• thermal switches are simple and expensive devices used to activate or deactivate an electrical circuit at a set temperature.
• they are commonly used to activate cooling fans or to deactivate a device that is running hot
• the switch body is affixed to the object¹⁾ being measured
• regardless of type (flat bar or button) the switch connectors are spliced into the positive leg of the existing circuit in order to make/break the connection

The switch used below to illustrate specs is: KSD9700 NO 45C Temperature Control Switch Thermostat Thermal Protector Switch - Normally Open/250V 5A/Metal/2Pcs

A thermal switch can either make (close) or break (open) a circuit when the temperature setpoint is reached.

• a Normally Closed (NC) switch is closed (circuit is made) until the set temp is reached. At that time the switch opens, breaking the circuit. Common use: LiFePO4 warming pad shut off at 10C (50F). Or turning off alternator charging when alternator case temperature reaches a given temp.

• a Normally Open (NO) switch is open (circuit is broken) until the set temp is reached. At that time the switch closes, making the circuit. Common use: cooling fan blowing over a DC-DC charger turns on at 40C (104F).

Our example switch is normally open; the listing says both NO and Normally Open.

The main temperature spec is the temp (usually in Celsius) at which the switch is triggered. Our example is triggered at 45C.

There is also a temperature at which the switch no longer triggered. If our use case turns on a fan when temps hit 40C then the when temps cool the fan is eventually turned off again. This “hysteresis” needs to be wide enough that the switch doesn't cycle ON/OFF rapidly, but narrow enough that the circuit recovers fast enough to meet our criteria. You don't want the fan running forever or you would have just hardwired it.

The example switch does not give this “fallback” temperature but we can deduce it from comments. For example:

Switch activities at 103-105°F, deactivates at 102°F (38.9C)

That's a rather narrow swing, but maybe that's what you want. Others might be 5C or more wide.

These devices are quite small and meant to carry small currents like trigger signals or very small loads. Our example switch can handle up to 5A.

Since many are rated >100v, this spec usually won't make a difference for 12v systems. Our example is rated to 250V.