Electricity - Schematic Diagrams I - Teacher's Notes  
  schema
A Greek word, meaning plan or diagram. The original schema were drawings of the heavens, showing the position and relationships of the stars.
 
diagramma
A Latin word, meaning a line drawing used to explain a concept or plan.
 
Showing Connections
Not Connected Our notation for representing wires that cross without connecting has changed within the last 50 years. In earlier times, two wires that did not connect would be shown as "not touching", like the top figure to the left. Later, some draftsmen would show "not touching" like the lower figure to the left.
 
Likewise, our notation for representing wires that connect has also changed. In earlier times, it was only legal to show a connection with three wires, and the dot at the connection was not required (see the top right). A connection of four wires at the same point was not legal, and so draftsmen would use the notation shown at the lower right when they needed to show four wires connecting.
Connected
 

  Poles and Throws
Every new subject comes with its own vocabulary! Sometimes words that you already know are just used in a new way. The word "pole", for example, has a number of obscure meanings. You already know that magnets have a "north pole" and a "south pole". We don't say so any more, but in the old days batteries had a "positive pole" and a "negative pole". We still say that components have a "polarity", meaning that it matters which end is more positive and which end is more negative.
 
Even though we don't refer to battery terminals as poles anymore, we still refer to switches as having poles. In this case, we mean how many circuits can the switch control independently of each other. A single pole switch can control one circuit. A double pole switch can control two circuits, and so on.
 
Ever heard someone say "throw the switch"? We use the word "throw" to describe how the switch can be used. Just as a switch has some number of poles, it also has some number of "throws". If a switch has a single connected position (i.e. it can be either on or off) then we say it is a "single throw" switch. If a switch has two different "connected" positions, then we say it is a "double throw" switch.
 
Switches are made in all combinations of poles and throws. A single circuit, single position switch is called "single pole - single throw" or SPST for short. Likewise, there are SPDT (single pole - double throw), DPST (double pole - single throw), and DPDT (double pole - double throw) types. I own an 8P10T (8 pole - 10 throw) switch that was once used by the telephone company.
 
Momentary Switches
A push-button switch is considered to be "momentary"; it doesn't stay in place when you remove your finger. Your door-bell button is a "normally open" (N.O.) momentary switch. When you push it, the circuit is connected and so the bell rings. When you release it, it disconnects the circuit, and the bell stops ringing. The switch for the light inside your refridgerator is a "normally closed" (N.C.) momentary switch. If nothing is pushing on it, the circuit is connected (when the door is open, the light is on). When the door is closed, it pushes against the switch, disconnecting the circuit.
 
A Cell or a Battery?
The word "battery" means a collection or arrangement of several items. Thus a single D cell is not really a battery; it's not a collection, it is a single item. A 9 volt transistor radio battery on the other hand, is actually 6 cells inside of a single can, so it is properly called a battery. Over the years the distinction has been fading. Now single cells are often called batteries too.
 
Questions for your students:
 
Q: Why do we use schematics instead of pictoral drawings?
A: Even the ancient Greeks and Romans understood that not everyone is an artist. A diagram is fairly easy to draw, and a lot easier to understand that a photo-realistic drawing. Diagrams leave out some of the information we don't need or care about. For example, who cares what color the battery is? The circuit should work with batteries of any color! How long is that wire? Long enough to connect the two components! Any wire that reaches is long enough!
 
Q: How many symbols are there for electrical and electronic components?
A: I don't know. Probably no-one does know. New symbols are invented when necessary to represent newly invented components. For electricity, there are about a dozen important symbols. You've already seen many of them. For electronics their are probably 40 or 50 that everyone knows and uses, and another 30 or 40 that are only used by specialists.
 
 

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