Video: Circuit Symbols and Diagrams

In this lesson, we will learn how to identify basic circuit symbols in circuit diagrams.


Video Transcript

In this video, we’re talking about circuit symbols and diagrams.

As we start out, let’s first talk for a bit about just why it is that such symbols and such diagrams exist. Imagine that you were designing a house and that, as part of the job, it was up to you to describe just how the house would use electricity. You were gonna make some plans and then hand them on to the builders of the house. Say that you came up with something that looks like this. The question is, will the builders of the house be able to understand this sketch and then create what it shows?

We can see that if there are agreed-on conventions for what different parts of an electrical circuit will look like, there is a much better chance of understanding what’s going on. And that, really, is our topic, understanding circuit symbols and diagram conventions.

Say that we start out with this. What we’re looking at is a loop of wire and nothing else. There’s nothing powering current. There are no resistors. There’s nothing in this circuit. It’s simply a loop of conducting wire. Notice though that we draw this wire in straight lines from one point to another. What we mean is that we could draw connections between points in our circuit like this, like a wire might look like in real life. But the convention, both for tidiness as well as clarity, is to make wires look like this.

Now, as we said, as it is, this isn’t a very exciting circuit. There’s no current flowing and nothing is going on because there’s nothing to power current. What we need is an electromotive force, an EMF, in order to push charge along. And this device is designed to do just that. It’s designed to push current in a circuit. The name for this device might be a bit of a surprise. In everyday life, we often refer to this as a battery. But, in fact, a battery is what’s formed when we link up multiple of these individual units together. By themselves, each one of these units that’s used to link up to create a battery is called a cell. So the name for a single unit providing electromotive force is a cell. But when we link multiple cells together end-on-end to create a compounded effect, that’s called a battery.

The reason we get into this is that the symbol, the electrical symbol, for a cell is different than the symbol for a battery. As we mentioned, a cell refers to a single unit. If we were to put a cell into the circuit that we have here, that cell symbol would look like this. We can see that one of these parallel lines is longer than the other. And the longer line indicates the positive terminal of the cell. If, however, we were to take our cell and start to link up other cells with it, those creating a battery, then the circuit symbol that represents this would be different. We said that a battery is some number of cells strung together. And this symbol gets that idea across. It shows us that there is at least two cells involved here and possibly more than that. So anytime you see this symbol on a diagram, that indicates a cell. On the other hand, this symbol indicates a string of cells, that is, a battery.

We have then a source of electromotive force in this loop of wire. That means that current will start to flow. Conventional current will start to move in the anti-clockwise direction. As it travels around the closed loop, this current will encounter any circuit elements there may be. One of the more common elements we find in circuit diagrams is what’s called a resistor. As the name implies, the function of a resistor is to resist the flow of current in the circuit. And the symbol for a resistor in a circuit looks like this.

Now, it maybe that one day you happen to cross a circuit diagram which shows you an element like this. This element may look unfamiliar. But, in fact, it’s another way of symbolizing a resistor. These two symbols are completely equivalent to one another. They mean the same thing. When you do see the resistor’s symbol that looks like these shark’s teeth, it’s not important the exact number of up and down lines that the symbol has. There is variation within this convention. Simply, recognizing that it does look like a shark’s-tooth pattern means that there’s a resistor there in the circuit.

Now, before we go much farther, it’s helpful to know that, sometimes, symbols on a circuit diagram have labels on them. For example, if the resistor we were working out had a particular resistance value, say 10 ohms, we might see that printed above the symbol for the resistor. Likewise, if our battery had a particular voltage supply, say 25 volts, we might see that written above that symbol. Sometimes these additional labels exist and sometimes they don’t. But, in either case, we’ll know now what they mean.

When we consider resistors in a circuit, one way they often show up is as light sources. For example, an incandescent bulb is a resistor in a circuit. What if we wanted to specify that this particular resistor wasn’t any ord. resistor, but was in fact a bulb, something that could light up. The symbol for that looks like this. This is the symbol that indicates a bulb in an electric circuit. So then, we have this bulb. And with the bulb in the circuit as shown, it will be lit up.

But what if we wanna create a control for that bulb, a switch so it can be on or off. The symbol for a switch when it’s open looks like this. And it clearly shows that current now stops flowing through this loop. And therefore, anything in it that requires current would not be powered. This is the symbol for an open switch. And if we wanna close the switch and so complete the circuit again, the symbol for doing that looks like this.

Now that current is once more flowing, what if we wanted to measure that current. We would do that using a device called an ammeter. The symbol for that looks like this, an 𝐴 with a circle around it. Now we’ve got the ability to measure the current running to this circuit. What if we wanted to figure out the voltage, the potential difference across our bulb? In that case, we would attach a voltmeter in parallel with the bulb. Similarly to the symbol for an ammeter, that for a voltmeter is a circle with a 𝑉 in it.

Now that we’re measuring quantities in this circuit, let’s say we want to limit the circuit so that if the current exceeds a certain value, the whole thing will shut down. In that case, we install what’s called a fuse into the circuit. It’s a device which breaks when the current in it exceeds a certain value. The breaking of the fuse means that there is no longer a closed loop for current to travel through and the whole circuit shuts down. The symbol for a fuse commonly looks like this. It’s a box with the circuit running through it.

Let’s say that in addition to our fuse, we also wanted to install a valve in this circuit so that current could only flow in one of two directions. The electrical component that does that is called a diode. It only allows current to flow through it in one direction. This here is the symbol for a diode. And we can tell which way it allows current to flow by looking at the direction the arrow points. The way this diode is set up, it allows current to flow in a counterclockwise direction but not in a clockwise direction. It would block any current trying to move that way.

