With today’s components, it is possible to make thousands upon thousands of useful circuits. These range from the very simple to very complex. But, did you know that there are only three basic types of circuits anyone can create? That’s right, every schematic you’ve ever seen, every circuit or project you or anyone else has ever built is one of three types of circuits (or a combination of them). First, we’re going to talk about two of them: series vs parallel circuits. The we’ll talk about the third.
If you’re new to electronics, this information is need-to-know info. If you’re a veteran, you’re likely familiar with the three types, but this may serve as a good review of the basics, so I implore you to read on.
Series vs Parallel Circuits
Series vs Parallel: Series Circuits
For our comparison of series vs parallel circuits, let’s start by talking about the simplest circuit of all — the series circuit.
Sneak a peek at figure 1. Here we have a series circuit with a battery, an LED and a resistor. You probably know that as long as the battery doesn’t die the LED lights up.
Figure 1: a simple series circuit with an LED and a resistor.
The battery provides the voltage, or the electrical pressure. This pressure pushes current, which is simply moving electrons, through the wire, resistor, and LED causing it to light (for a more detailed explanation of voltage and what it is see Back to the Basics: What is Voltage?). The resistor limits this flow of current. If we left it out of the circuit, the LED would burn bright for a little while and then burn out due to excess current.
Which brings us to the first take away: just about any circuit usually needs some way to limit the amount of current or bad things can happen.
As we’ve seen, electronic components can be destroyed by excessive current. And since current flow generates heat, it can also be a fire hazard.
Back to the subject at hand, we know that our LED circuit is a series circuit. But what does that mean?
In a series circuit, current only has one path to flow through.
Take a peek at figure 2. If we remove the resistor (or it burns open), current ceases to flow because it is no longer a complete circuit. Note that the battery voltage still appears where the resistor was, but current cannot flow in an open circuit, so the LED does not light.
If you’re new to electronics, this may seem odd, but you can build something like this, grab your meter and measure the voltage from point 2 (at the top right of figure 2) to ground and see for yourself. The meter is going to show the battery voltage.
Figure 2: Looks just like figure 1, only I used MS Paint and my superb graphic design skills to remove the resistor.
When comparing series vs parallel circuits, it’s important to remember that the same amount of current flows through the whole circuit in a series arrangement (assuming it’s not an open circuit!).
So, since the current only has one path to follow, the same current that flows through the resistor will flow through the LED in figure 1.
Figure 3: the same current I flows through all the components in a series circuit. Here we’re using conventional current flow by the way. For more explanation on conventional vs electron current flow see Conventional Current vs. Electron Flow: Which is Correct?
It’s the voltage across each component in a series circuit that is usually going to be different, unless we have a string of resistors with the same value. In fact, the sum of all the individual voltage drops equals the source voltage. This is known as Kirchhoff’s Voltage Law or KVL for short.
This may seem confusing if you’re a new Maker, but fear not. Figure 4 can clear things up.
Figure 4: multiple resistors in series. The voltage drop across each one will be different since their values are different.
Figure 4 shows a series circuit with four resistors of different values and a 12 V battery. Some voltage will drop or appear across each resistor. The larger the resistor, the larger the voltage drop across it. If the resistors were all the same value, the same voltage would drop across each one. But, since they’re all different the voltage drops will be different. However, if we were to add up the voltage drops across each resistor, they would sum to 12 V, which is the battery voltage. This is KVL in a nutshell.
See that wasn’t so bad, was it?
Like everything in life, series circuits have their weaknesses. One of them is that if one element — such as a resistor — breaks or maybe a wire gets loose, the circuit no longer works.
Sometimes this is done purpose. A common example would be a fuse. Fuses are usually in series with the rest of the circuit. If the fuse opens (due to excessive current) the circuit dies, thus saving your house, car, or whatever from going up in flames. This is so important that I strongly suggest you include a fuse in series with every single project you build, even if it operates on 1.5 V batteries.
Series vs Parallel: Parallel Circuits
So, we now know that series circuits have a weakness. The solution to this is the parallel circuit.
In a parallel circuit, the current has more than one path to follow. So, if one of the resistors in the simple parallel circuit from figure 5 blows open, current still flows through the other resistors. In fact, this is the reason the wiring in your house and all buildings is in parallel. If it weren’t and a lightbulb burnt out, the rest of the bulbs on that circuit would go out. Anyone who’s used cheap Christmas lights before is familiar with this debacle.
Figure 5: a simple parallel circuit. If one or two resistors burn open current still flows through the circuit rendering it operational.
Another difference in a series circuit vs a parallel circuit is that in a parallel circuit, the voltage across all legs of the circuit is the same. It’s the current that divides up and flows though the various elements in proportion to their value. The amount of current through each element depends on the resistance of the element. The higher the resistance, the less current flows through it. Figure 6 depicts this.
