Testing circuit intuition

This post will serve to assess whether you've developed a good intuition when interpreting circuits, as well as to check whether the concepts were well understood. Although some examples may seem a bit absurd, the reality is that these are scenarios muy occur, not by themselves, but rather as part of larger problems or complex situations. That's why it's very important to clear up any doubts so you can focus on the real problem. We're not going to do any math; it's about understanding the concepts. I suggest you approach each circuit as a challenge and think about it (thinking is important) before reading the explanation. That said, let's begin.

The importance of understanding topology.

Take a look to the circuit before proceeding.

At a quick glance, the circuit looks normal. A voltage source, some resistors... Easy stuff... Or not. If I told you this circuit is the same as the first example we saw in the previous post, does it seem obvious to you, or do you think I'm misleading you? Take a look and see if you get inspired. Still not? Here are some clues:

• R2 and R3: Are they in series or parallel? Does it really matter?
• Does the current flow from the source to ground? Which loop does the current follow?
• The mass symbol is lying in a weird place.

Okay, let's unravel the circuit. The first thing you should know is that the ground symbol doesn't make one node more special than any other. We could have drawn it anywhere and nothing would change in the circuit. In short, I put it there to make you think about it. Since the ground symbol is (almost) always drawn at the "-" of the power supply, this can create the false idea that the current has to flow from the "+" to ground, but that's false. What the current should do is circulate through a loop, mesh, or loop—call it what you will—but keep in mind that the current leaves a point and returns to that same point, whether there's a ground symbol or not.

Okay, first clarification made, and we haven't solved any problems, but bear with me, this isn't a race. The next thing we need to think about is which loop the current will follow. It clearly leaves source V1, passes through R1, and reaches a node with a junction. By Kirchoff, we know that the current entering a node is equal to the sum of the currents leaving it. Here, the current can be divided between going to R2 or continuing through a cable with no resistance. So, the question is: Will it go through R2 and R3 or through the cable? The current is always distributed among the paths based on the resistance of each of these. More resistance implies less current. Therefore, if the cable has zero resistance, all the current goes through it.

This brings us to the next topic: if there's no current through R2 and R3, what voltage is dropped? Well, nothing. Therefore, if there's no voltage difference across the Rs, it means the voltage across both terminals is equal. You're seeing where I'm going with this, right? Indeed, it's as if R2 and R3 didn't exist in the circuit! So, what's left? Just the sources V1 and R1!

Where is the current going?

As the great Pepe Cuenca (a brilliant spanish chess caster) would say: pero piltrafilla, cierrabares, qué basura de circuito es este!

Let's see, a current source pointing clockwise, a voltage source "pointing" to the left, which, on top of that, is negative... What's the meaning of life? Why do we see this circuit? Well, so you can check whether you already have an electronic head or not. If you have an electronic head, this circuit can be solved in 10 seconds maximum and without using paper or calculators. Don't you see it that way? No problem! We're here to help you get there. I'll explain how I process it in my head, and then we'll break it down. Be careful, the following sentences have (more) incorrect punctuation (than the rest of the text). Let's get down to business:

Ok there is only one amp loop the R is one Ohm so I have 1V in Vr so if I drop -10 to the left I get -9V.

Now it's time to process it thoroughly to understand the basis of the previous conclusions. In this circuit, we see a current source and a voltage source. Is this a problem? Can a circuit have more than one source? Well, the answer is… almost always yes. Sources are normal components that exhibit specific behavior. Only when the circuit topology prevents them from exhibiting that behavior will we have problems. In the current source's specific case, the current source guarantees that one ampere flows through it. This is true regardless of the rest of the circuit, including voltage sources. Since there is only one loop, all current passing through the current source must necessarily pass through the voltage source and the resistor. By Ohm's Law, we know that 1 V drops across the resistor (V = I * R = 1 * 1). So, at the "Vr" node, to the right of the source, there is 1 V. This was the easy part, but what about the voltage source? First, let's recall what it is: a voltage source ensures a potential difference between its terminals. That difference always occurs from the "-" terminal to the "+" terminal. Look closely at the circuit: the "-" terminal is on the right. What is the voltage difference imposed by the source? Well, -10V. That is, between the "-" terminal and the "+" terminal, we're going to "gain" -10V. But Miguelón, can the voltage be negative? Of course, voltages are potential differences. That "one" potential is higher than "another," or that "another" is higher than "one," is perfectly possible, 100% legal. But back to the circuit: the source guarantees -10V from the "-" terminal to the "+" terminal. So, if "Vr" was 1V, to the left of the source there will be -9V.

