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.
Switch Basics — What’s Covered
- Switch Actuation — The physical action that “flips” the position of a switch.
- Switch Function — The positions that a switch can be flipped to, such as ON or OFF.
- Momentary vs. Maintained — Does the switch remain in its flipped-to position?
- Poles and Throw — What do SPST, SPDT, DPST, and DPDT mean?
Actuation
An actuator is the mechanical component of a switch that is used to manually open and close a circuit. Toggles, buttons, keys — anything that changes the state of the switch. This physical action is more formally called the switch actuation.
Although actuation is a defining trait, choosing whether you want to flip, push, slide, or key-turn your switch is more a matter of preference. Most MGI switches are of the toggle, rocker, pushbutton, or key-turning variety.
Switch Function
The switch function describes the positions or states that the actuator may be moved to. The most simple switch function consists of on and off positions, typically denoted as ON/OFF. More versatile switches may boast additional positions, such as an ignition key switch with typically 4 or 5 positions.
| Some examples of switch functions: |
|
Maintained vs. Momentary
Take any switch and actuate it: What happens to the toggle or pushbutton after you let go? Does the circuit stay ON? There are two distinct categories for switches: momentary or maintained.
Maintained switches — like a common light switch — stay in the actuated position until once again acted upon. ON/OFF is the simplest maintained switch function, though ON/OFF/ON is another popular choice. These switches are ideal to power electric accessories that don’t require constant supervision.
Momentary switches, on the other hand, only remain active while physically actuated. Think of a keyboard button or doorbell. Most pushbuttons are of the momentary type, although that’s not a general rule. The most simple momentary switch function is (ON)/OFF, where the brackets denote momentary action. That is, the switch will only remain (ON) while being pushed, pressed, turned, etc. An ignition key switch — ACC/OFF/RUN/(START) — with momentary action on the start function only, is another great example.
| Maintained | Momentary |
| Examples of maintained and momentary switches: | Some switches, like toggle and rocker switches, are not strictly limited to maintained or momentary action. |
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.
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.
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.
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.
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.