A dimmer is an electronic device that controls the amount of power used by a load. It turns off parts of the main waveform to reduce the amount of time the lamp receives main voltage. Switching on a variable portion of the main waveform controls the amount of power sent to a lamp. This is known as phase control. The devices that switch the main waveform are collectively known as thyristors. These are divided into two groups: triacs and siliconcontrolled rectifiers (SCRs).