Understanding schematics

Schematics represent a physical circuit just like a map represents real roads. One big difference is that schematics do not map directly to the physical position of the electronic circuit and its components. Schematics make it easier for us to follow the logic behind the design of a circuit in a way that would be difficult if you were having to figure it out from looking at the physical board.

As I teach different components, we will add to the number of symbols you will see on the schematics. For now, we are going to start simple with a battery, wires, and a resistor. A resistor is a component that is manufactured to have a specific resistance such as 100Ω or 1000Ω. We will dive deeper into resistors soon, but for now, you just need to know that a resistor has a specific amount of resistance, its unit of measure is the Ohm and this is shortened to the Omega character Ω.

Above, you can see the schematic symbol of the battery. The battery represents a real battery and has a positive and negative terminal. You will also notice the top line is long, which represents positive, while the bottom line is short, which represents the negative terminal.

Now in electronics theory, there are two ways of describing current flow. One is called conventional current, which states that current flows from the positive terminal to the negative terminal. There is a second which is called electron flow which states the exact opposite, that current flows from the negative terminals to the positive terminal.

On this site, we will simply be referring to a source and a return. This is because that is how schematics are drawn.

In this diagram, we will consider the positive terminal as the source and the negative terminal as the return. An important point I would like to make here is that the current leaving a source and returning is always the same.

For current to keep flowing, there must be a complete path from one terminal of the battery, through the circuit, and back to the other terminal. The same amount of current that leaves the battery returns to it. Components do not use up current.

A big part of being a good technician is the ability to see the current flowing in your mind. When you look at the schematic above, you should be thinking about how current is leaving the source (positive terminal in this case), flowing through the top wire, through the resistor, through the bottom wire, and then back to the battery where it returns and completes the loop.

Another point I would like to make is that a wire is often just a line of copper on a circuit board. This is called a trace and electricity flows through traces just like a wire.

Now let's look at a simplified schematic.

At first it may seem like this is not a simplified schematic because we simply removed two wires and replaced them with symbols; however, this type of simplification makes a big difference when working with larger schematics. Imagine drawing supply and return connections to every section of a large schematic. Those lines could crowd the page and make the circuit harder to follow. Using these symbols let us show those connections without drawing every connecting line.

The two symbols at the top of the schematic, labeled +9V, gives the same information as a line that represents the wires would. The big difference is that now we can separate them across multiple pages and still understand that they are connected. All power rail symbols that are connected will have the same label. A complex circuit may have multiple voltage sources all at different voltages. It is very common for circuits to require +5V, +12V, -12V, and many other sources. We understand that all of the supplies that are labeled the same are connected.

In this circuit, ground is our 0V reference point and is connected to the battery’s negative terminal. It does not mean a connection to the earth. Repeated ground symbols represent connections to this same point, even when the connecting lines are not drawn.

The symbol at the bottom of the schematic is called a ground. There are other ground symbols, which we will cover later. The point is, just like all the supply rails that have the same label are connected, all grounds that use the same symbol and label (if it has a label) are also connected.

Now that you understand the ground symbol, we will always consider the ground to be the path where current returns to its power source.

Can you still imagine how the current flows from the positive battery terminal, through the +9V rail, through the resistor, and back to the return through the ground? Even though we do not see the lines that represent the wires, we know that the real circuit still has them.

Now we will make one more simplification to the schematic.

Now we see that the power supply / battery has been removed from the schematic. We only see the power rail, resistor, and ground. This is a common way to draw a section of a larger circuit. The schematic will usually have a separate section or page that shows the power supply. We can simply infer that the supply exists because of the power rail and the ground.

You should still be able to imagine current leaving the positive terminal of the battery, following the connection marked +9V, passing through the resistor, and returning to the negative terminal through the ground connection. The +9V label tells us the voltage at that connection relative to ground.