Let’s imagine now that instead of having a fuse here in our circuit, we want to replace it with a resistor, but not just any ord. resistor. We’ll put in a resistor that changes based on its environment. There are a few different varieties of this type of resistor. One type is called a thermistor. This is a resistor whose resistance value depends on its temperature. The symbol for a thermistor is nice because we start out with the symbol for a resistor, like we saw before, and then we put a line like this through it that has a little band at the end. This is the symbol for a thermistor, that is, a resistor whose resistance value depends on its temperature.

But there’s another parameter that resistance value can depend on. That is the light incident on a resistor. This component is called a light-dependent resistor or LDR, for short. Notice that it starts out with the basic resistor symbol then surrounds it with a circle. And then we have these arrows going to it indicating the incident light on the resistor.

While we’re on the topic of electrical components and light, going back to our diode, one species of diode is a type that emits light, called a light-emitting diode or LED. The symbol for that also involves a circle around the base electrical component. And this time, the arrows indicate that light is leaving the device because it’s light emitting rather than the light-dependent resistor which absorbs light. But, going back to our resistor, in this case a light-dependent resistor, another kind of resistor, symbolized in a different way, is one that’s simply called a variable resistor. This is symbolized using the regular resistor symbol and then adding a diagonal arrow over top of it. A variable resistor is one that can be tuned, perhaps, by hand so that the resistance value is adjusted.

By the way, this symbol of the diagonal arrow to represent variable is a general symbol for electrical components. If we put that same arrow over top the symbol of some other component that could be varied, say a battery, then that would indicate a variable battery. There are still more symbols for electrical circuits that we could learn. But this is a good set to start with. Now that we’ve had a look at these symbols, let’s get some practice using them through a couple of examples.

The diagram shows four circuit symbols. Which symbol represents a variable resistor?

We see here the four symbols shown. And we’re told that one of them represents a variable resistor in a circuit. Recalling what circuit symbols mean is really a memorization task. Let’s just start from the top and remember what each of these symbols represents.

Option A shows us the symbol for an electrical fuse. This is a device used to limit the maximum current running through a circuit. In option B, we recognize the shark-tooth pattern of this symbol, telling us that it’s a resistor. Option C is like option B. But we see it has the additional diagonal arrow running through the resistor. That tells us that the value of this resistor can be varied. It’s a variable resistor. Option C is our answer then. This shows us the symbol for a variable resistor.

Just out of interest though, let’s consider what option D shows us. We see that sometimes a resistor is represented using this box and sometimes it’s represented using the shark-tooth pattern. The representations are equivalent. And which one we use just depends on which convention we’ve learned. In any case, this line with a slant to it going through the resistor indicates that this is a thermistor, a resistor whose resistance depends on temperature. Our answer though is answer option C. This is the symbol that represents a variable resistor.

Let’s take a look at a second question involving circuit symbols.

The diagram shows an electrical circuit. How many light-emitting diodes does the circuit contain? How many thermistors does the circuit contain? How many voltmeters does the circuit contain?

Looking at this diagram, we see indeed it represents an electrical circuit with a number of electrical components in it. In order to answer these questions about how many of certain components are in the circuit, we’ll need to know what those components look like as symbols. In other words, in order to answer how many light-emitting diodes are in this circuit, we’ll need to know the symbol for a light-emitting diode. And likewise for thermistors as well as voltmeters, we’ll need to know the symbols for each of these components. Once we recall the symbols, it’s just a matter of counting the number of times that symbol appears in the circuit in the diagram.

So starting with our first question, how many light-emitting diodes does the circuit contain? A light-emitting diode is, first of all, a diode whose symbol looks like this. And when that diode emits light, we modify the symbol to have a circle around it with two arrows leaving the circle. So the question becomes, looking over at our circuit diagram, how many times do we see this symbol appear? Looking the circuit over, we see it appears here, that’s one time, and then here. That’s another time. Now, this symbol over here looks a bit like a light-emitting diode. But it’s lacking the arrows leaving the circle. Those arrows indicate the light being emitted. So this doesn’t count as a light-emitting diode in the circuit. So then we’ve got two light-emitting diodes in this circuit. And we’ll write that down as our answer.

The next question asks about the number of thermistors contained in the circuit. A thermistor, remember, is a resistor whose resistance depends upon its temperature. A common way to symbolize a thermistor is like this. It starts out with a basic resistor symbol and then adds this diagonal line with a little slope to it. Looking back to our circuit, we see a somewhat similar-looking symbol right here. But that symbol has a diagonal arrow over a resistor whereas the thermistor symbol is a little bit different. Overall then, we don’t see any thermistors in this circuit. So we’ll write that down as our answer.

And finally, we want to know how many voltmeters are here in this circuit. Recalling the symbol for a voltmeter, that’s a circle with a 𝑉 in it. Our circuit has a circle with an 𝐴 in it. But that’s an ammeter, a device for measuring current, rather than a voltmeter. We don’t see any of those in the circuit shown. So once again, we write down an answer of zero.

In this electrical circuit then, there are two light-emitting diodes, zero thermistors, and zero voltmeters.

Let’s summarize what we’ve learned so far about circuit symbols and diagrams. In this lesson, we saw that a cell, symbolized this way, is the single unit from which a battery, symbolized this way, is made. We also saw that a resistor, a very common element in electrical circuits, can be symbolized this way, using what we called a shark-tooth pattern or this way. And along with all that, we learned a bunch of circuit symbols. We learned, as well, the symbols for a fuse, a thermistor, an open switch, a light-dependent resistor, a variable resistor, a diode, a light-emitting diode, a bulb, an ammeter, and a voltmeter. These are all circuit symbols that we learned and can now put to use.

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