What do SPST, SPDT, and DPDT mean?
SPST, SPDT, DPST and DPDT are abbreviations for popular types of switches.
These terms define the internal circuit structure and contact form of a switch. So what’s the difference? SP refers to single-pole while DP refers to double-pole. Similarly, ST refers single-throw while DT refers to double-throw.
- 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. Think ON-OFF (single throw) vs ON-OFF-ON (double throw).
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.
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. Keep in mind 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 actuator (ie. 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 (or two outputs for each individual circuit). A DPDT switch controls two separate circuits with the same actuator, which is generally designed for on-on or on-off-on function.