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Full wave rectifiers

Half-wave rectifiers only keep the circuit on for half the time. To get more output we need a full wave rectifier In this video, let's see how we can build a full wave rectifier with just 2 diodes. Created by Mahesh Shenoy.

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Video transcript

in a previous video we saw that a single diode acts like a rectifier meaning it makes sure that charges only flow in one direction but the problem is it does that by switching off the circuit when the judges are trying to flow in the opposite direction which means for half the time it is switched on and half the time it is switched off that's why it's called the half wave rectifier but the problem with this is if you're charging a phone say for example with this then it'll take twice the amount of time for your phone to get charged because half the time it was a circular just cysts off it in this video we'll see how to build a full wave rectifier a full wave rectifier is the circuit will be on all the time switched on all the time and still the current should flow or the child should flow only in one direction through our through our phone let's see how to do that well if we just have one diode and then you place it wherever you want you place it here or here or whatever you want you will say you can't make a full wave rectifier it'll just act like this to build a full wave rectifier you at least require two diodes so let's bring in a second Dyer over here and let's think about how can we orient this diode and maybe we can get a Foley rectifier let's just play with this for a while let's try and explore maybe we take this diode and orient the same way as a diode we have down over here what do you think will happen if we had a circuit like this I want you to pause the video and think about what will happen well it turns out in this case nothing happens there'll be no current at all through the battery and the reason for this is you see when the current tries to folk rock wise over here this diode will conduct so this diode will have no problems it will be green but this dad won't conduct so it'll be red and now similarly when the current tries to flow anti-clockwise notice this dad will be green that'll be forward by us so this will be green but this will be red because this time you see the current trying to flow anti-clockwise won't work and so this will be red and that's why when one is followed by us the other one will be reverse and when one is reverse biased the other one will be forward and so this won't work at all hmm maybe we can flip this Dyer so let's see what happens if we flip that Dyer will flip that Dyer over and I want to again pause this video and see what do you think will happen in this case well now when current is trying to be clockwise notice both the diode will be forward biased both the diet will allow that so board dad's will be green and as a result will have a current flowing yay but when the current tries to flow anti-clockwise ah no you see again both of these will be red and so the current won't flow hen again she'll get the same thing as a half wave rectifier so this is still a half wave rectifier we just to tie Hertz hmm what do we do so what people really did to get a full wave rectifier is something not very obvious to me so I don't think we can work towards that logically so I'll just tell you what they did so their idea was well first bring this dad back to the original orientation both of them will put them in the same way okay so their idea was to to take these two diodes and put them in a different circuit altogether right that was the idea so how do we do that the right now they're in the same circuit right so they thought to remove this you know this phone from here so let's do that so let's remove that phone from there from this circuit and remember we want the current in this phone downwards that's when the battery is charging right that's what we assumed so let's flip it let's flip it like this and let's make sure the orientation is exactly same as the diode itself and we'll keep it somewhere over here so as of now the phone is at removed from the circuit and then we'll connect this wire will connect this wire over here and we'll take an additional wire from here and connect it to the phone and another wire from here to here so this is the circuit that people came up with oh now you may be wondering where did this wire come from well this wire is attached to the center of the secondary winding you got that so you can just imagine that we are taking one wire and soldering it to the center of the secondary winding that's it and so what this does now is it allows two path for the current to flow so when the current is trying to for clockwise for example you can have a clockwise flow here or you can also have a clockwise flow here that's there are two paths and similarly when the current is trying to go clockwise you can have a anti-clockwise fool here or you can have an anti clockwise flow here both are possible so now I really encourage you to again pause this video and try to see if you can figure out how this behaves as a fully rectifier just think about this alright let's see when the charges are trying to flow clockwise notice in the above circuit the diode will not allow that it's in the opposite direction right so the above circuit diode will be red it won't allow but notice in the below circuit the clockwise flow the Dyer allows and so during the clockwise motion this circuit will be active and so the current will flow only in this circuit similarly when the terrorists are trying to for anti-clockwise the exact opposite happens see when the Chinese are trying to for anti-clockwise this circuit will be switched off you see this dad will not allow that so that will be green so that will be red but in the circuit above and the judges try to flow anti-clockwise notice this diode allows so that will become green and so the charges flow anti-clockwise in the above circuit so when the Chinese are flowing clockwise this circuit will be on and when the tears are flowing anti-clockwise this circuit will be on so now let's look at the animation and see what happens you you you and now look carefully to the charges that are flowing through the battery through the phone notice that in both the half cycles the Chinese are flowing in the same direction can you see that same direction we have achieved full wave rectification right a last thing we'll do is to predict what will be the graph of the current versus time that's flowing through the phone in this fully rectifier again I want you to pause the video one last time and predict what the graph would look like well the first thing is the current is flowing all the time which means the circuit is conducting during both the half cycles both the half cycles but since the current is flowing always in the same direction always in the same direction what will happen well let's let's pause this animation for a while so we just saw that the Chinese are only flowing through this phone in one and only one direction if we call that direction is positive then you will get positive current and you will again get positive current regardless of what is going on over here does it matter whether it's flowing anti-clockwise or clockwise the current through the phone is always going to be in one single direction so you'll still get a current like this even here will again get a current like this so this will be the output of the fully rectifier and so this is no longer half wave rub this and this now is full wave rectifier full wave rectified alright one last technical detail is that you see the transformer we used for this is a special one usual transformers only have two output wires but now we have three output wires as an additional one that's taken from the center so such transformers are called center tapped transformer center tab transformers so if you want to build a full wave rectifier with just 2 diodes then we have to use this special kind of center tap transformer you you you