Video: Determining the Biasing of a LED

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? [A] An LED can only be used in logic circuits. [B] An LED cannot be placed in series with a resistor. [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. [E] The current through the LED is too large. LEDs can only work with small currents.

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Video Transcript

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.

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