Yesterday's lesson covered bimetallic strips that bend gradually with temperature. But a slowly-bending strip makes a terrible switch: as the contacts drift together, they arc, weld, chatter, and burn. The fix is a piece of geometry borrowed from a child's toy popper — the snap-action bimetallic disc, invented by Spencer Thermostat Company in the 1930s and made famous under the Klixon brand.

A snap disc is a small, dished (concave) bimetallic disc, typically 10–25 mm in diameter. At room temperature it sits stable in one curvature — say, bowed downward. As it heats, the higher-expansion layer wants to grow but is constrained by the dish geometry. Stress builds elastically until, at a precise temperature, the disc buckles through center and snaps to the opposite curvature in under a millisecond. Cooling reverses the process at a lower temperature (the reset point). The gap between trip and reset — the differential — is inherent to the geometry, typically 15–40°F.

Why buckling beats bending:

• Speed: Contact separation happens in ~1 ms regardless of how slowly temperature is rising. No arc-time, no contact welding.
• Repeatability: Trip temperature is set by disc geometry (dish depth, diameter, alloy stack) during stamping — typically ±3°F over millions of cycles.
• Hysteresis: Built-in differential prevents chatter near the setpoint. You cannot get a snap disc that trips and resets at the same temperature.
• Force: The snap delivers real mechanical work — enough to move contacts through debris, ice, or light corrosion.

Real-world example: The thermal cutoff in a coffee maker or clothes iron is almost always a snap disc — trip at 100°C, reset at 75°C, wired in series with the heating element. If the thermostat sticks or the pot boils dry, the disc trips instantly and stays tripped until the appliance cools. Automotive radiator fan switches, refrigerator defrost terminators, and hair dryer overheat protection all use the same mechanism. HVAC limit switches on gas furnaces are snap discs rated 150–250°F.

Design rule of thumb: Choose the trip temperature 20–30°C above your maximum normal operating temperature, and confirm that the reset point is below ambient extremes plus self-heating. A disc that trips at 90°C and resets at 70°C in a 60°C ambient will cycle indefinitely once tripped — bad for auto-reset applications, useless for manual-reset ones. For safety-critical shutdown, spec a one-shot thermal fuse instead: the disc resets, but a Y-alloy fuse doesn't.

Failure modes: Contact erosion from arcing (limits life to 10K–100K cycles at rated load), fatigue cracking of the disc at millions of thermal cycles, and creep-induced setpoint drift if held near trip for years. Never wire two snap discs in parallel expecting redundancy — one will always trip first and take all the wear.