Most users associate switches with turning lights on and off, but these simple devices are at work in nearly every electronics application. Electromechanical switches, for instance, break or form connections in electrical circuits. Let's look at the most common types of switches and how to apply them in your next circuit design.
Types of electrical switches
The four most common types of switches in circuit design are:
- Single Pole Single Throw (SPST)
- Single Pole Double Throw (SPDT)
- Double Pole Single Throw (DPST)
- Double Pole Double Throw (DPDT)
Manual switches are more common in day-to-day life, but you can apply the same concepts to relays. Relays are electrically actioned electromechanical switches. Every type of switch has two main components that control the circuit: a switch pole and a switch throw.
- Switch pole: A switch pole describes the number of separate circuits that the switch or relay controls. As we will see later, a Single Pole Single Throw (SPST) switch controls a single circuit.
- Switch throw: A switch throw describes the number of output connections each pole of the switch can have. For example, a Single Pole Double Throw (SPDT) switch has a single pole and two different switch output options.
What is an SPST switch?
The simplest form of a switch is an SPST switch. An SPST switch embraces a basic "ON/OFF" control of a single circuit and consists of two terminals that serve as electrical connection points. Power the switch "ON" to establish a connection between the two terminals. Turn the switch to "OFF" to eliminate the terminals' connection.
The light switch in the room you're in right now probably uses an SPST switch. When in the "OFF" position, the circuit breaks and the light turns off. When the switch is set to "ON," the two connected terminals complete the circuit and the light turns on.
What is an SPDT switch?
An SPDT switch consists of three terminals and connects the source terminal and one of two output terminals. An SPDT switch allows for an "ON/ON" configuration, which means the switch's input terminal is always completing one of the two possible circuits that the switch controls.
- If the switch is in the "ON-1" position, the connection between the input terminal is established with the first output terminal, thus completing the first circuit.
- If the switch is set to "ON-2," the second output terminal establishes the connection with the input terminal, thus completing the second circuit.
Since both circuit options rely on the input terminal, this switch is a single pole switch because only one completed circuit can exist at any given time. Confusingly, a third configuration exists for some SPDT switches. This third switch can be set to an "OFF" status in which neither circuit one or two is complete. This SPDT switch configuration is noted as "ON/OFF/ON".
What is a DPST switch?
A Double Pole Single Throw (DPST) switch has four different terminals and is often used to connect two source terminals to their respective output terminals (but never to each other). A DPST switch can be used in an "ON/OFF" configuration, where its terminal pairs are either connected ("ON") or not connected ("OFF").
Imagine the switch has four terminals, numbered 1, 2, 3, and 4:
- Terminals 1 and 3 are the first pair of terminals, and they are completely electrically isolated from terminals 2 and 4.
- Terminals 2 and 4 are the second pair.
- If the switch is in the "ON" position, it creates a connection between terminal 1 and 3, completing circuit 1.
- Terminal 2 and 4 simultaneously connect to complete the isolated circuit 2.
Functionally, a DPST switch is the same as two SPST switches actuated in tandem. The main advantage of a DPST switch is that both circuits are completely isolated from each other. This setup can be advantageous in applications with different voltages. One practical example of a DPST switch is when a status light circuit indicates the connection of a higher-voltage load circuit.
What is a DPDT switch?
A Double Pole Double Throw (DPDT) switch consists of six terminals, two of which are independent input terminals. Each of the poles can complete two different circuits. In other words, each input terminal connects with two output terminals, and all four output terminals are separate. DPDT switches are the most advanced type of switch we'll cover here, so be sure to consult a datasheet when you order a DPDT switch.
The image below is from NKK Switches' dual-seal, waterproof-packaged S-series S7AWB switch.
Note the "Throw & Schematics" section at the bottom of the datasheet. This section shows the two input terminals (2 and 5) and their respective output terminals (1 and 3 and 4 and 6). The S7AWB model consists of three switch positions (up, center, down). When the switch is in the "UP" position, input terminal 2 connects to output terminal 3. Input terminal 5 is electrically isolated but connected to output terminal 6.
As you design and shop for switches and relays, remember two important points:
- Manufacturers will often save manufacturing costs by using a more complex switch's packaging for a simpler model. For example, S21AWB is a simpler DPST switch, but it is housed in the same dual-seal waterproof package as the other S-series switches. The rest of the models in the datasheet are all DPDT switches. In some unique scenarios, switch terminals may go completely unused, so reference the relevant datasheet when wiring your switch.
- Switches often have "momentary tactics" that are ideal for specific applications. Think of momentary tactics like a car's automatic window button ― the switch position is only active while the user actuates it. Once the user disengages, the switch returns to its original position. The datasheet above uses parenthesis to denote the momentary tactic feature. Switches that feature momentary switch configurations may be listed as "(ON) OFF ON" or "(ON) OFF (ON)".
Conclusion
Many other types of switches feature various pole/throw configurations, such as 6PDT toggle switches or DP3T toggles. Rotary switches consist of nearly unlimited amounts of poles and throw combinations. Understanding the fundamental types of pole and throw combinations will be hugely beneficial as you design simple and complex circuits.
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