Luyện nói tiếng Anh bằng Shadowing qua video: Diodes Explained - The basics how diodes work working principle pn junction

Hey there guys, Paul here from Engineering mindset.com In this video We're going to be looking at diodes to understand the basics of how they work as well as where and why we use them so What is a diode? A diode looks something like this and it comes in different sizes.

They typically have a black cylindrical body as a stripe at one end as well as some leads coming out to allow us to connect It into a circuit this end is known as the anode and this end is the cathode But we're going to see what that means later on in this video You can also get other forms such as the Zener diode or an LED which is a light emitting diode but we're not going to cover those in this video a diode allows current to flow in only one direction in a circuit if We imagine a water pipe or the swing valve installed as water flows through the pipe It will push open the swing gate and continue to flow through However, if the water changes direction the water will push the gate shut and it will prevent it from flowing therefore water can only flow in one direction This is very similar to a diode we use them to control the direction of current in a circuit Now we've animated this video using electron flow which is where the electrons flow from the negative to the positive However, you might be used to seeing conventional flow which is traditional in electronics engineering and this is where the electrons flow from the positive to the negative Electron flow is what's actually occurring But you might come across Conventional current still as these explanations are easier to understand just be aware of the two on which one we're using So if we connect a diode into a simple led circuit like this one we see that the LED will only turn on when the diode is installed the correct way and That's because it allows current to flow in only one direction So depending on which way the diode is installed. This will act as either a conductor or an insulator In order for the diode to acts as a conductor The stripe end is connected to the negative and the black end is connected to the positive This allows current to flow we call this the forward bias If we flip the diode, it will act as an initiator and the current can't flow and we call this the reverse bias So, how does the diode work?

As you may know electricity is the flow of free electrons but atoms we use copper wires because copper has a lot of free electrons, which makes it very easy to pass electricity through We use rubber to insulate the copper wires and keep us safe Because rubber is an insulator which means its electrons are held very tightly and they can't therefore move between our atoms If we look at the basic model of an atom for a metal conductor We have the nucleus at the center and this is surrounded by a number of orbital shells, which hold the electrons Each shell holds a maximum number of electrons an electron has to have a certain amount of energy to be accepted into each shell the electrons located far east away from the nucleus hold the most energy The outermost shell is known as the valence shell and a conductor has between 1 and 3 electrons in its valence shell The electrons are held in place by the nucleus, but there's another shell known as the conduction band If an electron can reach this then it can break free from the atom and move to another With a methyl atoms such as copper the conduction band and the valence shell overlap. So is very easy for the electron to move With an insulator the outermost shell is packed. There's very little to no room for an electron to join The nucleus has a tight grip on the electrons and the conduction band is far away So electrons can't reach this to escape therefore electricity cannot flow through this material However, there's another material known as a semiconductor Silicon is example of a semiconductor with this material. There's one too many electrons in the outermost shell for it to be a conductor So it acts as an insulator But as the conduction band is quite close if we provide some external energy Some electrons will gain enough energy to make the jump from the valence and into the conduction band to become free Therefore this material can access both an insulator and a conductor if your silicon has almost no free electrons So what engineers do is dope the silicon with a small amount of another material to change the electrical properties?

