form a: single pull-single throw (spst), normally open (no) magnetic reed switch configuration
This is the most common configuration of magnetic reed switch operation. The Form A magnetic reed switch has two contacts which are Normally Open (NO). Having NO contacts means that the introduction of a magnetic field will cause these contacts to close. That closure, in turn, also closes the circuit, thereby conducting electricity. Upon removal of the magnetic field, the contacts will return to their Normally Open (NO) state, thereby opening the circuit and stopping the flow of electricity. A Form A magnetic reed switch can be wired without consideration as to the direction of electrical flow.
form b: single pull-single throw (spst), normally closed (nc) magnetic reed switch configuration
Form B is the least common magnetic reed switch configuration and operates in the opposite fashion as a Form A. The Form B magnetic reed switch also has two contacts, but they are Normally Closed (NC). This NC designation means that, in its default state, the circuit is closed and will itself conduct electricity. The introduction of a magnetic field will cause the contacts of a Form B magnetic reed switch to open, or separate, thereby opening the circuit and breaking the flow of electricity. By removing the magnetic field, the contacts can return to their Normally Closed (NC) state, thereby closing the circuit. A Form B magnetic reed switch can also be wired without consideration as to the direction of electrical flow. re
form c: single pull-double throw (spdt), normally open/normally closed (no/nc) magnetic reed switch configuration
The Form C magnetic reed switch configuration is also common. A Form C magnetic reed switch is comprised of three unique contacts. Those contacts include both a common contact and one each, Normally Open (NO) and Normally Closed (NC).
Essentially, a Form C magnetic reed switch has all of the performance capabilities of both a Form A and Form B magnetic reed switch combined within a single, compact, hermetically sealed package. In its default state, electricity flows from the common contact through the Normally Closed (NC) contact. Upon introduction of a magnetic field, the common contact moves from the Normally Closed (NC) contact to the Normally Open (NO) contact. This movement results in the opening of one circuit and the closing of the other, thus redirecting the flow of electricity. Removal of the magnetic field will then cause the common contact to revert back to its initial Normally Closed (NC) position.
Unlike a standalone Form A or Form B magnetic reed switch, the direction of electrical flow during the wiring and installation of a Form C magnetic reed switch, specifically, must be considered. For typical SPDT functionality, the electrical source is attached to the common lead. This is not to say that a configuration where switching between two electrical sources, including on the NO and NC leads over to the common lead, is not possible. It simply is not the norm.