Understanding Switch Pole and Switch Throw: Choosing the Best Option for Your Design

Modern switches come in a wide assortment of forms. From simple push buttons to industrial toggles, switches are available to perform anything from simple tasks to complicated circuit control. Despite this, almost all switches share basic characteristics that can be described as poles and throws. You will frequently see acronyms such as SPDT (single pole, double throw) associated with switches. Before selecting a switch, it is crucial to understand these attributes and the differences between poles and throws as well as the differences between latching and momentary switches.

A switch is a simple device that allows an operator to open or close an electrical circuit. In the open condition, the circuit is broken, and no current can pass. Once closed, the electrical circuit is complete and current can flow.

This is the simplest kind of switch. It has one input, or pole, and a corresponding single output, or throw. When the switch is closed, it completes the circuit, and the current flows from the input to the output. This arrangement is therefore known as a single pole, single throw switch (SPST). The tasks for which this type of switch can be used are seemingly endless, from the light switch in your living room to the firing circuit of a rocket. 

A double pole, single throw switch (DPST) shares a key feature with the SPST type. For every input, there is a single output. However, instead of one input the DPST has two (double pole), each of which has a single corresponding output (single throw). The two circuits are independent of each other but are opened and closed by the same action. Therefore, a DPST switch is ideal for isolating a machine safely or for activating two motors at the same time.

Conversely, a double throw switch features two outputs for each input. The switch works by connecting the input to either of the two outputs. Actuating the switch moves the contact from one to the other, so that as one circuit opens, the other closes.

A single pole double throw switch, or SPDT switch, is ideal for use in any application that requires an operator to change between two options. An SPDT switch is useful in a situation where a motor needs to reverse its direction, or to change a light between two brightness levels.

Just as there is a double pole variation of the single throw switch, there is also a double pole, double throw switch, or DPDT. A DPDT switch features two inputs, each with two outputs. As with the DPST switch, there are two independent circuits, each of which is activated by a single switch. A DPDT switch is ideal for controlling multiple devices simultaneously, whether in the domestic environment or industrial control.

Switches are not just designated by their poles and throws. It is important to understand how the switch behaves once it is released. Some switches remain in the new condition, which are known as latching switches. This is typical of a switch used to turn on a light or activate a motor. 

The opposite of a latching switch is a non-latching or momentary type. In this case, the switch returns to its previous condition when released. Once the actuator is released, the circuit is broken. The simplest example of a non-latching switch is a doorbell.

With the vast array of switches available, selecting the right product can feel like a daunting task. However, by asking a few key questions, an engineer can gain a critical understanding of his requirements. Does the application demand an on/off switch, or an either/or switch? Single throw switches are ideal for on/off applications, whereas double throw types are more flexible and give the designer the option to change between two circuits in an either/or manner.

Secondly, the engineer needs to know how the switch needs to behave once it is released. Does it need to remain in the new state, or should it return to its previous state? The answer to this question will suggest whether the designer requires a latching or momentary switch. 

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