Lesson Video: Light-emitting Diodes Physics

In this lesson, we will learn how to identify the action of LEDs in circuits.


Video Transcript

In this video, we’re talking about light-emitting diodes, also called LED, is for short. Light-emitting diodes are very common components in modern electrical circuits. In this lesson, we’ll see how they work and just what it is that makes them so useful. One way to start understanding LEDs is to think of them, according to their name, that they’re diodes that emit light. So first, let’s talk about what a diode is. A diode is an electrical circuit component that acts as a one-way switch. In a circuit, a diode lets current flow one way, but it blocks any current trying to flow in the opposite direction.

By way of analogy, say that we have a pipe with water flowing through it in one direction. Then imagine that, at a midpoint along the pipe, we put in a component called a valve. The job of the valve is to let water flow one way, in this case, from left to right. But if the water flow were to reverse direction, trying to go from right to left in this case, then the valve would prevent that. This is just how a diode works in an electrical circuit. The symbol for a diode looks like this. And it’s the way that the arrow head on the diode points that shows us the direction that conventional current is allowed to flow through the diode.

If we change this, say by flipping the diode around, then no current could flow in the circuit because the diode’s direction is resisting that flow. So the job of a diode in an electrical circuit is to serve as a one-way valve for current. And we can see that because an LED is based on a diode, it will behave the same way. We can see though that there’s something else an LED does as well. It also emits light when current is flowing through it. We see that there are basically two things that an LED does. First, it serves as a one-way valve for current flow in a circuit. It allows current in one direction and blocks it in the other.

Then also an LED gives off light when current is flowing through it. We’ve seen that this symbol is the symbol of a one-way current switch, a diode, and the symbol for an LED builds on the diode symbol. If we take that symbol for a diode, put a circle around it, and then have two arrows going off from that circle, that shows the light that’s given off when current is traveling in the right direction through the LED. Since the allowable current flow direction through a diode or an LED is in the direction the arrow on the symbol points, in this case, that would mean that when current is flowing for right to left this device, this light-emitting diode, gives off light.

So that’s what an LED does. But the question naturally comes up. Just how is it that a light-emitting diode gives off light? To understand that, we’ll need to look down at the microscopic scale. Light-emitting diodes are made up of electrical materials that are neither conductors nor insulators. In other words, this material is neither very good at letting electrons pass through. That would be a good conductor. But it’s also not very bad at doing that, either. That would be an insulator; it’s somewhere in the middle. And this material is called a semiconductor. Light-emitting diodes in particular are made up of two different types of semiconductor.

And it’s the things that happened at the boundary between these two materials that really lead to the way a light-emitting diode functions. More specifically, this particular semiconductor material has a number of electrons that freely move about in this material, while this semiconductor material over here has the opposite thing going on. Instead of there being lots of mobile electrons, like on the other side, this side has mobile electrons absences, sometimes called electron holes. So on the one side of our diode, we have these negatively charged electrons moving about, while on the other side, we have moving around these electron absences, which are effectively positively charged gaps.

When we connect our light-emitting diode up to a power supply and current starts to flow in the circuit, then what we’ll have is negative charges coming in from the left and effective positive charges coming in from the right. Since like electrical charges repel one another, push one another away, this influence has the effect of pushing some of our mobile negative charges into our mobile effectively positive holes. When this happens, when an electron moves into an electron hole, the term for that is recombination. When this takes place, the system overall reaches a lower energy state. And the excess energy, we could call it that our mobile electron and our mobile electron hole had before being recombined, is then released in the form of light photons.

The particular wavelength, and therefore the particular color of the light that’s released, depends on the specific properties of these semiconductor materials. But whatever the specific color of light given off, this is the mechanism by which that happens. The technical name for this process is electroluminescence, so we can see it has to do with electrons and light being created. Now, here’s something interesting about light-emitting diodes. If we take a light-emitting diode and we connect it up to a power cell so that current is allowed to flow, then the LED by itself doesn’t limit the amount of current it allows through. Rather, it calls for more and more current from the power supply. This could be damaging to the LED though. If too much current runs through it, it will burn out.

So if we want to limit the current allowed to flow through the LED to a safe level, what component might we put in the circuit to do that? A great electrical component for doing this is a resistor. Recall that if we think of electrical current, like a river of water flowing down through a riverbed, then putting a resister in the circuit is a bit like putting rocks and sticks and leaves in the way of the river. It slows down and limits the current that’s able to flow through. And that’s exactly what we want to do here in the case of our led, which would take more current than it can handle if we let it. So when we see a light-emitting diode in a completed circuit diagram, often we’ll see a resistor there as well. The two go hand in hand in order to safeguard the light-emitting diode from drawing too much current.

Now, we mentioned earlier that, in modern electrical circuits LEDs, are fairly common components. This has to do with the advantages that light-emitting diodes offer. For one thing, LEDs are able to create light through this process of electroluminescence without also creating much heat. We may know from experience, for example, that a light-emitting diode gives off light but is not hot to the touch. Along with this, LEDs can serve as indicators that a circuit is functioning properly. It can show, by giving off light, that current is flowing the way that it’s intended to.

And along with all this, we can vary the brightness of the LED by varying the amount of resistance in the circuit with it. The lower the value of a resistor, the more current an LED can draw and the more light it can emit. But then, on the other hand, if we would prefer a dimly LED light-emitting diode, we can have that too. We accomplish it by raising the value of a resister in the circuit with the LED. So light-emitting diodes emit light while generating very little heat. They can serve as indicators for when a circuit is functioning properly, and they’re also tunable. They could be made brighter or dimmer. Knowing all this, let’s test our understanding of light-emitting diodes through an example question.

The diagram shows a circuit containing a cell, a resistor, and an LED. The LED does not turn on. Which of the following reasons explains why?