Figure 6: current and voltage in a parallel circuit.
All individual branch currents sum up to the source current. This is one way to word Kirchhoff’s current law or KCL.
In other words, the total current is equal to I1 + I2 + I3. Since R1 in figure 6 has the least resistance, most of the current will flow through it with R3 having the second most and R2 the least current since it is the largest resistor.
Series vs Parallel Circuits: Combining Both
Understanding the difference between series vs parallel circuits is essential knowledge for any electronics enthusiast.
But, most of the circuits you’ll face in real life are neither series nor parallel. They’re a combination of both.
Enter the series-parallel circuit.
Most useful circuits you’ll work with will be of this type. However, the same concepts we already discussed apply.
Voltage stays the same in parts of the circuit that are parallel to each other and the same current flows through the parts that are in series.
Figure 7 depicts a series-parallel circuit.
Figure 7: a simple series-parallel circuit.
Let’s do a quick qualitative analysis of the series-parallel circuit above.
Since R1 and R2 are in parallel, we know the voltage across both will be the same. In fact, we can even combine R3 and R4 to make one “pretend” resistor (let’s call it R3,4) since they are in series. More on combining resistors shortly. So now R1, R2, and R3,4 are in parallel and the voltage will be the same across all of them.
Let’s uncombine R3 and R4 again. Since they’re in series, we know that they will both carry the same current regardless of their values. However, the total current will split between R1, R2, and the series combination of R3 and R4. Piece of cake.
Series vs Parallel Circuits: Combining Circuit Elements
Now that we know the basic rules of how voltage and current behave in series and parallel circuits, it may be helpful to know how circuit elements such as resistors add up and combine. Again, many of you probably know this and if so, I implore you read on for a review. But there may be some newbies reading this, so I’m going to quickly go over combining circuit elements in series and parallel circuits.
If you study electronics in any formal way, you’ll learn about three basic circuit elements: resistors, capacitors, and inductors. This is because all circuit elements including more exotic ones like diodes, transistors, and crystals exhibit some combination of resistance, capacitance, and inductance. So, it’s these three basic elements that are often used to model the more exotic ones.
Let’s start with resistors.
Resistors in series simply add up. For example, the total resistance in the circuit in figure 8 is:
Rtotal = R1 + R2 + R3 + R4
Figure 8: series resistors.
The total resistance in a parallel circuit is always lower than the lowest-value resistor. So, if R1 were the smallest resistor in the circuit in figure 9, the total resistance would be less than R1.
To get the total resistance in a parallel circuit, we add up the reciprocal of all the resistances and then take the reciprocal of this sum.
For example, the total resistance of the circuit in figure 9 is:
Figure 9: looks just like figure 5, but I’m showing it here again to save you from excessive scrolling.
And that’s really all there is combining resistors in series and parallel circuits.
But what about capacitors and inductors?
Combining Capacitors and Inductors
Inductors in series add up just like resistors do. You simply sum them up.
However, parallel inductors add just like parallel resistors. Have a look at figure 10.
Figure 10: inductors in parallel.
First, like resistors in parallel, the voltage across all inductors will be the same. It’s the current that splits.
Next, to add them up, we use the same (ok, similar) formula for parallel resistors, as we can see below.
Let’s tackle capacitors now.
Capacitors in series add up like resistors in parallel.
So, if we have 3 caps in series and want the total capacitance, all we need to do is use the formula above and replace the L’s with C’s.
Capacitors in parallel combine like resistors and inductors in series, they simply sum up.
Ctotal = C1 + C2 + C3 … +… Cn
This may be surprising to those who are new to electronics, but there’s a good, logical reason for it. Have a look at figure 11.
Figure 11: capacitors in parallel.
The amount of capacitance a capacitor sports depends partly on the size of its plates. Wiring caps in parallel effectively increases the size of plates of the single equivalent capacitor it produces. Hence, the total value of capacitors in parallel is greater than any individual capacitor and they simply add up.
Series vs Parallel Circuits Wrap Up
Knowing the difference between series vs parallel circuits and how they behave is the first step in becoming competent in circuit analysis.
There’s not enough space to go into other detail concerning circuit analysis here, but the good news is there are two other posts that do cover circuit analysis.
Simple Circuit Analysis Techniques You Should Know covers some very basic stuff like Ohm’s Law and Kirchhoff’s Laws in addition to going over some of the stuff from this post. There are also a few practice problems.
3 Ninja Circuit Analysis Tricks discusses shortcuts for combining resistors (ones we didn’t cover here), voltage division, current division, and superposition. The last one is a bit more advanced but useful. You’ll also find a few more practice problems.
Meanwhile, leave us a comment and tell us how long you’ve been dabbling in electronics. Are you a noob? A veteran? Something in between? I’d love to know!