Electrical circuits

If you've just read the previous article in the series, you might think you haven't even begun learning electronics yet. Don't worry, because that feeling will soon pass. In this second part, we'll understand what an electrical circuit is and how it's represented. If this is your first time learning electronics, I recommend reading the previous article first.

Electrical circuits

The concept of a circuit is very simple. It's any association of components through which a current flows (or can flow) in a closed loop. It's a somewhat abstract phrase, so let's break it down a bit. Let's start: association of components. In electronics, components are used that perform different functions. The names will surely sound familiar: resistor, capacitor, inductor, transistor, diode... There are several types, and we'll look at them in detail in the future. If we connect two or more components, we have a network of components. Notice that I wrote "network of components," not "electric circuit." The reason is that a set of connected components doesn't meet the definition. The second part of the circuit definition is key: through which a current flows in a closed loop . Therefore, a network of components will only be a circuit if the components in the network are associated in a way that allows the current to flow. And how do they have to be associated for a current to flow? Well, so that they form a closed loop. That is, the current must leave a point and, passing through the circuit elements, be able to return to that point. If this isn't possible, the current won't flow (unless we introduce electromagnetic fields 🤪, which we won't do).

Let's forget about concepts and look at some examples. One of the simplest circuits you use every day is the socket-switch-light bulb circuit. Current leaves the socket, passes through the switch if it's in the "ON" position, then passes through the light bulb and returns to the socket. There's a path out and a path back. The current leaving the socket and returning is the same .

If you've understood what we've discussed correctly, something must be rattling inside you. We've said that current flows out of and returns to the socket, and we've also said that current flows in closed loops. So, you might ask, where did the current in the socket come from? Has it always been circling in that circuit? Did a magician bring it there? There are two important ideas that emerge from this situation. Let's look at them.

The first idea is that we can have electrical circuits within circuits. Our example circuit, the lamp, is part of a much larger circuit that involves the electrical grid of the building where the lamp is located, the local distribution grid, the transformer stations, the long-distance grid, and so on, all the way to the electricity generating plant. If we look at it from a distance, the current makes a huge "journey" to light the lamp. However, none of that matters when analyzing the switch-lamp circuit. And this is precisely the second idea. In electrical circuits, it's common to replace a set of elements with a "summary" or fictional version of them. In fact, what we've done is consider the plug to be a magical thing (or a battery) from which current flows. By doing this mental reversal, we can forget about all the other elements of the "large circuit" and work with far fewer elements. This is very common when working with circuits.

You already have a good understanding of what a circuit is, but before moving on to other things, let's look at the typical circuits that most electronics technicians work with. The most common approach is to place the components and connect them together on a flat surface. In professional environments, PCBs ( printed circuit boards ) are used, which are "cards" made of fiberglass onto which the components are soldered. The components are connected using copper tracks or traces . The advantage of using PCBs is that they are reliable and very cheap when manufacturing in large quantities. In hobby or home environments, breadboards are often used. A breadboard is a PCB that has a series of factory-made connections (always the same ones). The PCB is then covered with plastic with holes into which the components can be inserted. This makes assembling a circuit very simple, as it is not necessary to solder the components. The downside is that very large circuits cannot be assembled, and the result is always a bit sloppy. A third option, used both in hobby and professional prototyping, is stripboard. This is a perforated card with pre-printed copper tracks. These can be easily cut to create circuits by simply "trimming off the excess."

If you look at the previous image, you can see that the components are connected to each other in completely different ways, with traces used in some cases and cables in others. The same circuit can be assembled on any physical medium (with enough patience 😄). In the images above, you've seen a physical representation of how components are connected. However, looking at the images, it's not entirely clear what's going on. Jump into the future and imagine you already know a ton about electronics and want to explain your circuit to someone on paper. How do you give them the information? Do you draw something similar to the one above? Do you describe it to them? That's what we're going to look at next.