Okay, before we get to those reasons, let’s take a look at this diagram. We see in it that there’s a power cell, a resistor, and this symbol here, which represents an LED, a light-emitting diode. We’re told the led is not turning on with the circuit set up as is. Let’s look now at some possible reasons why.

Option A, an LED can only be used in logic circuits. Option B, an LED cannot be placed in series with a resistor. Option C, an LED can only be used with alternating current power sources. A cell provides direct current, so the LED will not work. D) an LED is a type of diode. Current only flows in one direction through a diode, and in this circuit, the diode is connected in the wrong direction to allow current through it. And finally, option E, the current through the LED is too large. LEDs can only work with small currents.

All right, of these five answer options, we want to pick which one correctly explains why the LED is not lit up. Option A claims that LEDs can only be used in logic circuits. Now, a logic circuit is one where binary inputs, inputs that are either one or zero, are combined together to give a single binary output, an answer or a result also of one or zero. It’s true that LEDs could be used in logic circuits. Say an LED was placed so it would light up when current flows through a certain part of the circuit, indicating on or true. But this doesn’t mean that LEDs can only be used in such circuits. It’s perfectly fine to put an LED in a circuit that doesn’t involve input and output ones and zeros.

For example, if we designed a circuit with the goal of creating light emitted by an LED, then that simple circuit need not be a logic circuit, but it’s simply a practical circuit designed to give off light. So while LEDs can be used in logic circuits, they need not only be used that way. So option A isn’t our choice. Option B then says that an LED cannot be placed in series with a resistor. Looking back at our diagram, we see that this is indeed the case here. An LED is in series with a resistor. But actually, an LED and a resistor being part of the same circuit is more of a good sign than a bad sign. That’s because LEDs tend to draw more current than they’re capable of handling. And so resistors do a good job of limiting the current that an led is exposed to and thereby safeguarding the LED from burnout. So light-emitting diodes cannot be placed in series with a resistor. And often, that’s a helpful approach.

Moving on to option C, this says that an LED can only be used with alternating current power sources. This option points out that the cell in our circuit provides a direct current. And that’s the reason the LED will not work. As we analyze this answer option, let’s take a closer look at the symbol for the light-emitting diode. If we were to forget for a moment these two arrows here, as well as the circle that’s part of the LED symbol, then we would have a symbol that looks like this. And this is the electrical circuit symbol for a diode.

Now, a diode, and a light-emitting diode is a type of diode, is designed so that it only allows conventional current that is the flow of positive charge to pass through in one direction. And that direction, by the way, is indicated by the way that this triangle points. In this case, current can move through the diode, left to right. Current trying to flow the other way will be blocked. Keeping that in mind, if we think about an alternating current or AC power supply, this is a power source where the current direction flips or reverses rapidly, often many times a second. Based on the way a diode works, it would prevent current from flowing in a circuit with an AC power supply any time that power supply try to send current in the direction opposite the way the diode pointed.

Since current in an AC circuits spends about half its time running one way and half its time running the opposite way, we might expect a diode in an AC circuit to be rapidly turning on and off, on and off over and over every time the current change direction. In order for a diode, and by extension a light-emitting diode, to operate steadily, we would want the diode to be in a circuit with a direct current supply, one that’s always pointing in the same direction. And one that’s able to pass through the diode. So it’s not true that LEDs can only be used with AC power sources. So we cross off option C.

Option D then says that an LED is a type of diode. Current only flows in one direction through a diode. And in this circuit, the diode is connected in the wrong direction to allow current through it. Well, we’ve seen that, indeed, this description of a diode is correct. It only allows current to flow one way through it. And looking again at the cell in our circuit, we see that, based on its orientation, this cell would create conventional current that flows in a clockwise direction. But then look at this. The triangle or the arrowhead on our light-emitting diode points the opposite way. This means that the way this LED is oriented in the circuit will prevent it from allowing any current to pass through. As this answer option states, the diode is connected in the wrong direction. If we were to flip it around to reverse its polarity, it would allow current to flow through. But as is, current is blocked.

So it looks like option D will be our answer. But let’s take a look at option E just to see what it says. This option says the current through the LED is too large. LEDs can only work with small currents. But looking back at our circuit, since the LED is not lit up, that means no current is flowing through it. And that means no current is flowing in the circuit at all. So the current running through the LED can possibly be too large when that current is zero.

And then further, this answer says that LEDs can only work with small currents, but that’s not true either. While there is an upper limit to the current that a given LED can sustain without breaking down, it is possible to make LEDs which are capable of handling relatively large currents. So Option E is not an accurate description of why the LED isn’t turning on. Our answer then is option D. An LED is a type of diode. Current only flows in one direction through a diode, and in this circuit the diode is connected in the wrong direction to allow current through it.

Let’s take a moment now to summarize what we’ve learned in this lesson about light-emitting diodes. We saw it first that light-emitting diodes, called LEDs for short, do two things when they’re placed in a circuit. First, they limit current flow to one direction in the circuit. And second, when current does flow, they give off light. We saw further that the circuit symbol for an LED, which looks like this, is based on the circuit symbol for a diode, which looks like this. And we saw further that the direction of the triangle, or we could call it the arrowhead in the LED symbol, indicates the direction of allowable current flow.

We learned that light-emitting diodes are made from semiconductor materials, that is, materials which are neither very good conductors nor very good electrical insulators. And that the mechanism by which they give off light, by which they are light-emitting, is called electroluminescence. Lastly, We looked a bit at the properties or advantages of light-emitting diodes and saw that they’re able to generate light while generating very little heat, that they can serve as indicators for whether a circuit is operating properly, and also they’re tunable. They could be made brighter or dimmer by varying the current that flows through them.

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