SPSTNO

SPSTNO stands for single-pole, single-throw normally open. It refers to a switch configuration with one circuit path (pole) and one contact path (throw). In the resting state the contacts are open, so no current flows; actuation closes the contacts to complete the circuit.

Mechanical SPSTNO switches achieve closure by an actuator such as a pushbutton, toggle, or lever. The arrangement can be momentary (push-to-make) or maintained (latching), in which case the circuit remains closed after actuation until it is reset. Electronic variants can realize SPST NO behavior using devices such as transistors, thyristors, or solid-state relays, controlled by a separate input signal.

SPSTNO switches are used for basic on/off control of a single circuit. They are common in consumer appliances as power switches, in door or limit interlocks, in control panels, and in automotive or industrial equipment where a simple, low-cost opening/closing path is sufficient.

Ratings and characteristics vary widely by device. Specifications typically include nominal voltage and current, contact resistance when closed, mechanical life, actuation force, and environmental sealing. The defining feature is the normally open state: the circuit is open until the switch is actuated.

Compared with SPST-NC (normally closed), the SPST-NO provides default off behavior. Related configurations include SPDT (single-pole double-throw) and other multi-pole variants used for more complex switching tasks.