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Half wave rectifiers

How do you convert AC to DC? The circuits that do that are called Rectifiers. Turns out there are different kinds of rectifiers, some simple and some more advanced. In this video, we will learn the simplest kind of rectifier, a half wave rectifier. Created by Mahesh Shenoy.

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Video transcript

today we charge the batteries of our mobile phones by directly connecting them to the socket but here's the problem if you were to directly take a couple of wires from the batteries and connect them directly to the socket and say close the switch then your phone will probably blow up because the world is that you get from here is very high you may have already learned it's about 200 volts but even if you could solve the problem of the voltage maybe we could somehow lower the voltage even then your battery will not get charged there's a very good chance your battery will just get spoiled and the reason for that is because the current that we get from the socket from the wall is an alternating current which means the charges are going to keep changing their direction of flow so once therefore maybe clockwise and then maybe they'll flow anti-clockwise and as a result when the current is flowing in one direction your battery might get charged but when it flows in the opposite direction will get discharged we get charged and then discharged and so your battery will not get recharged at all it might even get spoiled so in order to successfully charge the battery we need to do two things one is we need to lower the voltage that something will not focus on in this video but the second thing is we need to convert this alternating flow into a direct flow a one directional flow and that's what we will see in this video how to do that alright let's open the switch and our first step is going to be to lower the voltage and that's done by using a device called transformer now you may have already learned about this or may not it doesn't matter because we don't really care about how this works all this does is it lowers the voltage that's all that matters all right we don't have to worry too much about what it is so maybe the voltage over here is 200 volt alternating the voltage that we might get over here would be 5 volt alternating so that problem is solved don't worry too much about that now comes a big problem though the current that we're gonna get if we close the switch let me just do that let me just do that let's just close the switch and see how our battery will get spoiled and let's assume these blue dots represent positive charges inside the wire now of course in reality it's the electrons and negative electrons that are doing the motion but since the direction of the current is the same as that of positive charges let's assume some kind of positive charges are moving in the wire then it would look something like this this is what they will do they will once flow in the clockwise direction and then the anti-clockwise direction clockwise anti-clockwise your batteries getting charged discharged charged and discharged and that's what we need to solve now so let's open the switch that's what we need to solve so how do we do that well one solution would be to hire someone ask them to sit right next to the switch and then a third person to close the switch only when the current flows in the clockwise direction maybe that's what we want maybe that's when the battery gets rich hard let's say and open the switch when the current tries to flow in the anti-clockwise direction so if we look someone like this so you close the switch when the current is flowing clockwise and I was about to flow anti-clockwise open wait close clockwise open wait close open easy right the battery will get charged no problem but you can see that this is not a very practical method I mean in fact the current is alternating 50 times per second and no one is going to be able to do that right if only we had a way to automate this if only we had some kind of an automatic switch it only allows the current to flow this way but not in the opposite direction that would be awesome but we do have a device a PN Junction diode this is exactly what we want it is something that only allows the flow of charges in one direction but blocks the flow in the opposite direction and we will not be very familiar with this circuit symbol sometimes people might tend to get confused that which side is P and which side is N or remember this why this is the P side and this is the N side but guess what the circuit symbol is so great you don't really have to think about which is P n which is n you see an electronic engineers doesn't even care which is P n which is and here's there's the way that person would think about it you see the arrow mark over here is telling us that this diode in this orientation allows the flow of charges upwards all right and it doesn't allow the flow of charges downwards that's it we don't even have to worry which is P and N so if you want this diode to conduct where to put a positive here and negative here forcing the current this way if you would a positive here and you hear well the current won't flow that's all think of it this way don't worry about the pian and when you look at a circuit symbol like this all right imagine we want the current to flow through our battery downwards can you think how should we attach this diode over here there are two options one we might not be able to attach we should attach it this way or we should attach it this way can you think about how to attach that pause the video for a while and think about it alright let's do this since we know that the current has to be