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Current time:0:00Total duration:10:10

Video transcript

suppose we have a very tiny bulb like an LED which requires say about two wars to turn it on and let's say that if you put any voltage across it more than two words then you're going to blow it up maybe three or four volts the LED gets blown up gone and let's say that to power this up we use a power supply which is not a very reliable power supply let's say it produces a voltage it gives out a voltage anywhere between one world to five world let's say how do we connect this led to this power supply and make sure that it doesn't blow up one way to do this is by using a Zener diode in a previous video we've spoken about this thing called the Zener diode we saw that it was a very heavily doped diode and due to which in the reverse bias it can break down at much lower voltages it can start conducting very heavily at much lower voltages than a conventional diode and we've also seen that the more you dope it the more the doping impurities lower will be it's it's breakdown voltage all right and this mechanism is called the Zener mechanism it's a little different than what happens in regular diodes and we spoke a lot about this in a previous video so if you need more clarity it would be a great idea to go back watch that video and then come back over here but in short Zener diodes can break down at very low voltages under reverse bias okay so let's say we have a Zener diode with us normal diodes are represented this way in a circuit symbol right how do we represent a Zener diode well all we do is put a zero over here to tell us that this is not a normal diode this is a very heavily doped Zener diode okay we mentioned like this now suppose we have a Zener diode whose reverse breakdown the Zener breakdown voltage let's say is two volts that's what we'll be requiring for our purpose let's say we have a breakdown voltage of exactly two words now let's see how this diode behaves under various circumstances so first of all to make sure it is in reverse bias you have to make sure this is positive and this is negative right we should try and force the current in the opposite direction now notice as long the reverse bias is less than tools when we are over here less than tours notice that the current flowing through the diode is extremely tiny over here we are which means it's almost as if the diode doesn't conduct at all so we could assume this diode acts like an open circuit when the voltage across that is less than tours all right so that's one way to think about our diode so that's one behavior it'll act like an open circuit like an open circuit when the world is across it and the voltage this voltage is less than tours and similarly once you hit two words not is what happens once you hit two words then regardless of what current flows through it notice this becomes almost vertical regardless of what current flows through it the voltage pretty much remains a constant all right and the heavy current starts flowing through it which means we can assume that once the voltage hits two wars so let's say let's use purple for this once the voltage hits two words we can pretty much assume that our Zener diode behaves like a short circuit because it offers extremely low resistance you see it allows it readily allows the flow of charges so we could assume it acts like a short circuit between two points between two points and the voltage across it remains a constant that's important the voltage doesn't change the voltage across it just gets fixed at tours so this is the VI characteristics of the Zener diode and this is how we can interpret that characteristics and now using this let's see how we can attach our LED to that power supply so here's our LED its voltage shouldn't exceed 2 volts a tiny bulk you can imagine and here's our supply which can supply anything between 1 volt to 5 volt and we have to connect this to the LED we don't have anything else and we need to power it up so if we directly connect the wires from the led directly to the supply like this and there's a chance that our LED can blow up because the voltage can exceed towards how do we make sure the voltage over here never exceeds too old how do we do that well can you see that we can use a Zener diode over here because the voltage across I would under reverse bias remember under reverse bias can never exceed two words so let's bring a Zener diode over there let's bring a Zener diode over here and we can attach it across the LED right to make sure that the voltage across every doesn't exceed too old now I want you to think for a while and I want you to pause the video and think for a while what direction how should I connect this error the Zener diode do you think I should connect the Zener diode this way like this or do you think I should connect it this way I want you to pause the video and think about this for a while all right let's see if you attach it this way then you're attached putting a positive over here and negative over here pushing the current downwards hey that means your forward biasing your Zener diode don't do that because we don't want our Zener diode in the forward bias its speciality lies in the reverse bias it's in the reverse bias that the voltage will not exceed - all right so let's flip the diode this is not how we should connect it we should connect it this way tada and there we have it there we have it now we know for sure that the voltage across this diode is not going to exceed two words so yay done right all not really I want you to now think about what will happen under various circumstances I want you to just pause the video and again think about this is very interesting what will happen if the supply voltage is less than towards pause and think and similarly what will happen when the supply voltage hits two words think about these two cases we have the behavior over here all right let's see if the supply voltage is less than Tour's right then we are over here we are over here which means our Zener diode acts like a an open circuit right so let's let's do that let's put that so if it's less than 2 volts then our Zener diode acts like an open circuit in such a case the current directly will fall this way through the LED powering up the LED nicely our LED will glow as long as the voltage is less than two words excellent excellent now if the voltage exceeds towards or let's say the voltage equals two words supply voltage becomes Tour's then our Zener acts short circuit like this and therefore now it will be in this part of the graph we are now over here and so from now onwards the voltage across the Zener diode will be remaining constant as towards and so this ability will have a constant voltage of tours yayyy problems all right no it's not solved because in this case when the voltage is about two words we are short-circuiting our entire circuit which means an extremely heavy current starts flowing over here I don't know what will happen to Zener diode but our battery will blow off so we've avoided our LED film proved blowing up sure but this is bad for us apply you should never ever short-circuit any power supply a very strong current like that can blow up this battery so this is the right mode I mean we have the right circuit but we need to just improve it and make sure it doesn't undergo short-circuit what do you think we should do again I want you to pause the video one last time and think about this all right just to make sure that it doesn't undergo a short-circuit what we can do is a put a resistance over here right we can apply a resistance in our circuit an extra resistance like this we usually call that resistance as RS saying that it's this in series with this circuit right we just put a series resistance over here and our problem is now solved again when the voltage is less than two words this will act like an open circuit and only the current flows this way so again let me do that as long as the voltage is less than two words it'll act like an open circuit kernel for this way if the voltage equals two words now then this will act like a short circuit and the current will start flowing this way and now guess what even if the voltage exceeds tours the voltage over here will always remain two words so let me just write that down that voltage across this Zener diode this is positive this is negative this voltage across the diode can never exceed two words this will be only towards because we've seen over here now of course in reality this is not exactly vertical so the voltage will exceed a little bit all right as the current changes but we'll assume that pretty much doesn't change at all and as a result if the power supply supply say three words then that extra one volt will be dropped across this resistor towards get dropped over here if the supply voltage becomes four volts there will be two walls here and two volts over here so notice whatever happens as long as the voltage supplied by the power supply is more than two volts the voltage across the LED will stay at tours because of this zener mechanism you understand and that's the whole idea behind voltage recognition so this this circuit is called as a voltage or this zener Dyer is called as a voltage regulator voltage regulator and that's one of the applications of a Zener diode voltage regulator just means that it keeps a constant voltage across it it makes sure the voltage doesn't exceed a particular value well of course if the input voltage goes below below two volts then the Zener will act like a open circuit and you know the voltage across the LED will also go below tools that it can't do but can make sure that the voltage cannot won't exceed tools that's the idea behind a regulator or a voltage regulator so long story short a Zener diode in reverse bias has a constant voltage and as a result can be used as a voltage regulator