ground is okay now a laser diodes my specialization optical fiber communication I deal with this all the time now laser diodes are nothing but let's say special LEDs where the fabricated such a way that the light coming out is coherent so that's not what you can think of them as let's say special LEDs and today laser diodes are used extensively and all communications they use but a more than that pointers okay we all we are all familiar with the standard the kind of laser pointers okay any idea what is the kind of power you're seeing here like if you read it they'll say why do they sell this in fact the what is the power of let's say a tube light this standard tube light 40 watts any idea what's power here 500 micro watt maximum yeah I don't think it's even that okay why is it looks so bright whereas the light 40 watt 500 micro watt looks so bright but the 40 watt doesn't look that bright why coherent so this is where it is coherent you have two types of coherent one is what is called a facial coherent which means it's so sharp focused okay so that's an angular or spatial so it's extremely narrow beam whereas the the tube light it spreads all over the place so there is no special coherent number one second thing is what is called a temporal coherent or the spectral coherent if you look at this light it looks so sharp red if you if you see the spectrum of it you would see that you have a extremely narrow you know wavelength region so that is called the temporal coherent so a laser diode LED also does that but LED is much broad much broader than an a laser diode so let's not get into that but anyway before we have crossed the time solar cells and just let me just stop with this now again another very good thing you can tell students is about where it is like I'm sure all of all students would have done you know fine to radio and fine tuning you know and so if you say that where actors are special diodes which are used in the reverse bias where if you change the voltage the capacitance of changes in a very small maybe in the in the sub picofarad range which you cannot change otherwise you can change the tuning and then they really understand I think we'll stop here so there's a very broad kind of we talked a lot of things the diodes as you said it's a topic well understood and a lot of applications and conceptually couldn't find this not that difficult yeah thank you