Switch Configurations: SPST, SPDT, DPDT, and Their Roles in Pressure Control Systems
Opinion | May. 5th, 2025
Joshua Matthew Williams
Pressure Product Line Manager
This article delineates the operational principles and distinctions among SPST, SPDT, and DPDT switches, emphasizing their roles in pressure control systems.
Core Definitions
Single Pole Single Throw (SPST)
A fundamental two-terminal switch with a single conductive path, operating in a binary mode (open/closed). Applications focus on basic circuit interruption, such as de-energizing pumps when pressure exceeds preset limits in hydraulic systems.
Single Pole Double Throw (SPDT)
A three-terminal switch using one movable contact to alternate between two stationary paths. This enables dual-output control, commonly seen in pressure switches managing sequential actions (e.g., activating a cooling fan at 100 psi while maintaining primary compressor operation).
Double Pole Double Throw (DPDT)
A six-terminal configuration controlling two isolated circuits simultaneously. Critical for fail-safe operations in pressurized environments, such as shutting down dual boilers and engaging emergency vents upon detecting overpressure conditions.
Functional Differentiation
SPST: Simplifies on/off control with no path alternation. Ideal for low-complexity pressure cutoffs (e.g., residential water pumps).
SPDT: Provides path-switching capability with hysteresis control, optimizing stability in pneumatic systems with fluctuating thresholds (±0.5 bar deadband typical).
DPDT: Synchronizes multiple subsystems, ensuring redundancy in high-risk industrial settings (e.g., oil refinery pressure relief).
Pressure Switch Implementation
Pressure switches integrate these configurations to convert mechanical displacement (from diaphragms or bourdon tubes) into electrical signals:
SPST: Directly breaks circuits at fixed setpoints. Example: 10A/250VAC contacts for HVAC pressure cutoffs.
SPDT: Enables bidirectional control, such as switching between alarm triggers and process adjustments in chemical reactors.
DPDT: Coordinates multi-stage responses – simultaneously disabling actuators and initiating backups in power plant safety protocols.
Key design parameters include:
Contact Rating: ≥15A for industrial hydraulic systems
Repeatability: <±1% full scale (FS) for SPDT pressure switches
Environmental Sealing: IP68 for submersible or high-humidity applications
Anwoll pressure switch
Anwoll is a leading developer of pressure measurement and applications, with a mature series of pressure switches suitable for daily and cutting-edge applications.
If you would like to learn more about our pressure switches, please contact us directly.
