Skip to main content

Section 1.3 Electrical schematics

An electrical schematic is a concise visual representation of a circuit which shows how electrical components are connected together. Circuit elements are represented with various symbols, and the wires connecting them are drawn as lines.

Subsection 1.3.1 Elements and nodes

Circuit elements are the components that make up a circuit, such as power supplies, switches, motors, capacitors, and so on. The symbols for a few common circuit elements are shown below, and we'll introduce more as we continue:

Circuit nodes are the interconnections between elements, typically made with wires or metal traces on a circuit board. A connected group of wires forms a single node.

Subsection 1.3.2 Junctions and crossings

If two wires cross on a schematic, they are not electrically connected. Sometimes you'll see designers put a little "jump" on one wire when it crosses another. This is a nice visual reminder that the wires aren't connected, but if you have a lot of them it can just clutter the page. To indicate that crossing wires are connected, a small dot is placed at the junction:

If wires join with a T, then they're connected whether or not there is a dot. There would be reason to have a T if the designer didn't mean for there to be a connection! However, sometimes it's visually clearer to put a dot on a T-junction, or maybe you're on a roll drawing dots and just feel like it. That's fine too.

Subsection 1.3.3 Series and parallel

Two elements are in series if and only if they exclusively share a node. Said another way, all of the current that flows out of the first component must flow into the second component, because there are no other paths. TODO: figure here

Why does this matter? If two elements are in series, then the current through each of them must be equal. This insight will allow us to simplify the analysis of many circuits. [TODO: which of these are in series?]

Two elements are in parallel if they are directly connected to each other at both ends. If two elements are in parallel, then the voltage across each of them must be the same. Again, this will simplify the analysis of many circuits. [TODO: which of these are in parallel?]

Subsection 1.3.4 The zero reference point, aka "Ground"

Hopefully you've drilled it into your head that voltage is a potential difference between two nodes. It makes no sense to talk about the voltage at a point; it's always a voltage between two points.

However, as our circuits get larger it will be helpful to designate a common voltage reference point, and define all voltages as relative to that reference. This reference is called "ground" and is represented by one of the symbols below.

Once we've defined "ground", then we can say things like, "the voltage at node A is 5 V", which implicitly means "the voltage at node A is 5 V relative to the ground node".

You may also come across the symbol for earth ground, which is a little different. It represents a connection to the actual ground (the dirt beneath your feet). This is the third prong on a standard US electrical outlet, and it connects to a metal pipe or stake that goes into the earth outside of the building. The external cases of most appliances are connected to earth ground for safety. In the event that something breaks inside such that the external case gets accidentally energized with mains voltage, the electricity will travel through this path to ground (instead of through an unsuspecting person who touches the appliance), and quickly trip the circuit breaker.

Once we've defined a ground point, we can also define a voltage source without showing it explicitly. The two schematics below describe the same circuit. The representation on the right is particularly useful when there are many connections to Vcc (power) and ground and drawing them all would clutter the diagram. Instead we use a ground or power symbol wherever necessary, which indicates that all of these part of the same node even if the wires aren't shown.

Subsection 1.3.5 Congruent schematics

A schematic does not indicate anything about the spatial layout of componenents; it only describes electrical connections. As a result, the components can be rearranged or redrawn on the page however you'd like as long as the connections are preserved.

Subsection 1.3.6 A few conventions

  • Ground should be toward the bottom of the sheet, and Vcc toward the top. Don't ever draw the ground or Vcc symbols upside down, that's really confusing. [ TODO: figure illustrating what and what not to do ]

  • Lines representing wires should be straight with right angles. You can occasionally use a 45-degree angle if you have a really good reason.

  • Circuits usually have logical organization, so try to group related components together. If you're tracing out the schematic for a device that already exists, it may be very hard at first to figure out what is related. But once you've drawn the schematic out and begun to figure out its operation, go back and re-draw the circuit in a more logical way.