Circuit diagrams

Before looking at what circuit diagrams look like, we need to briefly reflect on topology. TopologyWHAT? In the context of electronics, topology refers to the relationships between components, from a logical, rather than a physical, point of view. That is, which components connect to which one another. Let's first illustrate this with something you're probably familiar with. Take a look at the following image:

On the right, there is a map of Barcelona with the actual route of the metro lines superimposed, and the stations and stops marked. On the left, there is a topological diagram of the lines and stations. What information does the diagram on the left give us? It is not the actual location of the stations, but rather the relationship between stations. The metro network described in the image on the left could have been implemented in many other ways (trains could pass through other streets, for example) without affecting users, as long as the position of the stops and possible line transfers at them are maintained. We will use these ideas when creating circuit diagrams. Now is the time.

AAAaaabefore we start, it's important. If you want to learn electronics, you have to understand and know how to draw circuit diagrams well. It's not worth taking shortcuts or relying on simplified representations. Believe me, you won't get far that way. I mention this because it's very common to find tutorials on the internet with physical representations of circuits. That said, let's get to it. And, by the way, circuit diagrams are often called "schematics" or "schematics."

Remember the socket-light bulb circuit we saw earlier? Let's see it drawn in a circuit diagram:

Well, how's that for you? There's not much to it, really. This diagram represents the same elements we saw before: the plug, represented by a voltage source ; the cable, represented by black lines; and the light bulb, represented by that strange symbol. This diagram only gives us information about the connection points of the two circuit elements (plug and bulb) and says nothing about the physical form of the circuit. Be careful, this is extremely important. It doesn't matter how we draw the "cable," which doesn't represent a "cable" but rather a connection. The black line between the voltage source and the bulb is, in fact, a single point in space. Before moving on to something more complex, you have to consider what the current does in this circuit. If you look closely at the cable connections, you'll see that the circuit is closed, which is an essential condition for there to be current. The current leaves the plug, passes through the bulb, and returns. As long as the circuit is closed, the current will flow, causing the bulb to glow. There are a few key questions: How much current is flowing? What voltage is needed? Be patient. These are very important questions, but we won't delve into them yet. Before that, let's look at a few more circuits, just to make sure we're clear.

Parallel and series associations

Ale, now you have two identical light bulbs! And not only that, but they're both connected to the socket. How do you connect them to the socket? We don't care! That's not what the diagram is supposed to show. Let's look at some interesting things about this circuit. The first thing to realize is that two light bulbs could illuminate twice as much as one. But notice that I used the conditional tense. For that to happen, twice the current is needed. If you're up to date with your electricity bills, that shouldn't be an impediment. The current flows through the circuit, but now it does so in a more complex way than before. Notice that it has to be divided between the two bulbs and then brought back together. We said that the two bulbs are connected to the socket, represented in the diagram as a voltage source. This way of connecting things is called parallel association. In other words, when two or more elements share voltage, they are in parallel. Do a mental check to see if you understand: think of everyday things that are connected in parallel. Have you thought about it yet? Well, it shouldn't have been difficult, because almost everything in your house is connected in parallel. Your house is a large circuit. When you plug things into outlets, you're actually adding elements in parallel to others already connected (even if they're connected at the other end of the house!).

How else could we connect the light bulbs? Well, in series. Series association is another way of making connections. In this case, we say that two or more elements are in series if the current passing through them is the same. How can we make this connection? We see it in the following diagram:

Now, we don't have both bulbs connected to the socket. Each bulb has one end connected to the socket, and the other end connected to the other bulb. Unlike the previous case, the current doesn't have to be divided between the bulbs; rather, it passes first through one and then through the other. What is divided in this case is the voltage. Half of the voltage falls across each bulb . The concept of voltage is more complex than that of current, so we'll delve into it later.

