# Does an AC lightswitch dimmer/light combination consume the same power no matter its setting?

I'm hoping to resolve an argument here. Someone else said that lightswitch dimmers don't actually cut back on power used, since they're just resistors and just divert the power to additional heat. I agree that dimmers are just resistors, but from my thinking, V=IR and P=IV gives us P=V^2/R. Voltage is constant in this system, so as resistance increases between the dimmer and the lightbulb in series, power will go down. Who is correct?

• Many dimmers are synchronized with the AC mains power and adjust the phase angle during which they trigger operation. For example, at 50% setting they may trigger half-way through the AC half-cycle. The light would only dissipate for half the time. Since they use semiconductor switches, they don't dissipate much power themselves. Sure, there's a resistor involved. But it is only part of an RC timing circuit and not directly in-line with the load. There are types that only start after a delay from the zero-cross and others that cut things off after a delay. But the basic ideas are similar. – jonk Jul 23 '20 at 2:43
• ac light dimmers are not just resistors ... the "someone" does not know what they are talking about – jsotola Jul 23 '20 at 3:00
• Does this answer your question? electronics.stackexchange.com/questions/38031/… – vu2nan Jul 23 '20 at 3:20
• "since [dimmers] are just resistors and just divert the power to additional heat" - If that were true, wouldn't dimmers get incredibly hot, especially when dimming bigger incandescent bulbs? – marcelm Jul 23 '20 at 6:59