Introduction

Switches are an integral, but often overlooked, component of auto electrics. And when it comes to purchasing switches or customizing an MGI Switch Control Panel, it’s important to understand how different switches will affect your project. With this switch basics tutorial, we strive to provide you with the right tools to attain your goal.

Poles and Throw

In dealing with switches, you may have come across terms like single-pole single-throw (SPST), single-pole double-throw (SPDT), double-pole single-throw (DPST), and double-pole double-throw (DPST). These define the internal circuit structure and contact form of a switch.

  • Poles describe how many circuits the switch can control. A single-pole switch controls just one circuit while a double-pole switch controls two separate circuits. A double-pole switch is essentially two single-pole switches that are operated by the same actuator.
  • Throw indicates how many outputs each switch pole can connect to. The most common types are single-throw and double-throw, and these are closely related to the switch function.

Single-Pole Single-Throw SPST

SPST switches consist of a single input and output — the circuit is either open or closed — and are ideal for on-off applications. These can have either maintained or momentary action. When it comes to SPST type switches, MGI products typically consist of ON/OFF and (ON)/OFF switches.

spst diagram and switch

Single-Pole Double-Throw SPDT

With one pole and two throws, the SPDT switch has three terminals — one common input and two outputs — and is suited for on-on or on-off-on applications. When it comes to SPDT type switches, MGI products typically consist of ON/OFF/ON and (ON)/OFF/ON switches. It should be noted that the throw-count does not necessarily relate to the number of positions on a switch; ON/ON function is a valid SPDT, and so is ON/OFF/ON/OFF. As long as there are two outputs for which current to flow, the switch is of double-throw type.

spdt diagram and switch

Double-Pole Single-Throw DPST

The DPST switch turns two circuits on or off using only a single toggle, pushbutton, etc. With two poles and one throw for each pole, DPST switches have four terminals — two inputs and two outputs. These are essentially two SPST switches built together, affecting separate circuits but controlled by the same actuator.

dpst diagram and switch

Double-Pole Double-Throw DPDT

With two poles and two throws for each pole, the DPDT switch has six terminals — two inputs and four outputs (two outputs for each individual circuit). A DPDT switch controls two separate circuits with the same actuator, which is designed for on-on or on-off-on function.

dpdt diagram and switch

A Few More Things About Switch Basics

Although uncommon, switches with more than two poles and/or throws do exist. For example, a 4-position ignition key switch has three throws: accessories, on, and start. But for the most part, single and double pole/throw switches will get the job done. MGI SpeedWare boasts a wide selection of these switches for individual sale or for installation on switch control panels.