How Switches Work - SPST, DPDT, ... Switch Configurations Explained
In this article, we will explain the basics of how different switch configurations work. We will cover the common terms such as SPST, DPDT, MOM, (ON), I-O, and more. When you need a switch, the first thing you will find is that there are a lot of switches to choose from. There are rocker switches, toggle switches, slide switches, button switches, rotary switches, pull switches, and more.
Although all of these switches operate a little differently from each other, they share common switch configurations that determine how they operate and what kind of devices they are suited to switch. Switching configurations are almost as diverse as the types of switches are, and we will not cover all of them here. We will, however, cover the common ones such as single pole and double pole, single throw and double throw, and momentary vs on-off.
What does Single Pole and Double Pole Mean?
The term "pole" when used to reference switches, indicates the number of circuits, or devices, that the switch can control. If a switch is designed to operate a single circuit or electrical device, it is called a single pole switch. If a switch can operate two separate circuits or devices that are electrically isolated from each other, it is called a double pole switch. Single pole switches usually have two or three terminals on the back, all in a row or column. Double pole switches usually have four or six terminals on the back, in either two columns or rows of two or three terminals. Double pole switches can simply be looked at as two switches combined together into one switch body so the same lever or rocker operates both switches at the same time.
What Does Single Throw and Double Throw Mean?
If you have ever heard the term "throw the switch", then you probably have an idea of what single throw and double throw mean. "Throwing" a switch simply means the physical act of operating the actuator to turn a switch on (or off). Using this simplified definition of switch throw, a single throw switch has just one "ON" position. A double throw switch will have two "ON" positions, often times an up or a down position of the switching actuator. Single throw switches will usually have two or four terminals on the back, while double throw switches will usually have three or six terminals on the back. Double throw switches are commonly used to switch devices that have complimentary functions, such as a motor that goes up or down, or a winch that goes in or out.
What do the Terms Momentary, MOM, (ON), and (I) Mean?
They all indicate that the switch operates in a momentary fashion, meaning that after a switch is turned on or "thrown", it requires that it be held in that position in order to keep the switch activated. When you remove your finger from the switch, it will revert back to it's original position, turning the switch off. This is an important feature of switches used for electrical devices such as horns, lifts, winches, and outriggers, where you don't want a continuous on circuit because it's annoying, or could potentially cause damage or injury.
Single pole single throw ON-OFF switches are the most common of the switches. SPST ON-OFF switches are designed to turn a single device or circuit on or off. These switches will have two terminals on the back to connect your wires. DPST ON-OFF switches are designed to two devices or circuits on or off from the same switch. These switches will have four terminals on the back to connect your wires. Switches that have an indicator light built in will have an extra terminal for the indicator light. If it is a 12 volt or 24 volt DC switch, the extra terminal is for connecting a ground wire so the indicator will light up. If it is an 120 volt AC switch, the third terminal is for connecting the neutral wire so the indicator will light up.
Like ON-OFF switches, MOM-OFF switches can be single pole or double pole. Single pole switches will operate a single electrical circuit or device and have two terminals on the back for connecting wires. Double pole switches will operate two separate circuits or devices and have four terminals on the back for connecting wires. These switches will be turned on momentarily when the switch is activated. When you release the switch actuator, it will turn itself back off.
ON-OFF-ON- switches are available in either a single pole double throw (SPDT) or a double pole double throw (DPDT) configuration. SPDT ON-OFF-ON switches will have three terminals on the back of the switch, while a DPDT ON-OFF-ON switch will have 6 terminals on the back for connecting your wires. These types of switches have a center "OFF" position. If the switch has any built in indicator lights, there will be an extra terminal for each indicator light that you will hook a ground wire to if it is DC, and a neutral wire if it is AC.
ON-ON Switches are identical to the double throw ON-OFF-ON switches, except there is no official "OFF" position. Instead, the input terminal(s) of the switch is(are) always connected to at least one of the outer terminals of the switch. These switches are available as either Single Pole or Double Pole. This is a useful switch configuration if you want to toggle between two different modes, such as Low and High, Fast and slow, where there is no third "OFF" state. It is also useful for creating 3-way switches, where two switches located in different places are use to turn on or off a circuit from either switch. Wiring up a 3-way switch is beyond the scope of this article, but we may cover it in the future.
MOM-OFF-MOM switches are single pole or double pole switches that have two "Momentary ON" positions that will spring back to the "OFF" position when you release the switch. Single Pole MOM-OFF-MOM switches will have three terminals on the back, and double pole MOM-OFF-MOM switches will have six terminals on the back. The momentary operation of these switches make them ideal for use with devices such as windows, lifts, winches, electric jacks, and other circuits or devices that should only be operated on a momentary basis.
Beyond the basics, switching configurations can be found for more specialty circuits. We won't cover there operations in this article, but there are reversing switches that are designed to reverse the polarity of the electricity passing through the switch for motor reversing applications. There are triple pole switches, and triple throw switches. There are OFF-ON-ON switches, OFF-ON-MOM switches, (OFF)-ON switches, and much more. Check out or selection of switches here.