In the next two examples we will see more unusual source combinations.

Are those sources well connected?

We're going to talk about two fairly simple circuits. The feature I want to highlight is that each has two voltage sources. I want you to look closely at both and ask yourself if they're possible and functional circuits. What do you think of the parallel sources in circuit "A"? And the opposing sources in circuit "B"? Think about it before you continue reading.

Okay, let's see how you did. To determine if these sources are positioned so that they are compatible with each other, we just need to refer to the basic behavior of a voltage source. By now, you should have it in your head: a voltage source guarantees a potential difference across its terminals (you have to imagine that phrase sounds like members of a Renaissance sect in robes and hoods repeating it in unison).

So, let's move on to circuit "A." There are two sources in parallel. If both sources provide a potential difference, it's clear that they can only coexist if the voltage they provide is the same. Otherwise, we would end up with a metaphysical implosion of the universe. If V3 provides one thing, but V4 provides another, and they share terminals, that is, they share voltage, that voltage, which is common, cannot have two values. Therefore, circuit "A" cannot exist unless the sources are equal.

Let's see what happens with circuit "B." There we have two voltage sources in series, and on top of that, they're placed somewhat randomly. Is there a problem with this circuit? No! Note that each source can perform its function freely without impeding the other. If we analyze the circuit clockwise with ground on the "+" terminal of V6, we can interpret that V6 reduces the voltage and V5 increases it. There's no problem with that. We already mentioned in the previous example that having negative and positive voltages is 100% legal. In fact, voltages aren't positive or negative until we establish a reference point. Let me see if I can explain. Maybe you could use some paper and pencil.

Imagine the ground is on the "wire" below V6. If that were the case, we would have a voltage drop from V6 to the common node between V6 and V5. Subsequently, a voltage increase from the common node to the node above V5. However, we can place the ground anywhere (although, if you're working on a circuit with someone, be careful to choose the same spot, or you'll go crazy), and one possibility would be the node between V5 and V6. Go back to the drawing and imagine the ground symbol on the node between the two sources. In this case, you would have a positive voltage produced by V5 going up, and another positive voltage produced by V6 going down. However, the circuit is exactly the same as before. There are the same currents, the same voltage drop across the resistor, and the same potential differences across the sources. It's absolutely crucial that you fully understand this . If you have any questions, go to Part 3 of the series and review the section on voltage.

Does the circuit work?

We're almost done, but I wanted to twist things a bit more before letting you go. Anyway, is ther anything better to do than learn electronics? With the following circuits, we'll proceed in the same way as in the previous example. You'll have to take a look and try to figure out if they're legal topologies. I'll give you the explanation below the circuit, but I insist you should think about it before reading.

Let's start with circuit "B." We have a loop with two current sources. By now, you should have all your alarms going off. Let's recall what a current source does: a current source guarantees a current flowing through it. In our circuit, we have I2, which would impose a certain current, and I5, which also wants to impose a current. Since the two sources are in the same loop, they cannot coexist, as a loop can only have one current value at a given time.

Okay, let's move on to circuit "A." You may have some more questions here, and that would be reasonable. However, we just need to refer to the definitions we've seen so many times about voltage sources and current sources. If we use those definitions, we won't encounter any absurd or impossible situations. In other words, such a circuit can perfectly exist. Note that the voltage source can impose it without any problem. That voltage would be directly what would fall across the resistor. When the voltage falls across the resistor, a current will be generated by it. That current will be supplied, in part, by the current source, and in part by the voltage source. The current that doesn't go to the resistor (excess or missing depending on the values of the sources) remains circulating through the sources.

Conclusions of the series

If by some miracle you've made it this far, congratulations. The concepts we've covered aren't easy to grasp. Even if you think you've understood them, I assure you that you'll encounter situations that will catch you off guard, but you'll surely have the resources to eventually understand them by giving it a little thought. If you're coming from the first entry and didn't know anything about electronics, this will have been a huge effort, and you may feel like you can't apply it yet. I have good news for you: the most boring part is over. However, I'd be lying if I told you there wasn't much left to do. Electronics is a vast field, although you won't need to know everything. The next steps will be to understand how other circuit elements work: inductors and capacitors. These elements allow you to control voltages and currents over time , and open up a ton of applications.

I finished writing this text on Christmas morning 2024 (who knows when I'll publish it), so now I'm going to continue eating like a pig (where I live there are celebrations from the 24th to the 26th) and I wish you a Merry Christmas, whenever you read it.