circuit basics of automotive electrical systems

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Now that you've gotton your Laws dowen, it's important to understand the difference between the different types of circuits---specifically the series circuit, parallel circuit, and series-parallel circuit. Furthermore, it's important to understand how Ohm's Law and Kirchhoff's Law govern these basic circuits.

Series Circuits

A series circuit is one that has all components connected in an end-to-end (or daisy chained) fashion. Operation of the circuit is dependant on all of the individual components of the circuit being in working order. A series circuit is only as good as its weakest link--if one component fails, so does the entire circuit. This is how strings of Christmas lights were once made. Problem was, when one bulb failed the entire string wouldn't light.

In  a series circuit, the follwing apply:

• The sum of voltages across the individual components is equal to the total voltage applied.
• The current flowing through the individual components is the same throughout the circuit. (Note the tie to Kirchhoff's Law!)

Parallel Circuits

A parallel circuit is one that has all the components connected in a parallel fashion. Operation of the circuit is not dependant on the individual components of the circuit--one or more components could fail and the remaining components will still work. The typical parking light circuit is a good example of a parallel circuit.

In a parallel circuit, the following apply:

• The voltage flowing across the individual components is the same throughout the circuit.
• The sum of the currents across the individual components is equal to the total current flowing through the circuit. (Again, Kirchhoff's Law.)

Both figures illustrate the differences between these two circuits. Both are simple lighting circuits. Note that in both, I've outlined voltage and current across the various components as well as totals in the circuits so that you can see how Kirchhoff's Law governs them.

Series- Parallel Circuits

You guesed it; there can be combinations of series and parallel circuits. These types of circuits, although not very common in automobiles, are referred to as series-parallel circuits.

The rules above apply to the individual parts of the circuit. The figure above is an example of a series-parallel lighting circuit. Again, note how Kirchhoff's Law governs this as well.

It is important to understand the differences between series, parallel, and series-parallel circuits. Examples of each of them are found in nearly every vehicle on the road.

Short Circuit

For such a simple idea, this is one of the most misunderstood circuits that I know of. Quite simply, a short circuit is any circuit that has a circuit path between the battery (+)  and battery (-). Certainly, these are not intentional and can be quite hazardous. This figure will show a short circuit that bypasses the lights altogether. Even though they are connected, they will not light.

How exactly does this happen? Any number of reasons. Let's say that you were mounting something under the vehicle's hood and you unintentionally pinched the parking light wire or pierced it with a screw in the process. All would be fine until you turned on the parking light switch, at which point, the parking light circuit fuse would blow as the switch just connected 12 volts directly to the vehicles chassis. The blown fuse just protected the wiring and vehicle from any serious damage. ( The value of a fuse should be fairly clear at this point! ) Of course, you would now have to determine what caused this and fix the problem. This process is called troubleshooting, and you'll learn more about how to do this later as I blog on.

The Open Circuit

Now, this is the one that eludes folks--after all, any vehicle wiring problem is referred to as a short! In many cases, that just isn't true. An open circuit is a break in the circuit that does not allow the circuit to operate. Recall the example I gave above in the series circuit explanation of the way strings of Christmas lights used to be manufactured. Should a bulb fail, and the string go out, this is an open circuit condition. Now, we don't install many of these in vehicles, so let's look at the next figure---note that it is also based on a figure from when we spoke of Kirchhoff's Law.

Again, there are any number of reasons for this to happen. Let's say that you were installing new carpeting in your vehicle and you used a razor knife to cut an opening along the sill plate for the big Torx bolt that holds your seat belt down. If you're not careful, you could cut through the wiring harness in this area, thereby removing the pareking lights in the rear of the vehicle from the circuit. The front lights would still work fine because they are still connected to the circuit. Obviously, this is a simple fix, and it's covered later nin this blog.