A method of dimming that reduces the amount of time that full voltage is applied to the lamp is referred to as?

• A. Phase control
• B. Pulse-width modulation
• C. Voltage reduction
• D. Current limiting

Answer: (A)

Dimming by phase control works by cutting parts of each AC cycle so the lamp is energized only during a portion of the cycle. By delaying the point in the cycle at which the switching device (like a triac) turns on, the lamp receives full mains voltage for less time in each half-cycle. This reduces the average voltage across the lamp, which lowers brightness. It’s the classic method used in most household dimmers because it directly controls how long the lamp is connected to full voltage without changing the overall frequency.

Pulse-width modulation is a different approach that switches on and off at a much higher frequency, producing a variable duty cycle. While it can dim loads, it’s not the typical method for mains AC lamp dimming and can introduce flicker or EMI with incandescent lamps. Voltage reduction is too vague a term, as it doesn’t specify how the voltage is being manipulated. Current limiting would change the current drawn but isn’t the mechanism used to dim lamps by slicing portions of the AC waveform.