We call this p-type and n-type doping we combine these don't materials to form the diode so inside the diode we have the two leads the anode and the cathode which connect to some thin plates and Then between these plates there is a layer of p-type doped silicon on the anode side And the layer of n-type types of cone on the cathode side The whole thing is enclosed in a resin to insulate and protect the materials Let's imagine the material hasn't been doped yet. So it's just pure silicon inside Each silicon atom is surrounded by four of our silicon atoms Each atom wants eight electrons in its valence shell But the silicon atoms only have four electrons in their valence shell So they sneakily share an electron with their neighboring atom to get their eight They desire this is known as covalent bonding when we add in the n-type material such as phosphorus It will take the position of some of the silicon atoms The phosphorus atom has five electrons in its valence shell So as the silicon atoms are sharing electrons to get their desired eight. They don't need this extra one So there's now extra electrons in the material and these are therefore free to move With p-type doping we add in a material such as aluminium or aluminium This atom has only three electrons in its valence shell so it can't provide its four neighbors with an electron to share So one of them will have to go without there is therefore a hole created where an electron can sit and occupy So we now have two Doak pieces of silicon one with too many electrons and one we've not enough electrons The two materials join to form a PN Junction at this Junction We get was known as a depletion region in this region Some of the excess electrons from the n-type side will move over to occupy the holes in the p-type side This migration will form a barrier with a buildup of electrons and holes on opposite sides The electrons are negatively charged and the holes are considered therefore positively charged So the build-up causes a slightly negatively charged region and a slightly positively charged region this creates an electric field and prevents more electrons from moving across the potential difference across this region is about 0.7 volts in typical diodes When we connect a voltage source across the diode with the anode a p-type Connected to the positive and the cathode n-type connected to the negative This will create a forward bias and allow the current to flow The voltage source has to be greater than the 0.7 volt barrier. Otherwise the electrons can't make the jump When we reverse the power supply So the positive is connected to the n-type cathode and the negative is connected to the p-type anode The holes are pulled towards the negative and electrons are pulled towards the positive and this causes the bearer to expand There for the diode acts as an insulator to prevent the flow of current Diodes are represented in engineering drawings with symbols like these The stripe on the body is indicated with a vertical line on the symbol and the arrow points in the direction of conventional current When we look as a diode We see these numbers and letters on the body these identify the diodes so you can find the technical details on line The diode will have an IV diagram that looks something like this This diagram plots the current and the voltage characteristics and forms this curve line this side is how it should perform when acting as a conductor and this side when acting as an engine a tur You can see that the diode can only act as an insulator Up to a certain voltage difference across it. If you exceed this, then it will become a conductor and allow current to flow This will destroy the diode and probably your circuit. So you need to make sure the diode is sized correctly for the application Equally the dough can only handle a certain voltage or current in the forward bias The value is different for each node And you'll need to look up this data to find the details The diode requires a certain voltage level to open and allow current to flow in the forward bias If we apply a voltage less than this, it will not open and allow current to flow But as we increase past that the amount of current that can float will rapidly increase The diodes will also provide a voltage drop into the circuit For example when they added this diode into the simple LED circuit mounted in a breadboard I get a voltage drop reading of 0.7 1 volts So why do we use them as?

Mentioned we used diodes to control the direction of current flow in the circuit That's useful for example to protect our circuit if the power supply was connected back to front The diode can block the current and keep our components safe We can also use them to convert alternating current into direct current as you might know AC or alternating current moves electrons forwards and backwards creating a sine wave with a positive and a negative half But DC or direct current moves electrons in just one direction Which gives us a flat line in the positive region if we connect the primary side of a transformer to an AC supplier?

And then connect the secondary side to a single diode The diode would only allow half the wave to pass and it would block the current in the opposite So the secondary side of the circuit experiences only the positive half of the cycle So is therefore now a very rough DC circuit although the current pulsates, but we can improve this one way to do That is if we connected for Dov's to the secondary site we create a full wave rectifier the diodes controlled which paths for alternating current can flow long by blocking or allowing it to pass as We just saw the diodes allow the positive half of the sine wave to pass But this time the negative half is also allowed to pass Although this has now been inverted to turn it into a positive half Also, this gives us a better DC supply because the pulsating has greatly reduced, but we can still improve this further We simply add in some capacitors to smooth out the ripple and eventually get it to a smooth line closely mimicking a DC supply We've covered how capacitors work in great detail in our previous video do check that out links down below So how do we test the diode? So we take our diode and our multimeter?

We connect the black probe to the end of the diode with the stripe We then connect the red probe to the opposite end when we do this We should get a reading on the screen. For example, this model one n400 one diode gives us a reading of 0.5 1 6 volts that is the minimum voltage it takes to open the diode to allow some current to flow if We now reverse the leaves connected to the diode We should see oh L on the screen, which means outside limits that's telling us that it's not being able to make a measurement and that's a good thing because it means it can't complete the Circuit so the doubt is doing its job If we were to get a reading connecting on both configurations, then the component is faulty and shouldn't be used to test the diode in a circuit for voltage drop We simply move the multimeter into the DC voltage function and then we place the black probe on the striped end and the red probe On the black end. This will give us a reading for example of 0.7 1 volts, which is the voltage drop Okay, that's it for this video but to continual learning then check out one of the videos on screen now and I'll catch you there For the next lesson, don't forget to follow us on Facebook Instagram Twitter as well as the engineering mindset calm