Which component is used in controlling the brightness of lighting fixtures?

The component that is primarily used in controlling the brightness of lighting fixtures is the LED driver. LED drivers play a crucial role in the functioning of LED lighting by regulating the amount of electrical current delivered to the LEDs. They ensure that the LEDs operate efficiently and at the desired brightness level. By adjusting the input current, the LED driver can effectively control the intensity of the light emitted from the fixture.

In contrast, a thermal overload relay is designed to protect motors by disconnecting them from the power supply when it detects overheating, making it unsuitable for controlling brightness. A circuit breaker serves to protect electrical circuits from overloads and short circuits but does not have the capability to adjust brightness levels. A UPS (Uninterruptible Power Supply) provides backup power in the event of an outage and is not involved in lighting control. Thus, the LED driver is the correct answer, as it directly influences the operation and brightness of LED lighting fixtures.