Limit Switches

Limit switches are electromechanical or solid state devices that require the physical contact of an object with the switch’s actuator to make the contacts change state. There are two main types of products: rotary and linear. Rotary limit switches use physically-actuated levers to rotate a shaft and operate the contacts in a switch, making them change state. Linear limit switches are electromechanical devices that require physical contact between a target object and switch activator to make the contacts change state. Specialty limit switches are also available. 

Limit switches differ in terms of switch type and contact type. There are two choices for switch type: electromechancial and solid-state. Electromechanical limit switches have mechanical contacts, relays, or reeds. Solid-state limit switches are electronic and do not have moving parts.  There are three choices for contact type: momentary contact, maintained contact, and positive opening. Momentary contact means that the switch is open or closed only during actuation. Maintained contact means that the limit switch contacts remain in the triggered position even after the actuator has been released. The contacts are reset only by further mechanical action of the operating head. Positive opening means that the contact-point opens reliably and then remains open, in the activated position, even in the event of a mechanical failure.  

Normal state and switch configuration are important parameters to consider when selecting limit switches. There are two choices for normal state: normally open (NO) and normally closed (NC).   Normally open (NO) means that the switch contacts, when in their open position, interrupt the circuit. When actuated, the contacts close.   Normally closed (NC) means that the switch contacts, when in their normally closed position, establish a circuit. When actuated, the contacts open. There are four choices for switch configuration: single pole, single throw (SPST); single pole, double throw (SPDT); double pole, single throw (DPST); and double pole, double throw (DPDT). 

Limit switches differ in terms of orientation and performance. The orientation of the limit switch actuator is critical when determining sizing requirements. Some limit switches have a top-mounted actuator. Others have the actuator on the side of the limit switch. In terms of performance, limit switches carry specifications for maximum current rating, maximum AC voltage rating, maximum DC voltage rating, minimum mechanical life, minimum electrical life, and operating temperature. Some limit switches are designed for use in transistor-transistor logic (TTL) circuit. 

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Part Numbers for Limit Switches