downwards only downwards we only want the flow in the clockwise direction that's what we want so over here we've won the current only to flow towards the left so if we orient our diode this way oh that won't work because this diode will only allow the flow in the opposite direction oh we don't want that we want to keep our diode this way now it will only allow the charges to flow in this direction towards the left but it won't allow the times to flow towards the right and there we have it now if we close the switch let's just go ahead and do that so let's close the switch then when the current is trying to flow in the clockwise direction our diode is going to conduct and I'm going to give it a green signal it's going to conduct but now when the current tries to flow in the opposite direction nope it won't allow the diode will not allow the flow and so again even it tries to flow in the clockwise direction the diode conducts and again in the opposite direction the diode doesn't conduct and so now we've successfully converted AC to DC and this process in electronics is called rectification and the circuit is called as a rectifier so this diode itself is a rectifier it's something that converts AC to DC it's fun to watch right and by the way the green and the red color are only for our indications the diet is not really growing don't worry okay let's open the switch for a while and what we'll do now is we'll draw a graph here it is our graph is going to be the current along the vertical versus time along the horizontal and you might see there are two graphs over here and the current by the way is the current through the battery in the first graph it will be without the diode without rectifiers your battery will get spoiled but anyways the graph in the second graph will be with the diode alright so let's see again let's let's get rid of the diode for a while and let's see what the graph would look like if you had just closed the circuit over here so let's just go ahead and close that switch close that switch I've already seen the charges will flow in both ways once it flows clockwise and then it flows anti-clockwise so what will the graph look like well it's a graph of an alternating current will call clockwise as positive and let's call the anti clockwise current as negative well in that case well when the current is flowing in the clockwise direction the current increases becomes maximum decreases then goes to zero then the current starts flowing in the opposite direction becomes maximum goes to zero again clockwise anti-clockwise and so on you may have already seen this before but this is alternating current okay now let's see what happens with the with the dire on so imagine we just closed the switch now what do you think the graph will look like I want you to pause the video and try to think about this graph yourself alright let's see so during the clockwise the diode conducts it laws the flow of charges so the current flows through you get an exact copy of what you had before but now during the anti-clockwise direction when it's trying to four anti-clockwise our diet doesn't conduct it doesn't conduct so the current is zero can you see that the current is zero now again during the clockwise direction it'll conduct and so you get that exact copy of what you had before but during the anti-clockwise it doesn't conduct it doesn't conduct anti-clockwise at all so you get zero current and so by looking at the waveform you can clearly see that in the second case the current is only flowing that the current is only flowing in the positive direction only clockwise flow and so we will call this as the non rectified current and this is the rectified current this is the rectified output current now notice that the current that is flowing here when when the diode is placed in this wave form notice the circuit is only switched on during one half of the cycle right it switched off for the other half the Wahab that we don't want we just turn off the circuit again we turn on and we turn off this circuit is that this rectifier is called as a half wave rectifier for obvious reasons it only works during the positive half the half that we need and you switches off when we don't need it all right and so the problem with this is that your circuit is off for half the time which means if you're charging your phone with this half wave rectifier and say your phone takes about two hours to charge it means it was only the circuit the phone is actually only charging for one hour for the remaining one hour it was always it was just searched off so a lot of time gets wasted so of course we need to improve upon our design but anyways you get the idea of how a rectifier even works now and one last thing is whenever I should look at the circuits like this I should always wonder like how come there are two wires coming out of the phone there's only one wire coming out right well this is actually a schematic drawing okay more realistic circuit would look somewhat like this nobody the circuit is exactly the same as before but it is made more efficient so that you know you don't waste a lot of space notice that the wires are actually there are two wires actually bundled together and they come out of the phone together this is packaged together and where do you think are these transformers and the diodes oh they are found inside the adapter that we use that's right it's all packaged of course there are more stuff today because we have better rectifiers so there'll be more stuff but all of those things are packaged in that tiny adapter and again this is bundled together so that end-users like you and me we don't have to worry about the science behind it at all we can just plug in and charge your phones amazing isn't it