What is used to control heavy current that is unsuitable for electrical contacts in a thermostat?

In a thermostat, a relay is used to control heavy currents because it functions as an electromagnetic switch. Relays are designed to handle higher power loads than the thermostat’s electrical contacts can manage directly, allowing for the safe control of devices such as heating elements or air conditioning compressors.

When the thermostat detects a temperature change, it sends a small current to the relay's coil. This small current activates the relay, closing its contacts to complete the circuit for the heavier current that powers the heating or cooling system. By doing so, the relay ensures that the delicate components of the thermostat are protected from the high currents, which could otherwise damage them.

Additionally, other options serve different purposes. A transformer typically steps voltage up or down but does not control heavy currents in the way a relay does. A capacitor stores electrical energy for smoothing out voltage fluctuations and does not activate circuits. A switch can control circuits manually but may not be able to handle the more significant electrical loads safely or effectively without potentially causing damage or failure.