Troubleshooting Notes
Snap discs are used in a variety of ways and have many names, such as; Thermodiscs, Thermal switch, Bi-metal Switches, Fan Switches, Limit Switches, High Limit Switches, Low Limit Switches and Proof of Fire Switches. These switches have two or more wire terminals and inside the switch there is a disc that is coated with two dissimilar metals. Because all metals expand and contract at different temperatures, the surface tension of these metals together on a circular sheet of metal causes the disc to either have a convex or concave shape. Depending on which side of this disc is facing up toward the pin, the switch is either making a normally open or closed circuit at room temperature. When the switch detects heat, the metal on the disc will expand causing the disc to “flip” its convex or concave shape which allows the pin inside the switch to reverse its normally open or closed state. Bi-metal snap discs will automatically reset back to their normal state when enough of a temperature change is detected. Singular metal snap discs will have a button on the switch that must be manually pressed to reset the switch.
