Some switches have two entirely separate poles, so you can make two separate connections simultaneously when you flip the switch. These are called double-pole switches. Check the photographs in Figures 2-24 through 2-26 of old-fashioned “knife” switches (which are still used to teach electronics to kids in school) and you’ll see the simplest representation of single and double poles, and single and double throws. Various toggle switches that have contacts sealed inside them are shown in Figure 2-27.
Figure 2-24. This primitive-looking single-pole, double-throw switch does exactly the same thing as the toggle switches in Figures 2-23 and 2-27.
Figure 2-25. A single-pole, single-throw switch makes only one connection with one pole. Its two states are simply open and closed, on and off.
Figure 2-26. A double-pole, single-throw switch makes two separate on/off
connections.
Fundamentals
All about switches (continued)
Figure 2-27. These are all toggle switches. Generally, the larger the switch, the more current it can handle.
To make things more interesting, you can also buy switches that have three or four poles. (Some rotary switches have even more, but we won't be using them.) Also, some double-throw switches have an additional “center off” position.
Putting all this together, I made a table of possible types of switches (Figure 2-28). When you’re reading a parts catalog, you can check this table to remind yourself what the abbreviations mean.
Figure 2-28. This table summarizes all the various options for toggle switches and pushbuttons.
Now, what about pushbuttons? When you press a door bell, you’re making an electrical contact, so this is a type of switch—and indeed the correct term for it is a momentary switch, because it makes only a momentary contact. Any spring-loaded switch or button that wants to jump back to its original position is known as a momentary switch. We indicate this by putting its momentary state in parentheses. Here are some examples:
OFF-(ON): Because the ON state is in parentheses, it’s the momentary state. Therefore, this is a single-pole switch that makes contact only when you push it, and flips back to make no contact when you let it go. It is also known as a “normally open” momentary switch, abbreviated “NO.”
ON-(OFF): The opposite kind of momentary single-pole switch. It’s normally ON, but when you push it, you break the connection. So, the OFF state is momentary. It is known as a “normally closed” momentary switch, abbreviated “NC.”
(ON)-OFF-(ON): This switch has a center-off position. When you push it either way, it makes a momentary contact, and returns to the center when you let it go.
Other variations are possible, such as ON-OFF-(ON) or ON-(ON). As long as you remember that parentheses indicate the momentary state, you should be able to figure out what these switches are.
Figure 2-29. This evil mad scientist is ready to apply power to his experiment. For this purpose, he is using a single-pole, double-throw knife switch, conveniently mounted on the wall of his basement laboratory.
Fundamentals
All about switches (continued)
Sparking
When you make and break an electrical connection, it tends to create a spark. Sparking is bad for switch contacts. It eats them until the switch doesn’t make a reliable connection anymore. For this reason, you must use a switch that is appropriate for the voltage and amperage that you are dealing with. Electronic circuits generally are low-current, and low-voltage, so you can use almost any switch, but if you are switching a motor, it will tend to suck an initial surge of current that is at least double the rating of the motor when it is running constantly. You should probably use a 4-amp switch to turn a 2-amp motor on and off.
Checking a switch
You can use your meter to check a switch. Doing this helps you find out which contacts are connected when you turn a switch one way or the other. It’s also useful if you have a pushbutton and you can’t remember whether it’s the type that is normally open (you press it to make a connection) or normally closed (you press it to break the connection). Set your meter to measure ohms, and touch the probes to the switch terminals while you work the switch.
This is a hassle, though, because you have to wait while the meter makes an accurate measurement. When you just want to know whether there is a connection, your meter has a “continuity tester” setting. It beeps if it finds a connection, and stays silent if it doesn’t. See Figures 2-30 through 2-32 for examples of meters set to test continuity. Figure 2-33 offers an example of a toggle switch being tested for continuity.
Figure 2-30.
Figure 2-31.
Figure 2-32. To check a circuit for continuity, turn the dial of your meter to the symbol shown. Only use this feature when there is no power in the component or the circuit that you are testing.
Figure 2-33. When the switch connects two of its terminals, the meter shows zero resistance between them and will beep if you have set it to verify continuity.
Use the continuity-testing feature on your meter only on circuits or components that have no power in them at the time.
Background
Early switching systems
Switches seem to be such a fundamental feature of our world, and their concept is so simple that it’s easy to forget that they went through a gradual process of development and refinement. Primitive knife switches were quite adequate for pioneers of electricity who simply wanted to connect and disconnect electricity to some apparatus in a laboratory, but a more sophisticated approach was needed when telephone systems began to proliferate. Typically, an operator at a “switchboard” needed a way to connect any pair of 10,000 lines on the board. How could it be done?
In 1878, Charles E. Scribner (Figure 2-34) developed the “jack-knife switch,” so called because the part of it that the operator held looked like the handle of a jackknife. Protruding from it was a plug, and when the