In the last two circuits, we assumed that the bulbs are identical and, therefore, shine equally. The brightness of a bulb depends on the voltage across the terminals and the current flowing through it (higher voltage equals more current; we'll see this later). In series and parallel circuits, the bulbs will shine equally as long as we can provide each bulb with the voltage and current it needs. When you connect bulbs in parallel, more current automatically flows from the socket. If you connect them in series, this doesn't happen. The reason is that we said that, in a series connection, the voltage is divided among the bulbs. Since the socket voltage is fixed, this means that each bulb will have half the voltage it "would" have, and will shine half as brightly. The fact that the socket voltage is fixed is for practical reasons, but there is no physical limitation that imposes it. If you could double the socket voltage, then the bulbs would shine equally whether connected in parallel or in series. This last paragraph may be a little harder to digest, but that's okay because we'll revisit this topic when we discuss Ohm's Law soon. However, before that, let's talk about open circuits and short circuits.

Open circuits and short circuits.

One of the most undesirable, but common, things in electronics is that, unexpectedly, smoke comes out of your equipment while you get nervous and curse all your %=!os mu%|&/Fs. In short, what you were trying to do breaks. The most common cause is short circuits. A short circuit is, simply, the connection of two points in the circuit by a low-resistance path. Careful! We still don't know what resistance is, so for now, assume that low resistance = high current. A short circuit causes the current not to travel along the path you had planned, but along a different one. This usually ends in something burned.

Another common circumstance is an open circuit. This occurs when the current has no path to return to its point of origin. Remember that we've already seen that current can only exist in closed circuits. Without the current, nothing you intended to do with the circuit will happen. Let's look at two examples using the previous circuits to test your intuition.

Let's look at the previous diagrams again, but now they have some variations. The circuit on the left has suffered a short circuit in the first bulb. What is the effect of this? Let's think like an electronics expert. Bulb number 1 and the short circuit are in parallel, right? The current has to be shared between the red connection and the bulb. Since the short circuit has a much lower resistance than the bulb, almost all the current will go through it, and practically none through the bulb. Therefore, the bulb won't glow. Will bulb 1 break? It shouldn't, since no current flows through it. And what happens to bulb 2? If you remember what we saw in the series association, you'll see that we said that the two bulbs split the voltage of the socket equally. Now, since bulb 1 is short-circuited, all the voltage from the socket goes to bulb 2. It's as if, in fact, we had only connected bulb 2 to the socket. If bulb 2 was not designed to withstand the full voltage of the socket, it would have blown due to overvoltage.

Now we just have to ask ourselves what's going on with the plug. Let's recap: we represent the plug as a source. A current comes out of the source that depends on what's plugged into it. So far, so familiar, right? I mean, if you plug Christmas lights or a microwave into a household outlet, the current coming out of the plug is obviously different. And why is that? The real reason is as complex as hell, but we can simplify it by saying that the microwave has a lower resistance than the Christmas lights. Low resistance, more current, easy peasy. Let's go back to the short circuit. The plug (the source) was outputting a current that we'll call "X"—the number doesn't matter for now. That current depends on what's plugged into it, which was originally two light bulbs in series. Each bulb has a certain resistance; we don't care about the number either, but we do care that they're both the same. When the first bulb is short-circuited, the total resistance is halved. So, the current coming out of the plug doubles. This could break the water supply or trip a protection circuit. Everything we've said can be summed up in one concept you shouldn't forget:

• Anything in parallel with a short circuit is as if it didn't exist. You can erase it directly. Therefore, the previous circuit is as if it consisted only of the power supply and bulb 2.

Let's look at the image on the right. You see that I've erased part of the connection between bulb 2 and the source. The circuit, therefore, is no longer closed, or in other words, it's an open circuit. You already know what happens to the current in this state: there's no current. So, if there's no current, what does the circuit do? Well, it does nothing. Which brings us to the second thing you should remember:

• Anything in series with an open circuit is as if it weren't there. In our example, we'd be left without a complete circuit.

Well, this may have seemed a bit far-fetched, but it's reasoning used every day when working in the electronics world. If anything isn't clear, leave a comment with your questions, or help other users with theirs.

The next step we're going to take is to quantify what we've explained in this section and give some concepts a little more rigor (not much, though; I've never been a fan of rigor). We'll discuss Ohm's Law in the next part.