difference between SPST and SPDT micro switch
Sep 12, 2025
Single-Pole Single-Throw (SPST) and Single-Pole Double-Throw (SPDT) micro switches are two common configurations, differing fundamentally in contact structure, circuit control capability, and application scope-while retaining the micro switch's core advantage of low-force actuation.
1. Core Structural & Contact Configuration
An SPST micro switch features three terminals: one common pole (COM), the fixed input terminal connected to the power source or signal), and one normally open (NO) or normally closed (NC) terminal (the single output terminal). Its contact system only supports a single circuit path. For example, an SPST-NO switch has no connection between COM and NO in the unactuated state; actuation closes the contact to conduct the circuit. An SPST-NC switch, by contrast, maintains a closed path between COM and NC when unactuated, and opens the circuit when actuated.
An SPDT micro switch, however, has four terminals: one COM terminal, one NO terminal, and one NC terminal. Its key distinction is a movable contact linked to COM that "throws" (switches) between the NO and NC terminals. In the unactuated state, COM connects to NC (closing the NC circuit) and disconnects from NO (opening the NO circuit); actuation reverses this-COM disconnects from NC and connects to NO, switching the current/signal path.
2. Circuit Control Functionality
SPST switches are single-circuit, on/off controllers. They can only perform a binary function: either connecting or disconnecting one specific circuit. They lack the ability to redirect current/signal; their role is limited to activating or deactivating a single load (e.g., a small light or sensor).
SPDT switches are single-pole, dual-path diverters. They control one input (via COM) but can route it to two alternative outputs. This enables two complementary functions: for instance, cutting power to one component (via NC disconnection) while supplying power to another (via NO connection) in a single actuation-eliminating the need for two separate SPST switches.
3. Typical Applications
SPST micro switches are ideal for simple on/off scenarios, such as:
Power triggers for small household appliances (e.g., coffee maker power buttons).
Limit sensors in consumer electronics (e.g., laptop lid closure to deactivate the screen).
SPDT micro switches suit applications requiring signal/current redirection, such as:
Industrial position sensors (e.g., conveyor belt "start" when a part is detected, and "stop" when it passes).
Automotive controls (e.g., headlight dimmer switches to toggle between high and low beams).
